Effect of Calving Season and Timing Within Season on Performance and Economics of Cow-Calf Production in Southwest Missouri

Simple Summary

Cow-calf operations are the first step in beef production. The sustainability of cow-calf operations is greatly influenced by producers’ ability to plan management strategies that function most efficiently around variables such as weather, forage quality, and market conditions. These factors fluctuate semi-consistently throughout the year and can affect the performance of a cow and her calf, determining operational profitability. The purpose of this study was to evaluate the effect of the time of calving on cow and calf performance and, ultimately, net monetary returns. Cow and calf records were analyzed in conjunction with feed and forage data and market records to determine not only calf marketability but also cow efficiency and performance as affected by the timing of calving. Results indicated that later calving periods in spring negatively affected cow reproductive performance, calf weaning weight, total costs, and net returns compared with fall calving.

Abstract

A multitude of factors affect the optimum calving season, requiring site-specific systems analysis. The objective of this study was to determine the effects of the calving season (S1 and S2) and the timing of calving within the season on production parameters and economics in southwest Missouri. A five-year study was conducted from 2014 to 2018 using field data recorded for 1979 Hereford cow–calf pairs. Cows were categorized by calving season and 21 d calving periods within season (Periods 1–5) in which they calved. Data were analyzed under a completely randomized design with effects of season, period, calf sex, and two and three-way interactions with the calving year as a random effect. The Cattle Value Discovery System (CVDS) beef cow model was used to estimate required cow feed and feed intake. Historical cattle market prices were used, along with total feed costs, to calculate net returns. Calving in later periods reduced the pregnancy percentage in S1 but not S2. Cows calving earlier in both seasons weaned heavier calves. Feed costs were greater in S2, but replacement heifer costs were greater in S1, especially in later calving periods. Overall, net returns were not different between S1 and S2 cows, but net returns declined in later calving periods in S1, whereas there was no difference in S2. In conclusion, at the latitude of southwest Missouri for a forage species with a bimodal growth curve, the calving period affected cow productivity and profitability in Season 1 but less so in Season 2 cows, with no difference in profitability between calving seasons.

1. Introduction

The choice of calving season is based on many factors, such as the environment, availability of feed resources, availability of labor, market timing, and tradition [1]. The environment is most likely the single largest factor to consider and varies by latitude and longitude across the U.S. such that optimal calving season is not the same throughout the nation. For example, extreme cold affects neonatal calf survival [2]. At northern latitudes, late-spring calving is more advantageous than early-spring or fall calving [3,4], but fall calving is more advantageous in southern latitudes [5,6].

The chosen calving season dictates the timing of other aspects of the production cycle—specifically, the breeding season, which must be taken into consideration. Environmental conditions such as temperature and humidity influence heat and cold stress experienced by cattle during the breeding season and can have negative effects on male [6,7] and female [8,9] fertility. The breed affects the reproductive response to environmental stress and the photoperiod length [10], and the photoperiod affects puberty in heifers and postpartum anestrus in cows [11,12], which are critical to maintaining a 365-day calving interval. Additionally, beef heifers that calve early in their first calving season remain in the herd longer and have greater lifetime productivity [13,14,15] than those calving later in the season. Cows that calve early in the calving season generally wean heavier calves and have a greater pregnancy percentage than cows calving later in the calving season [16,17].

Calf prices fluctuate throughout the year with changes in supply and demand [18], impacting prices received when marketing calves born in different seasons and born at different times within a calving season. Labor supply affects the ability to monitor cows during calving and utilize advanced reproductive technologies in integrated crop and livestock operations during the growing season [19].

Nutrient requirements of beef cows are greatest during early lactation, leading up to the breeding season [20], and maintaining cows in adequate body condition (BCS ≥ 5; range = 1 to 9) during this time is critical to ensure maximum pregnancy percentages [21]. However, environmental factors including temperature, precipitation, soil type, latitude, and topography can affect forage species suited to the region and the timing of optimally nutritious forage availability [22], and the availability of feed resources interacts with mature size and milk production [23] to affect the optimum time to calve beef cows.

Milk production of beef cows is affected by energy intake such that the cow balances milk production with mobilization of body reserves [23,24]. Additionally, the calf becomes more reliant on nutrients from forage in mid and late lactation as milk production declines. Thus, aligning the calving season to coincide with the availability of grazed forage of greater nutritive value during early lactation and breeding can reduce supplemental feed costs [25]. However, having high-quality forage available in late lactation can increase calf growth [26,27]. All cows do not calve at the same time within the calving season, and alignment of nutrient demands of the cow and calf with the nutritive value of forage varies among cows.

Consequently, determining the optimum calving season is difficult due to the myriad factors involved and the site-specific nature of cow-calf production systems [28]. Additionally, timing of calving within a calving season impacts cow productivity due to the age of the calf and alignment of nutrient requirements with the nutritive value of forage. However, little research has evaluated the optimal calving season and timing within a calving season in central latitudes without extreme winters and summers or bimodal (spring and fall) forage growth curves. Thus, we hypothesized (1) that spring and fall calving seasons would be equally productive but that fall calving cows would be more profitable at the central latitude of southwest Missouri and (2) that cows calving earlier in the calving season would be more productive and profitable. The objective of this study was to determine the effects of the calving season and the timing of calving within the calving season on production parameters and economics of cow-calf systems in southwest Missouri.

2. Materials and Methods

A research protocol was submitted for approval by the Missouri State University Institutional Animal Care and Use Committee (Protocol #16-035.0); however, the committee deemed that a protocol was not necessary, since data collected on cows and calves were generally accepted as production data. The care and use of animals followed the Guide for the Care and Use of Agricultural Animals in Research and Teaching.

2.1. Cow/Calf Performance Traits

2.1.1. General Management

Research was conducted at Missouri State University’s Leo Journagan Ranch, located south of Mountain Grove, MO (36.993897, −92.258733). The five-year study, which was conducted from 2014 to 2018, utilized field data from Hereford cow–calf pairs (2222 calvings). Cows and calves were managed on 16 to 65 ha pastures year-round and divided into approximately 30-cow groups. Some groups continuously grazed a single pasture, and others were minimally rotated to alternate pastures throughout the year as grazing necessitated based on forage availability. The major forage species was Tall Fescue (Festuca arundinacea), along with other cool- and warm-season grasses and legumes. The full list is available in the Supplemental Material. Hay of similar plant species was produced on the ranch and fed during winter months (generally, December–April). Pasture management included fertilization every third year with urea, and hay pastures were fertilized every year.

Cows were managed as either spring or fall calving herds (Season 1 or Season 2, respectively). Season 1 cows gave birth in late January through early July, and Season 2 cows gave birth in late August through early December. The months composing Season 1 and Season 2 were determined based on the calf weaning date, as calves born January through July were usually weaned the following October/November and calves born August through December were usually weaned the following June. Most cows were bred via natural service, but about 100 cows/year (mostly heifers) were bred via artificial insemination. Bulls were released into pastures on April 1 for Season 1 and on November 1 for Season 2, then removed at weaning.

Cows were provided with supplemental feed during the winter and respective breeding seasons. Season 1 cows were supplemented 0.91 kg/hd/day from late January through March and 0.70 kg/week from April through June. Season 2 cows were supplemented with 1.13 kg/hd/d from August through December and 0.91 kg/hd/day from January through April. Season 2 calves were provided with creep feed from March until weaning (usually in May) via a self-feeder. Target creep feed intake at weaning was 2.27–3.63 kg as-fed/calf/day. The same grain mix was used for cow supplement and calf creep feed (Table S1).

2.1.2. Data Collection

Performance data were recorded by ranch employees as a part of normal production practices in a beef cattle seedstock operation. Collected cow data included American Hereford Association (AHA) registration number, dam tag ID, Body Condition Score (BCS) at calving and weaning, mature cow weight, and pregnancy status. Body condition scores were collected from 2016 to 2018 by visual appraisal using methods described in [29]. At weaning, the mature cow weight was recorded, and the pregnancy status was checked via palpation. Calf data collected included calf AHA registration number, date of birth, calf sex, birth weight, weaning weight, and weaning date. The calf birth weight was recorded within 24 h of parturition using a weight tape. Collected records were used to calculate the calving interval, calf weaning age, and 205-day adjusted weaning weight. Due to use of field data, data collection was partially incomplete; in these cases, some data gaps were able to be filled using the AHA online registry records. A total of 1979 complete records were used in data analysis.

2.2. Model Outputs

2.2.1. Forage and Supplement Sampling

Forage samples were collected monthly from all pastures in which study cows were residing from 2016 to 2018. Hay core samples were collected once a year from approximately 10% of total hay inventory in such a manner as to ensure all cuttings and fields were included in the yearly sample. Monthly pasture samples consisted of 4 to 6 combined sub-samples per pasture collected from locations chosen by observing the grazing patterns of cattle. Hay and pasture forage samples were dried in a forced-air oven at 55 °C, ground using a Cyclone Sample Mill (UDY Corporation, Fort Collins, CO, USA), and stored in plastic air-tight bags. Ground samples were analyzed for crude protein (CP), neutral detergent fiber (NDF), and acid detergent fiber (ADF) using near-infrared spectroscopy (NIRS SpectraStar, Unity Scientific, Milford, MA, USA). The total digestible nutrients measure (TDN) was calculated using Equation (S1) [30]. Metabolizable energy (ME) was calculated from TDN using Equation (S2) [20]. Samples of supplemental feed were collected from each load delivered to the ranch from 2016 to 2018. Samples were analyzed by Dairy One Forage Laboratory (Ithaca, NY, USA) for moisture, CP, NDF, ADF, and calculated TDN. The metabolizable energy concentration was computed using Equation (S3) [31]. Forage, hay, and supplement nutrient compositions are reported in Tables S2 and S3.

Forage metabolizable energy concentration (MEC) values were averaged for all pastures for each month of collection during the study for input into the model. The years of the study that preceded the forage collection period used a monthly average across all years of the forage collection period as the input forage ME values. Hay vs. pasture forage consumption was estimated based on ranch management practices. Consumption estimates were as follows: December to March = 100% hay, April = 75% pasture and 25% hay, and May to November = 100% pasture. A modification to the model was made to account for creep feed consumption by Season 2 calves. The model was modified to substitute 45% of non-milk intake with creep feed while assuming the other 55% of non-milk intake consisted of forage and that the calf would consume the entire daily milk yield of the cow. A rate of 45% was chosen because it gave an average creep intake for the group that matched the estimated intake of 1.81 kg/(calf/day) set by the ranch manager and accounted for variance in intake amount based on varying calf body weight.

2.2.2. Model Calculations

The Cattle Value Discovery System (CVDS) beef cow model was used to estimate nutrient requirements of cow–calf pairs, hay and supplemental feed consumption, and cow efficiency [32]. Performance inputs required for this model were cow weight at weaning, calf date of birth, birth weight, weaning weight, sex, weaning date, and date of cow weight. Values were obtained from performance data collected for the calf crops of 2014–2018. Other inputs included the estimated calf-equivalent shrunk body weight at harvest (ESBW), and relative milk production (RMP); these were applied to all cows as follows: ESBW = 550 kg and RMP = 5 (on a 1–9 scale). These values were chosen because they are average values for beef cattle in the USA, as actual values were unknown.

From performance and forage/feed inputs, the model calculated values for ME required for maintenance, gestation, and lactation throughout the production cycle. The net energy requirement for maintenance was based on cow weight at weaning using a net energy for maintenance (NEm) requirement of 77 kcal/kg shrunk body weight^.75 [20]. The net energy requirement was converted to the metabolizable energy requirement by calculating the efficiency of ME use for maintenance of the diet as diet NEm concentration divided by the diet ME concentration. The net energy requirement for gestation was computed from calf birth weight and day of conception, assuming 283 d gestation [20]. The net energy required for gestation was converted to ME required for gestation using an efficiency of ME use for conceptus of 0.13 [20]. The net energy requirement for lactation was computed from milk yield, which was calculated using the equation of Tedeschi et al. [32] and the National Academies of Science, Engineering and Medicine [20] milk yield equation based on the day of lactation by iteratively solving for the peak milk yield (PKM) such that predicted calf weaning weight matched the observed calf weaning weight. The net energy for lactation was converted to ME required for lactation assuming an efficiency of ME use for lactation of 0.64 [33]. The total ME required (MER) was calculated as the sum of ME required for maintenance, gestation, and lactation. The equations to predict milk yield also predict calf non-milk ME required (forage and/or creep feed).

The predicted ME intake was computed using the National Academies of Science, Engineering and Medicine [20] dry-matter intake equations for beef cows multiplied by the MEC of the forage (grazed and/or hay) plus supplemental ME intake. The energy balance was calculated for each day of the production cycle as ME intake minus MER and, as such, represents the deficiency of forage alone to meet energy requirements. The energy-balance nadir (EBN) is the lowest value during the production cycle. The time of EBN during the production cycle was calculated as days since parturition at energy-balance nadir (DSPN) and day of the year at energy-balance nadir (DOYN). Cow efficiency was evaluated using the energy efficiency index (EEI), which is calculated as the ratio of MER to calf weaning weight (Mcal/kg) based on cow MER alone (EEI1) or cow plus calf MER (EEI2).

The CVDS beef cow model uses cow weight, milk energy yield, and calf birth weight to compute the ME required for maintenance, lactation, and gestation, respectively, which are important variables that explain at least 85% of the variation in energy intake of beef cows [34,35]. An evaluation of the CVDS beef cow model using an independent dataset of drylot-fed cows found that MER explained 74% of the variation in observed ME intake, with a mean bias of 1671 Mcal (17% underprediction) over the entire production cycle [36]. Additionally, the predicted milk energy yield explained 25% of the variation in observed milk energy yield, with a mean bias of 105 Mcal (10% underprediction) over 240 d lactation, and EEI1 explained 39% of the variation in observed cow biological efficiency, with a mean bias of 5.35 Mcal MER/kg WW (17% underprediction). Thus, the CVDS beef cow model gives a reasonable estimation of cow energy consumption, milk yield, and biological efficiency.

2.3. Economic Outputs

Historic prices for weaned calves and cull cows were used to determine revenue. Prices for weaned calves and cull cows were obtained from USDA market report archives (https://www.ams.usda.gov/market-news/search-market-news, accessed on 17 September 2019). Weaned calf prices were from the USDA-MO Dept. of Ag Market News Feeder Cattle Reports for Joplin Regional Stockyards (report code jc_ls758), Springfield Livestock Marketing Center (report code jc_ls771), and Ozarks Regional Stockyard (report code jc_ls763), using the Medium and Large 1–2 feeder cattle grades. Calf revenue (CaREV) was calculated as the weaning weight multiplied by the calf price. Cull cow prices were from the USDA-MO Dept. of Ag Market News Slaughter/Replacement Cattle Reports for Joplin Regional Stockyards (report code jc_ls174) and Springfield Livestock Marketing Center (report code jc_ls140). Boner price was used for cows with a weaning BCS 5–7, and lean price was used for cows with a weaning BCS of less than 5. Cull cow revenue (CoREV) was calculated as cow weight at weaning multiplied by cull cow price.

Hay prices were gathered from USDA market report archives of the Missouri Weekly Hay Summary(report code jc_gr310) for June and July when hay was harvested. Cow supplement and creep-feed prices ($/ton) were collected from invoice records from South Central MFA Agri Services (Mountain Grove, MO, USA). Cow feed costs were calculated as predicted hay consumed multiplied by hay price plus supplemental feed fed multiplied by feed price and summed for the entire production cycle. Calf feed costs were computed as predicted calf ME required as hay or creep feed multiplied by the respective price and summed for the entire production cycle. Total feed cost (TFC) was computed as cow feed costs plus calf feed costs. Replacement heifer cost (RHC) data were collected from Show Me Select heifer sales at Joplin Regional Stockyards in the spring and fall of each year. It was assumed that all open cows were sold at weaning for this analysis. Spring replacement heifer prices were used for Season 2 cows, and fall replacement heifer prices were used for Season 1 cows. Revenue from the sale of calves and open cows and costs for feed and replacement heifers were used to compute returns to land, labor, and management (RET).

2.4. Statistical Analysis

For data analysis, cows were categorized as Season 1 or Season 2 and were grouped into calving periods based on 21 d periods to reflect cow estrus cycle length, starting with the date of birth for the first calf in a given calving season and year. Cows were grouped into 5 calving periods; period 5 included all calvings following period 4 due to low numbers in Season 2. Performance, model output, and economic data were analyzed using the mixed-model function (lmer in lmerTest package) in R statistical software (ver. 4.5.1, [37]). The model included the fixed effects of calving season, calving-period category, and calf sex, as well as the random effect of calving year. The Tukey–Kramer method was used to generate probabilities and standard errors for differences of least-square means between levels of each factor using the predictemeans function in R. Pregnancy and calf survival percentage were analyzed using the glmer function with the same fixed and random effects using a binary response distribution for least-square means. Means were considered significant at p ≤ 0.05. Treatment means of all variables are presented for all 2- and 3-way interactions to allow the reader to make inferences of animal biology, even though all variables do not have significant 2- and 3-way interactions and only the significant interactions will be discussed.

3. Results

For Season 1 cows, the average starting date of calving periods 1, 2, 3, 4, and 5 was 15 January, 6 February, 28 February, 22 March, and 13 April, respectively. For Season 2 cows, the average starting date of calving periods 1, 2, 3, 4, and 5 was 18 August, 9 September, 1 October, 23 October, and 14 November, respectively. Due to the nature of an observational study, there was an uneven number of recorded cow–calf pairs between calving seasons (1620 vs. 290 in Season 1 and 2, respectively), calving periods within seasons (308, 679, 556, 264, and 173 in Periods 1, 2, 3, 4, and 5, respectively), and calf sex within calving seasons and periods (1057 vs. 923 male and female calves, respectively).

3.1. Performance Traits

3.1.1. Cow Weight and Condition

The mature cow weight recorded at weaning (MCW) and body condition score at calving (CBCS) were significantly (p ≤ 0.05) affected by the calving season (

Table 1. ANOVA results (Pr > F) for calving season (CS), calving period (CP), calf sex, and two- and three-way interactions for performance traits, model outputs, and economics of Hereford cows in southwest Missouri.

Item 1	CS	CP	Sex	CS × CP	CS × Sex	CP × Sex	CS × CP × Sex
Performance
MCW	0.01	0.28	0.43	0.13	0.54	0.77	0.44
CBCS	0.01	0.66	0.95	0.57	0.71	0.76	0.76
WBCS	0.76	0.61	0.25	0.22	0.01	0.27	0.17
PREG	0.05	0.41	0.02	0.04	-	-	-
SURV	0.40	0.52	0.90	0.20	-	-	-
CAINT	0.08	0.01	0.22	0.60	0.04	0.35	0.12
BW	0.01	0.01	0.01	0.07	0.06	0.03	0.04
WW	0.01	0.01	0.01	0.01	0.01	0.41	0.04
WAGE	0.01	0.01	0.10	0.04	0.44	0.03	0.01
AWW	0.34	0.01	0.01	0.03	0.01	0.51	0.13
Model outputs
MER	0.01	0.24	0.01	0.14	0.26	0.40	0.10
PKM	0.01	0.01	0.01	0.02	0.01	0.35	0.07
EBN	0.01	0.01	0.01	0.01	0.01	0.01	0.01
DSPN	0.01	0.01	0.01	0.01	0.01	0.01	0.01
DOYN	0.01	0.01	0.17	0.01	0.18	0.52	0.69
EEI1	0.01	0.01	0.01	0.01	0.01	0.47	0.18
EEI2	0.01	0.01	0.01	0.01	0.01	0.57	0.29
Economics
CaREV	0.01	0.01	0.01	0.01	0.01	0.01	0.10
CoREV	0.37	0.31	0.30	0.07	0.87	0.04	0.04
TREV	0.45	0.14	0.10	0.01	0.04	0.01	0.12
TFC	0.01	0.01	0.02	0.01	0.32	0.88	0.25
RHC	0.39	0.24	0.24	0.04	0.92	0.02	0.04
COST	0.84	0.31	0.26	0.03	0.90	0.02	0.03
RET	0.37	0.01	0.01	0.01	0.13	0.04	0.04

¹ MCW = mature cow weight; CBCS = body condition score at calving; WBCS = body condition score at weaning; PREG = pregnancy percentage; SURV = calf survival percentage from birth to weaning; CAINT = calving interval; BW = calf birth weight; WW = calf weaning weight; WAGE = calf age at weaning; AWW = 205 d adjusted calf weaning weight; MER = metabolizable energy required; PKM = peak milk yield; EBN = energy-balance nadir; DSPN = days since parturition at energy-balance nadir; DOYN = day of the year at energy-balance nadir; EEI1 = cow energy efficiency index; EEI2 = cow + calf energy efficiency index; CaREV = calf revenue; CoREV = cull cow revenue; TREV = total revenue; TFC = total feed cost; RHC = replacement female cost; COST = total cost; RET = returns over costs.

). Season 2 cows had greater (p ≤ 0.05) CBCS (6.1 vs. 5.7 ± 0.1) and were lighter (p ≤ 0.05; 570 vs. 549 ± 8 kg) at weaning than Season 1 cows (

Table 2. Performance traits, model outputs and economics for effects of calving season (Season 1 or Season 2) and calf sex (males or females) of Hereford cows in southwest Missouri.

	Season 1		Season 2
Item 1	Males	Females	Males	Females	SEM
NC	920	770	137	153
Performance
MCW	574	565	550	549	9
CBCS	5.7	5.7	6.1	6.1	0.1
WBCS	5.5 ab	5.4 ab	5.0 b	5.5 a	0.2
PREG	-	-	-	-	-
SURV	-	-	-	-	-
CAINT	370 ab	374 a	371 ab	354 b	7
BW	36.2	34.9	34.5	32.0	0.3
WW	253 b	242 c	283 a	255 bc	7
WAGE	217	215	236	231	3
AWW	242 a	235 b	254 a	229 b	6
Model outputs
MER	7987	7857	7767	7484	106
PKM	6.3 a	5.9 b	5.8 b	4.7 c	0.2
EBN	−1.7 d	−1.6 c	0.0 b	0.8 a	0.1
DSPN	112 c	112 c	175 b	192 a	3
DOYN	181	181	112	118	6
EEI1	32.9 a	33.5 a	28.1 c	31.0 b	0.8
EEI2	36.3 ab	36.9 a	32.1c	34.8 b	0.7
Economics
CaREV	951 b	931 b	1067 a	915 b	114
CoREV	136	169	115	140	29
TREV	1086 a	1100 a	1182 a	1056 a	113
TFC	190	188	234	229	12
RHC	316	396	268	336	70
COST	506	584	502	564	65
RET	580	515	680	489	121

¹ NC = number of calves; MCW = mature cow weight, kg; CBCS = body condition score at calving (1–9); WBCS = body condition score at weaning (1–9); PREG = pregnancy percentage, %; SURV = calf survival percentage from birth to weaning, %; CAINT = calving interval, d; BW = calf birth weight, kg; WW = calf weaning weight, kg; WAGE = calf age at weaning, d; AWW = 205 d adjusted calf weaning weight, kg; MER = metabolizable energy required, Mcal; PKM = peak milk yield, kg/d; EBN = energy-balance nadir, Mcal/d; DSPN = days since parturition at energy-balance nadir, d; DOYN = day of the year at energy-balance nadir, d; EEI1 = cow energy efficiency index, Mcal/kg; EEI2 = cow + calf energy efficiency index, Mcal/kg; CaREV = calf revenue, $/cow; CoREV = cull cow revenue, $/cow; TREV = total revenue, $/cow; TFC = total feed cost, $/cow; RHC = replacement heifer cost, $/cow; COST = total cost, $/cow; RET = returns over costs, $/cow. ^abcd Means without a common superscript in the same row differ at p ≤ 0.05.

). The body condition score at weaning (WBCS) was significantly (p ≤ 0.05) affected by the interaction of calving season with calf sex such that Season 2 cows giving birth to males had lower (p ≤ 0.05) WBCS than those giving birth to females, but for Season 1 cows, WBCS was not different (p > 0.05) between cows giving birth to males and females (Table 2). Season 2 cows had more favorable temperatures (Figure S1) and greater forage nutritive values (Table S2) during late gestation than Season 1 cows. Conversely, Season 2 cows had more extreme temperatures and lower forage nutritive values during mid to late lactation than Season 1 cows.

3.1.2. Cow Reproduction

Cows giving birth to males had greater (p ≤ 0.05) pregnancy percentages than those giving birth to females (87 vs. 83 ± 3%), regardless of calving season or calving period. A calving season × calving period interaction was evident (p ≤ 0.05) for pregnancy percentage, with no difference (p > 0.05) among calving periods in Season 2 cows; but in Season 1, cows calving in period 1 had greater (p ≤ 0.05) pregnancy percentages than those calving in periods 3 and 5, with pregnancy percentages in Season 1, period 5 being lower than in all other periods (p < 0.05;

Table 3. Performance traits, model outputs and economics for effects of calving season (Season 1 or Season 2) and calving period (1 to 5) of Hereford cows in southwest Missouri.

	Season 1					Season 2
Item 1	1	2	3	4	5+	1	2	3	4	5+	SEM
NC	250	577	464	237	162	58	102	92	27	11
Performance
MCW	565	563	565	575	580	536	561	558	562	529	11
CBCS	5.5	5.6	5.7	5.7	5.9	6.1	6.1	6.2	6.2	5.8	0.1
WBCS	5.2	5.3	5.5	5.5	5.7	5.5	5.6	5.5	5.5	4.2	0.2
PREG	92.1 a	85.1 ab	82.6 b	84.0 ab	69.2 c	84.1 ab	89.0 ab	89.4 ab	75.9 ab	90.2 ab	4.9
SURV	98.1	96.1	98.3	98.0	97.1	100.0	98.0	93.7	96.5	91.8	2.0
CAINT	395	384	371	360	348	390	381	358	336	347	9
BW	35.0	35.9	35.6	36.1	35.2	31.2	33.4	33.6	34.2	34.0	0.5
WW	273 bc	264 cd	250 e	237 f	214 g	292 ab	296 a	256 de	250 cdef	252 bcdefg	8
WAGE	249 b	236 c	218 d	200 e	178 f	270 a	255 b	230 c	207 de	205 de	4
AWW	233 a	235 a	239 a	243 a	242 a	229 a	244 a	231 a	250 a	255 a	7
Model outputs
MER	7956	7908	7914	7954	7879	7523	7817	7620	7737	7429	127
PKM	5.9 a	6.0 a	6.1 a	6.2 a	6.2 a	4.6 b	5.1 b	4.7 b	5.6 ab	6.1 ab	0.3
EBN	−1.4 d	−1.5 d	−1.6 e	−1.8 e	−1.7 e	0.2 c	0.3 bc	1.0 a	0.8 ab	−0.3 c	0.1
DSPN	110 e	112 e	113 e	113 e	112 e	244 a	223 b	190 c	139 d	123 de	3
DOYN	139 e	157 d	178 c	199 b	234 a	141 e	135 e	107 f	90 f	103 f	6
EEI1	29.5 e	30.9 d	32.4 c	34.8 b	38.5 a	26.6 f	27.2 f	31.1 cde	31.8 bcde	31.1 bcdef	0.9
EEI2	33.7d	34.8d	35.9c	37.8b	41.0a	31.3e	31.6e	35.0cd	35.1bcd	34.4 bcde	0.8
Economics
CaREV	1004 b	963 bc	965 bc	936c	836d	1028 abc	1131 a	965 bc	986 abc	846 bcd	117
CoREV	65	130	159	134	275	134	98	96	201	109	40
TREV	1067 b	1092 b	1125 ab	1071 b	1111 ab	1160 ab	1228 a	1062 ab	1188 ab	955 ab	120
TFC	189 cd	191 c	192 c	189 cd	185 d	243 ab	244 a	237 ab	230 b	203 cd	12
RHC	156 c	312 bc	358 b	322 bc	633 a	324 abc	222 bc	217 bc	499 abc	249 abc	93
COST	344 c	503 bc	550 b	512 bc	819 a	567 abc	465 bc	454 bc	728 abc	451 abc	90
RET	725 a	591 b	573 b	559 b	291 c	600 ab	761 a	606 ab	455 abc	501 abc	130

¹ NC = number of calves; MCW = mature cow weight, kg; CBCS = body condition score at calving (1–9); WBCS = body condition score at weaning (1–9); PREG = pregnancy percentage, %; SURV = calf survival percentage from birth to weaning, %; CAINT = calving interval, d; BW = calf birth weight, kg; WW = calf weaning weight, kg; WAGE = calf age at weaning, d; AWW = 205 d adjusted calf weaning weight, kg; MER = metabolizable energy required, Mcal; PKM = peak milk yield, kg/d; EBN = energy-balance nadir, Mcal/d; DSPN = days since parturition at energy-balance nadir, d; DOYN = day of the year at energy-balance nadir, d; EEI1 = cow energy efficiency index, Mcal/kg; EEI2 = cow + calf energy efficiency index, Mcal/kg; CaREV = calf revenue, $/cow; CoREV = cull cow revenue, $/cow; TREV = total revenue, $/cow; TFC = total feed cost, $/cow; RHC = replacement heifer cost, $/cow; COST = total cost, $/cow; RET = returns over costs, $/cow; ^abcdefg Means without a common superscript in the same row differ at p ≤ 0.05.

). There was no effect (p > 0.05) of calving season, calving period, calf sex, or two- and three-way interactions on calf survival from birth to weaning (Table 1).

The calving interval (CAINT) differed (p ≤ 0.05) between calving periods, with cows giving birth in calving periods 1 and 2 having greater (p ≤ 0.05) CAINT values than those giving birth in calving periods 3, 4, and 5 (392, 383, 365, 348, and 348 ± 7 d, respectively). Additionally, the interaction of calving season and calf sex affected (p ≤ 0.05) CAINT; Season 1 cows giving birth to females had greater (p ≤ 0.05) CAINT values than Season 2 cows giving birth to females, but there was no difference (p > 0.05) in cows giving birth to males (Table 2).

In the current study, Season 1 cows giving birth in calving period 5 had lower pregnancy percentages and shorter calving intervals, suggesting that fewer cows became pregnant due to having fewer estrous cycles prior to the end of breeding, but cows that did become pregnant had shorter calving intervals because of a shorter anestrus period after calving. Season 2 cows giving birth in calving period 5 also had shorter calving intervals than those giving birth in earlier calving periods, but pregnancy percentage was not reduced.

3.1.3. Calf Growth

There was a significant (p ≤ 0.05) calving season × calving period × calf sex interaction for birth weight (BW), weaning weight (WW), and weaning age (WAGE) but not (p > 0.05) 205 d adjusted WW (AWW; Table 1). Females born in Season 2 in calving period 1 had lower (p ≤ 0.05) BWs than females born in Season 2 during calving periods 2 and 3; however, calving period did not (p > 0.05) affect the BW of females born in Season 1 or that of males born in Season 1 or Season 2 (

Table 4. Performance traits, model outputs and economics for effects of calving season (Season 1 or Season 2), calf sex (males or females), and calving period (1 to 5) of Hereford cows in southwest Missouri.

	Season 1											Season 2
	Males					Females						Males					Females
Item 1	1	2	3	4	5+	1	2	3	4	5+	SEM	1	2	3	4	5+	1	2	3	4	5+	SEM
NC	121	311	258	139	91	129	266	206	98	71	-	28	55	36	13	5	30	47	56	14	6	-
Performance
MCW	579	566	569	575	581	550	560	562	574	580	9	528	552	554	563	551	543	569	562	560	508	19
CBCS	5.6	5.6	5.7	5.7	5.9	5.5	5.6	5.7	5.7	5.9	0.1	6.1	6.1	6.0	6.2	6.0	6.2	6.1	6.3	6.2	5.6	0.3
WBCS	5.5	5.4	5.5	5.6	5.5	4.9	5.1	5.5	5.4	5.8	0.1	5.1	5.5	5.4	5.3	NA	5.8	5.6	5.6	5.7	4.9	0.3
PREG	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
SURV	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-
CAINT	396	384	366	362	339	394	385	376	358	358	11	402	373	364	339	NA	379	389	353	332	318	21
BW	35.6 abc	36.4 ab	36.3 ab	36.8 a	35.8 abc	34.4 cdef	35.4 bcd	34.8 cdef	35.4 abcde	34.5 cdef	0.4	32.6 fg	33.6 cdef	33.8 cdef	36.6 abcdef	36.0 abcdefg	29.8 g	33.1 f	33.4 ef	31.8 defg	32.1 abcdefg	0.9
WW	277 bce	270 cdeg	259 dfghi	245 ijk	213 l	269 bcdefgh	258 fhi	242 jk	228 kl	215 l	7	299 ab	306 a	260 cdefghij	260 bcdefghijk	291 abcdefghijk	284 abcdf	285 abc	251 dfghijk	241 cdefghijkl	213 eghijkl	12
WAGE	247 c	237 d	221 efg	204 h	182 i	252 bc	236 d	215 fg	196 h	179 i	5	268 ab	260 abc	229 def	200 ghi	166 cdefgh	272 a	251 bc	232 de	214 defgh	180 ghi	11
AWW	239	240	245	247	240	227	230	234	239	243	6	236	250	236	267	283	222	239	226	232	226	11
Model outputs
MER	8087	7974	8006	7989	7913	7826	7841	7821	7919	8040	144	7506	7794	7680	7886	6951	7543	7843	7559	7584	6595	372
PKM	6.2	6.2	6.3	6.4	6.2	5.7	5.8	5.9	6.1	6.4	0.3	4.8	5.3	4.9	6.2	6.8	4.3	4.9	4.5	5.0	3.8	0.8
EBN	−1.5 efg	−1.6 efg	−1.8 gh	−1.9 h	−1.8 fgh	−1.3 e	−1.4 ef	−1.5 efg	−1.8 gh	−1.9 efgh	0.1	0.2 cd	0.2 d	1.0 ab	0.6 abcd	−1.7 efgh	0.3 bcd	0.5 abcd	1.0 a	1.0 abc	1.5 abcd	0.4
DSPN	109 g	112 fg	113 fg	113 fg	113 fg	110 fg	112 fg	113 fg	113 fg	112 fg	4	235 ab	231 bc	187 d	124 efg	55 fg	253 a	215 c	192 d	154 e	158 ef	11
DOYN	140	157	178	199	220	139	157	178	198	222	7	139	134	106	92	34	142	136	108	88	124	13
EEI1	29.6	30.4	31.8	33.7	37.7	29.4	31.4	33.0	35.8	37.5	1.0	25.9	26.0	30.6	30.7	29.6	27.2	28.4	31.7	32.9	37.6	2.4
EEI2	33.7	34.3	35.3	36.8	40.3	33.7	35.3	36.4	38.7	40.0	0.9	30.6	30.5	34.4	33.8	31.9	31.9	32.7	35.6	36.3	40.4	2.4
Economics
CaREV	993	974	994	947	869	1016	951	936	925	849	119	1025	1199	1015	1033	868	1032	1064	914	938	869	162
CoREV	47 d	133 bcd	114 cd	122 abcd	262 ab	84 cd	127 bcd	203 abc	146 abcd	287 a	31	43 abcd	101 abcd	103 abcd	337 abcd	−8 abcd	226 abcd	94 abcd	89 abcd	64 abcd	226 abcd	78
TREV	1037	1106	1110	1069	1066	1098	1079	1140	1072	1126	125	1064	1300	1119	1373	863	1256	1156	1005	1005	902	220
TFC	193	192	193	189	185	185	190	191	190	186	12	243	244	240	234	202	242	243	233	226	196	15
RHC	114 d	302 bcd	265 cd	307 abcd	451 ab	198 cd	323 bcd	452 abc	335 abcd	663 a	113	82 bcd	246 abcd	216 abcd	814 abcd	−36 abcd	562 abcd	197 abcd	220 abcd	187 abcd	80 abcd	336
COST	306 d	494 bcd	457 cd	497 abcd	636 ab	383 cd	512 abcd	642 abc	526 abcd	849 a	109	327 abcd	488 abcd	457 abcd	1048 abcd	166 abcd	805 abcd	440 abcd	452 abcd	411 abcd	272 abcd	334
RET	733 a	614 ab	651 ab	573 ab	428 cd	716 a	567 ab	495 bc	546 abc	276 d	139	747 ab	806 a	661 abc	321 abcd	704 abcd	455 abcd	717 ab	549 abcd	586 abcd	611 abcd	258

¹ NC = number of calves; MCW = mature cow weight, kg; CBCS = body condition score at calving (1–9); WBCS = body condition score at weaning (1–9); PREG = pregnancy percentage, %; SURV = calf survival percentage from birth to weaning, %; CAINT = calving interval, d; BW = calf birth weight, kg; WW = calf weaning weight, kg; WAGE = calf age at weaning, d; AWW = 205 d adjusted calf weaning weight, kg; MER = metabolizable energy required, Mcal; PKM = peak milk yield, kg/d; EBN = energy-balance nadir, Mcal/d; DSPN = days since parturition at energy-balance nadir, d; DOYN = day of the year at energy-balance nadir, d; EEI1 = cow energy efficiency index, Mcal/kg; EEI2 = cow + calf energy efficiency index, Mcal/kg; CaREV = calf revenue, $/cow; CoREV = cull cow revenue, $/cow; TREV = total revenue, $/cow; TFC = total feed cost, $/cow; RHC = replacement heifer cost, $/cow; COST = total cost, $/cow; RET = returns over costs, $/cow. ^abcdefghijkl Means without a common superscript in the same row differ at p ≤ 0.05.

). Heat stress during late gestation, as would have been experienced by Season 2 cows in this study (Figure S1), decreases placental blood flow and reduces birth weight in dairy and beef cattle [38,39,40,41,42].

Females born in Season 2 during calving periods 1 and 2 had greater (p ≤ 0.05) WWs than those born in calving period 5 but not (p > 0.05) calving period 3 or 4, whereas Season 1 females born in calving periods 1 and 2 had greater (p ≤ 0.05) WWs than females born in other Season 1 calving periods. Males born in Season 2 during calving periods 1 and 2 had greater (p ≤ 0.05) WWs than those born in calving periods 3 and 4 but not (p > 0.05) calving period 5, whereas Season 1 males born in calving period 1 had greater (p ≤ 0.05) WWs than those born in calving periods 3, 4 and 5. These trends in WW are likely due to differences in age, as WAGE follows a similar pattern and the three-way interaction for AWW was not significant. Calves born in Season 2 consumed hay with lower CP and TDN than calves born in Season 1 consuming summer pasture (Tables S2 and S3).

In the current study, both calving period and calf sex interacted with calving season to affect (p ≤ 0.05) AWW (Table 1). Although the F-test was significant for the calving season × calving period interaction for AWW, there were no differences (p > 0.05) among treatment means; however, numerically, AWW increased 26 kg from calving period 1 to 5 in Season 2 calves but only 9 kg from calving period 1 to 5 in Season 1 calves, which likely caused the significant interaction (Table 3). Similarly, the AWW of Season 2 females was 25 kg less (p ≤ 0.05) than that of males, whereas the AWW of Season 1 females was only 7 kg less (p ≤ 0.05) than that of males (Table 2).

3.2. Model Outputs

3.2.1. Energy Required

Season 1 cows had greater (p ≤ 0.05) metabolizable energy required (MER) than Season 2 cows (7922 vs. 7626 ± 45 Mcal), and cows giving birth to males had greater (p ≤ 0.05) MER than cows giving birth to females (7877 vs. 7671 ± 96 Mcal), likely due to differences in peak milk yield and metabolizable energy required for lactation (Table 1 and Table 4). There was a calving season × calf sex interaction for peak milk yield (PKM) such that Season 2 cows giving birth to males produced 1.1 kg greater (p ≤ 0.05) PKM than those giving birth to females, whereas Season 1 cows giving birth to males produced 0.4 kg greater (p ≤ 0.05) PKM than those giving birth to females (Table 2). Given that fall forage and hay consumed by Season 2 calves had lower CP and TDN values than spring and summer forage available to Season 1 calves, the CVDS Beef Cow Model estimated greater PKM required for males to gain faster than females, achieving a greater difference in AWW in Season 2 (25 kg) compared with Season 1 (7 kg; Tables S2 and S3).

3.2.2. Energy Balance

For energy-balance nadir (EBN), there was a significant (p ≤ 0.05) calving season × calving period × calf sex interaction (Table 1). Season 1 cows that gave birth to males in calving period 4 had lower (p ≤ 0.05) EBN values than those that gave birth to males in periods 1 and 2, whereas, Season 2 cows giving birth to males in period 5 had lower (p ≤ 0.05) EBN values than in other periods, and those giving birth to males in calving period 3 had greater (p ≤ 0.05) EBN values than in calving periods 1 and 2 (Table 4). For cows giving birth to females, Season 1 cows in period 4 had lower (p ≤ 0.05) EBN values than in periods 1 and 2, with periods 3 and 5 being intermediate, but Season 2 cows in period 3 had greater (p ≤ 0.05) EBN values than in period 1, with other periods being intermediate. The interaction for EBN is likely due to the greater predicted PKM for cows giving birth to males and the timing of calving relative to forage nutritive value. In Season 1, cows giving birth in period 4 had lower forage nutritive values during mid lactation in May and June than cows giving birth in periods 1 and 2, with mid lactation occurring in April and May (Table S2). In contrast, in Season 2, cows giving birth in period 5 had lower forage and hay nutritive values in mid lactation in January than cows giving birth in previous periods, with mid lactation occurring in November and December (Tables S2 and S3).

Days since parturition at energy-balance nadir (DSPN) were not different (p > 0.05) between calving periods or calf sexes in Season 1 cows, indicating that energy requirement is the affecting variable, since EBN occurred at the same DSPN, regardless of changing forage nutritive value (Table 4). In Season 2 cows, DSPN decreased (p ≤ 0.05) progressively from calving period 1 through 4, with period 5 not being different from period 4, indicating that changing forage nutritive value affected the time in the production cycle when cows experienced EBN. This effect of changing forage nutritive value is confirmed by the calving season × calving period interaction (p ≤ 0.05) for day of the year at energy-balance nadir (DOYN; Table 1). In Season 1 cows, DOYN increased (p ≤ 0.05) for each successive calving period, which, again, indicates that energy requirement was the driving factor causing energy-balance nadir; but in Season 2 cows, DOYN decreased (p ≤ 0.05) from periods 1 and 2 to periods 3, 4 and 5, although not consistently (Table 3). The fact that the change in DOYN across calving periods in Season 2 cows is less than 21 days indicates that forage nutritive value affects the timing of energy-balance nadir, but the fact that the change in DOYN across calving periods is not 0 days indicates that energy requirement also affects the timing of energy-balance nadir.

3.2.3. Cow Efficiency

Calf sex interacted (p ≤ 0.05) with calving season to affect cow efficiency (energy efficiency index, EEI; Table 1) when based on metabolizable energy required for the cow (EEI1) or metabolizable energy required for the cow and calf (EEI2). A lesser EEI value indicates greater cow efficiency, as the cow requires less metabolizable energy per kg of weaned calf. In Season 1 cows, EEI1 and EEI2 were not different (p > 0.05) between cows giving birth to males and females, but in Season 2 cows, those giving birth to males had lower (p ≤ 0.05) EEI1 and EEI2 values compared to those giving birth to females (Table 2).

Additionally, calving period interacted (p ≤ 0.05) with calving season to affect EEI1 and EEI2 (Table 1). In Season 1 cows, EEI1 increased successively (p ≤ 0.05) from calving periods 1 through 5, and those giving birth in calving periods 1 and 2 had lower (p ≤ 0.05) EEI2 values than those giving birth in calving periods 3, 4 and 5, which then also increased successively (p ≤ 0.05). However, in Season 2 cows, those giving birth in calving periods 1 and 2 had lower (p ≤ 0.05) EEI1 and EEI2 values than those giving birth in calving periods 3 and 4 but not (p > 0.05) calving period 5 (Table 3).

3.3. Economics

3.3.1. Revenue

Calf revenue (CaREV) was affected (p ≤ 0.05) by calving season × calving period, calving season × calf sex, and calving period × calf sex interactions (Table 1). In Season 1 cows, CaREV was greater (p ≤ 0.05) for those giving birth in calving period 1 than those giving birth in calving periods 4 and 5, with calving periods 2 and 3 being intermediate, whereas in Season 2 cows, CaREV was greater (p ≤ 0.05) for those giving birth in calving period 2 than those giving birth in calving periods 3 and 5, with calving periods 1 and 4 being intermediate (Table 3). In Season 2 cows, CaREV was greater (p ≤ 0.05) for cows giving birth to males than females, but there was no difference (p > 0.05) in CaREV between calf sexes in Season 1 cows (Table 2). For cows giving birth to males, CaREV did not change (p > 0.05) across calving periods, but CaREV tended to decrease from calving period 1 to 5 for cows giving birth to females, with calving periods 1 and 2 being greater than calving periods 3 and 5 (p ≤ 0.05) and calving period 4 being intermediate (

Table 5. Performance traits, model outputs, and economics for effects of calf sex (males or females) and calving period (1 to 5) of Hereford cows in southwest Missouri.

	Males					Females
Item 1	1	2	3	4	5+	1	2	3	4	5+	SEM
NC	149	366	294	152	96	159	313	262	112	77
Performance
MCW	554	559	561	569	566	547	564	562	567	544	11
CBCS	5.8	5.9	5.9	5.9	6.0	5.8	5.8	6.0	5.9	5.8	0.1
WBCS	5.3	5.5	5.4	5.5	4.5	5.4	5.4	5.6	5.6	5.4	0.2
PREG	-	-	-	-	-	-	-	-	-	-	-
SURV	-	-	-	-	-	-	-	-	-	-	-
CAINT	399	379	365	351	359	387	387	364	345	337	9
BW	34.1 b	35.0 ab	35.1 ab	36.7 a	35.9 ab	32.1 c	34.2 b	34.1 b	33.6 bc	33.3 bc	0.5
WW	288	288	260	253	252	277	272	246	234	214	8
WAGE	257 ab	249 bc	225 d	202 ef	200 ef	262 a	243 c	223 d	205 e	183 f	4
AWW	237	245	240	257	262	225	235	230	236	235	7
Model outputs
MER	7795	7883	7844	7939	7925	7684	7842	7691	7752	7383	132
PKM	5.5	5.8	5.6	6.3	6.8	5.0	5.4	5.2	5.5	5.5	0.3
EBN	−0.7 bc	−0.7 c	−0.4 ab	−0.7 abc	−1.8 d	−0.5 abc	−0.5 abc	−0.3 a	−0.4 abc	−0.3 abc	0.1
DSPN	172 ab	171 b	150 c	119 e	106 e	182 a	164 b	152 c	133 d	128 de	4
DOYN	139	146	142	146	162	141	146	143	143	176	6
EEI1	27.8	28.2	31.2	32.2	33.2	28.3	29.9	32.4	34.3	36.4	0.9
EEI2	32.2	32.4	34.9	35.3	36.2	32.8	34.0	36.0	37.5	39.1	0.9
Economics
CaREV	1009 abc	1086 a	1005 abc	990 abc	955 abc	1024 ab	1007 b	925 c	931 bc	726 d	117
CoREV	45 a	117 a	109 a	229 a	127 a	155 a	111 a	146 a	105 a	257 a	43
TREV	1050 a	1203 a	1115 a	1221 a	1081 a	1177 a	1117 a	1072 a	1038 a	985 a	120
TFC	218	218	216	211	197	214	217	212	208	192	12
RHC	99 b	274 ab	240 ab	560 a	289 ab	380 ab	261 ab	335 ab	261 ab	594 ab	100
COST	317 b	491 ab	456 ab	771 b	484 ab	594 ab	476 ab	547 ab	468 ab	786 ab	97
RET	739 a	710 a	657 ab	448 bc	596 abc	585 ab	642 ab	522 bc	567 abc	196 c	131

¹ NC = number of calves; MCW = mature cow weight, kg; CBCS = body condition score at calving (1–9); WBCS = body condition score at weaning (1–9); PREG = pregnancy percentage, %; SURV = calf survival percentage from birth to weaning, %; CAINT = calving interval, d; BW = calf birth weight, kg; WW = calf weaning weight, kg; WAGE = calf age at weaning, d; AWW = 205 d adjusted calf weaning weight, kg; MER = metabolizable energy required, Mcal; PKM = peak milk yield, kg/d; EBN = energy-balance nadir, Mcal/d; DSPN = days since parturition at energy-balance nadir, d; DOYN = day of the year at energy-balance nadir, d; EEI1 = cow energy efficiency index, Mcal/kg; EEI2 = cow + calf energy efficiency index, Mcal/kg; CaREV = calf revenue, $/cow; CoREV = cull cow revenue, $/cow; TREV = total revenue, $/cow; TFC = total feed cost, $/cow; RHC = replacement heifer cost, $/cow; COST = total cost, $/cow; RET = returns over costs, $/cow. ^abcdef Means without a common superscript in the same row differ at p ≤ 0.05.

).

There was a calving season × calving period × calf sex interaction (p ≤ 0.05) for cull cow revenue per cow (CoREV; Table 1), corresponding to a function of pregnancy percentage and timing of marketing of open cows. In Season 1 cows, CoREV was greater (p ≤ 0.05) for cows that gave birth to males in calving period 5 than in calving periods 1 and 3, whereas CoREV for cows that gave birth to females in calving period 5 was greater (p ≤ 0.05) than in calving periods 1 and 2 (Table 4). In Season 2 cows, there was no difference (p > 0.05) across calving periods for cows giving birth to males or females.

Total revenue (TREV) was affected by calving season × calving period, calving season × calf sex, and calving period × calf sex interactions and followed a pattern similar to that for CaREV, likely due to CaREV being a large proportion (87.5% on average) of TREV (Table 1). When comparing the same calving period in alternate calving seasons, cows giving birth in calving period 2 had greater (p ≤ 0.05) TREV values for Season 2 than Season 1 cows, but there were no differences (p > 0.05) between other calving periods (Table 3). There were no differences (p > 0.05) among treatment means for the calving season × calf sex interaction for TREV (Table 2). The significant interaction is likely due to Season 1 cows giving birth to males with $14/cow lower TREV values than those giving birth to females, whereas in Season 2 cows, those giving birth to males had $126/cow greater TREV values than those giving birth to females. Similarly, there were no differences (p > 0.05) among treatment means for the calving period × calf sex interaction for TREV (Table 5). Total revenue had no discernable pattern from calving period 1 to 5 for cows giving birth to males; however, TREV numerically decreased from calving period 1 to 5 for cows giving birth to females, resulting in the significant calving period × calf sex interaction.

3.3.2. Costs

Total feed cost (TFC) was greater (p ≤ 0.05) for cows giving birth to males than those giving birth to females (212 vs. 209 ± 12 $/cow; Table 1). Additionally, TFC was greater (p ≤ 0.05) for cows giving birth in calving periods 1 and 2 than in calving period 5 in Season 2, but in Season 1 cows, the TFC was only lower in calving period 5 (p ≤ 0.05) than calving periods 2 and 3 (Table 1 and Table 5). The lack of reduced costs for cows giving birth in later Season 1 calving periods in the current study is likely due to all cows being managed for early-spring calving; thus, supplemental feed during January and February was not adjusted for the cows calving in March, April, and May (calving periods 3 to 5).

Replacement heifer cost per cow (RHC), which is a function of the pregnancy percentage and the time of year at which replacements were purchased, was affected (p ≤ 0.05) by a calving season × calving period × calf sex interaction (Table 1). In Season 1 cows, RHC increased (p ≤ 0.05) by $337/cow from calving period 1 to 5 in cows giving birth to males and by $465/cow from calving period 1 to 5 in cows giving birth to females. There was no difference (p > 0.05) in RHC among calving periods for Season 2 cows giving birth to males or females; however, RHC was numerically lower in calving periods 1 and 5 than in calving periods 2, 3 and 4 for Season 2 cows giving birth to males and numerically decreased by $482/cow from calving period 1 to 5 in Season 2 cows giving birth to females (Table 4).

Total cost (COST) followed a pattern similar to that of RHC and had (p ≤ 0.05) a calving season × calving period × calf sex interaction (Table 1). In Season 1 cows, COST increased (p ≤ 0.05) by $330/cow from calving period 1 to 5 in cows giving birth to males and by $466/cow from calving period 1 to 5 in cows giving birth to females, but there was no difference (p > 0.05) in COST among calving periods for Season 2 cows giving birth to males or females; however, COST was numerically lower in calving periods 1 and 5 than in calving periods 2, 3 and 4 for Season 2 cows giving birth to males and numerically decreased by $533/cow from calving period 1 to 5 in Season 2 cows giving birth to females (Table 4).

3.3.3. Returns

Return over feed and replacement cost (RET) was affected (p ≤ 0.05) by a calving season × calving period × calf sex interaction (Table 1). In Season 1 cows giving birth to males or females, RET was greater (p ≤ 0.05) in calving periods 1, 2, 3, and 4 than in calving period 5, but there was no difference (p > 0.05) in RET among calving periods for Season 2 cows giving birth to males or females (Table 4). However, Season 2 cows giving birth to males had the numerically lowest RET in calving period 4, whereas those giving birth to females had the numerically lowest RET in calving period 1, which likely caused the three-way interaction.

4. Discussion

4.1. Effect of Calving Season

In forage production systems with bimodal growth patterns, such as tall fescue, both spring and fall calving seasons are potentially productive and profitable. Peak forage nutritive value can align well with peak energy requirements of the cow, resulting in acceptable pregnancy percentages in both seasons. Generally, fall-calving cows would be expected to have greater body condition scores at calving than spring-calving cows [3,43,44] due to lower nutrient requirements, more favorable temperatures, and greater forage nutritive value during late gestation, as observed in the current study. Because fall-calving cows have greater body condition scores at calving than spring-calving cows, pregnancy percentage is expected to be greater [43,44], as in the current study, although this may not always be the case [3].

Reports on the effect of calving season on calving interval and pregnancy percentage are conflicting. Similar to the current study, Caldwell et al. [44] reported a longer calving interval for spring-calving cow, whereas Campbell et al. [45] reported a shorter calving interval for spring-calving cows; in both studies, cows were grazing endophyte-infected tall fescue pastures. Bagley et al. [5] reported no difference in calving interval or pregnancy percentage between spring- and fall-calving cows, but cows grazed ryegrass-overseeded bermudagrass during winter months. In contrast, King and Macleod [46] reported a shorter anestrus period postpartum and a greater percentage of cows pregnant by d 100 postpartum in fall-calving cows than spring-calving cows, which could be due to changing day length [12]; thus, latitude is likely a significant factor affecting differences in reproductive efficiency between spring- and fall-calving cows. This effect is dependent upon breed type (Bos Taurus vs. Bos indicus) [10], as Bos indicus cattle have less frequent estrus and longer periods of anestrus in months with the shortest photoperiods [8,47,48].

Early lactation through breeding is the time of greatest energy requirements during the beef cow production cycle, and matching this time with forage of high nutritive value is critical to ensuring successful rebreeding [6,28]. In the current study, only Season 1 cows were expected to be in negative energy balance during the breeding season, which may be due to a greater model-predicted PKM than Season 2 cows. Similarly, Caldwell et al. [44] reported that fall-calving cows were in positive energy balance during breeding in Arkansas, whereas spring-calving cows were in negative energy balance when grazing tall fescue pastures. However, Griffin et al. [3] reported that fall-calving cows were in negative energy balance during breeding in Nebraska when grazing native rangeland. McCarter et al. [49] reported no difference in milk yield of fall- and spring-calving cows over 6 months of lactation; however, milk yield is reduced when cow feed energy intake is reduced [23,24,50], as was likely the cause with Season 2 versus Season 1 cows in the current study.

Fall-calving cows would be expected to have lower body condition scores at weaning than spring-calving cows due to relatively greater nutrient requirements in mid lactation coupled with lower forage nutritive value and increased maintenance energy requirements during winter, as observed in the current study. In contrast to the current study, Caldwell et al. [44] reported that spring-calving cows were lighter and had lower body condition scores at weaning than fall-calving cows. The reason for the discrepancy between studies performed in such close proximity to each other is unclear but may be due to different precipitation patterns relative to the types of forage produced, genetics of the cattle, access to shade/ponds for cooling, or the effects of subclinical fescue toxicosis. For example, the majority of precipitation in the current study occurred from June to August but occurred in early spring and fall (ideal for cool season forages) in the study by Caldwell et al. [44]. The timing of rainfall, coupled with average daily temperature, may have resulted in greater availability of forage in the current study, allowing spring-calving cows to maintain their weight and condition better.

Maintenance energy requirements are affected by environmental factors (extreme temperatures, wind chill, humidity, etc.) [20,51,52,53]. At the latitude of the current study, maintenance energy requirements may be more balanced across seasons such that relative differences between energy requirements and forage nutritive value are reduced at different times of the year. Edwards et al. [54] reported no difference in body-weight nadir between low- and high-milking spring-calving cows, but high-milking cows had numerically 20 kg less body-weight nadir. In general, Season 1 cows were predicted to be in negative energy balance between calving and weaning, which agrees with the decrease in BCS between calving and weaning; however, Season 2 cows were rarely predicted to be in negative energy balance, which could partially be due to their lower PKM in Season 2. Additionally, the CVDS beef cow model does not currently adjust maintenance energy requirements for environmental effects associated with cold and heat stress as discussed by the National Academy of Science, Engineering, and Medicine [20], which likely explains the discrepancy between the positive EBN and decrease in BCS from calving to weaning of Season 2 cows.

Forage nutritive value can also align well with peak energy requirements of the cow, resulting in high milk production for calf growth in early and mid lactation, but lower forage nutritive value for fall-born calves in late lactation can result in less growth. Weaning weight is generally greater for fall-born calves than spring-born calves [5,44] due to age and nutrition. Fall-born calves are sometimes weaned at an older age (current study, [44]) to allow older calves with a functional rumen to capture the benefits of early-spring forage with greater nutritional value [55]; thus, age-adjusted weaning weight may [44,45] or may not (current study, [5]) differ. Also, fall-born calves may be creep-fed, as in the current study, to compensate for hay with lower nutritive value during winter months [56]. The effects of age and nutrition can be seen in the studies of Griffin et al. [3] and McCarter et al. [57]. August-born calves were older but had weights similar to those of March-born calves at weaning because August-born calves were not creep-fed and weaned in April prior to grazing spring pasture [3], and fall-born calves not provided creep feed and weaned at 240 d had weaning weights similar to those of spring-born calves at 205 d [57].

Calf and cull cow prices are typically greater in the spring, when fall-born calves are weaned and open dams are culled, than in the fall, when spring-born calves are weaned and open dams are culled, resulting in greater calf and total revenue for fall-calving systems [4,44,58]. However, feed costs are often greater for fall-calving cows due to increased nutrient requirements of lactating cows fed through winter and creep feed provided to calves (current study, [5]); however, Henry et al. [58] reported similar winter feed costs for spring- and fall-calving beef herds ($60.24 and $60.21/cow, respectively) in Tennessee, and Griffin et al. [4] reported lower annual feed costs for August-calving than March-calving cows in Nebraska. Shifting the calving date from early to late spring reduced winter feed usage by 52% [59] and reduced costs [4,60,61]. In the current study, calf revenue and feed costs were greater in Season 2, resulting in no difference in net returns between Season 1 and 2. In contrast, average net returns were greater for fall- than spring-calving cows in Louisiana, Nebraska, and Tennessee [4,5,58].

4.2. Effect of Calving Timing Within Season

The timing of when cows calve during a calving season can influence the alignment of forage nutritive value with peak nutrient requirements, affecting milk production, energy balance, and reproductive performance. In the current study, the timing of calving interacted with the calving season for almost all evaluated variables, which is likely due to alignment with forage nutritive value and photoperiod. Cows calving early in Season 1 would have lower forage nutritive values and shorter photoperiods through early lactation than cows calving late in Season 1, whereas cows calving early in Season 2 would have greater forage nutritive values and longer photoperiods through early lactation than cows calving late in Season 2. For example, pregnancy percentage was reduced for cows calving in later periods in Season 1 but not for cows calving in later periods in Season 2, e.g., period 5, possibly due to effects of photoperiod. Additionally, the calving interval was shorter in later calving periods in both Seasons 1 and 2.

Cows calving later in the calving season have less time to begin cycling and become pregnant prior to the end of the breeding season, which can be seen in the results of Deutscher et al. [62], who reported lower pregnancy percentages for cows allowed shorter breeding seasons. However, the time of year can also impact the resumption of estrous cycles in beef cattle [12,46]. Shorter breeding seasons had a greater impact in terms of reducing the pregnancy percentage in March-calving cows than April-calving cows in Nebraska [62], and calving in February versus June decreased pregnancy percentages in 2- and 3-yr-old cows in Montana [59]. Consistent with these results, females that calve in the first 21 days of the calving season as 2-yr-olds have longer postpartum and calving intervals, which increases pregnancy percentages and subsequently allows them to remain in the herd longer [14,15].

Additionally, the body condition score at calving and changes in body condition between calving and breeding are important factors [21,63]. Thus, cows calving shortly before or after availability of pasture with greater nutritive value in the spring, such as those calving in April to June in the northern U.S., are likely to have shorter anestrus periods and greater pregnancy percentages due to a more positive energy balance [59,62]. In contrast to the current study, several studies [3,59,62] have reported that early spring-calving cows were in greater negative energy balance than late spring-calving cows during breeding, which may be due to the timing of EBN, with Season 1 cows giving birth in calving periods 1 and 2 reaching nadir in May and June, when forage was of greater nutritive value than those giving birth in calving periods 4 and 5, reaching nadir in July and August, when forage was of moderate nutritive value. Additionally, as mentioned above, the CVDS beef cow model does not currently adjust maintenance energy requirements for environmental effects associated with cold and heat stress, as discussed by the National Academy of Science, Engineering, and Medicine [20]. The lower temperatures for cows calving in periods 1 and 2 of Season 1 would likely increase maintenance energy requirements, resulting in greater EBN than for cows calving in later calving periods and could have shifted the DSPN of cows calving in periods 1 and 2.

The timing of when cows calve during the calving season also impacts the age and weight of the calf at weaning, which impacts cow efficiency and profitability. Cows that give birth earlier in the calving season generally wean heavier calves, even when adjusted to a constant age [13,14,64]. Calves born earlier in the calving season were consistently heavier at weaning than calves born in later periods, and their dams had improved EEI. These findings support previous research indicating that cow biological efficiency is strongly correlated with calf weaning weight [35,65].

Similar to the current study, Damiran et al. [64] reported lower calf revenue for calves born later in the spring calving season, but calf revenue was greater for calves born later in the spring in studies by Stockton et al. [60] and Griffin et al. [4], although in these studies, calves were weaned at similar age rather than on a similar day of the year as in the current study. Thus, age at weaning appears to be the primary driver of differences in weaning weight and calf revenue between calves born earlier and later in the calving season. Feed costs were not different among calving periods in Season 1; thus, net returns decreased for cows calving later in Season 1. Alternatively, net returns were increased when shifting the calving season from early to late spring in Nebraska and Montana [60,61].

4.3. Interactions with Calf Sex

The effect of calf sex interacted with calving season and calving period for many outcome variables—WBCS, CAINT, BW, WW, WAGE, AWW, PKM, EBN, DSPN, EEI, CaREV, CoREV, TREV, RHC, COST, and RET. Many of these pertain to the biological differences between males and females, with females having lighter birth and weaning weights, leading to lower model-predicted milk production and negative energy balance of the cow, as well as to lower cow efficiency and net returns. These differences provide a biological understanding of the effects on cow performance, but calf sex is not a primary factor for producers to consider when deciding on a calving season.

4.4. Summary

Overall, fall calving systems have some advantages over early-spring calving systems, especially in southern latitudes, for several reasons. Fall-calving cows calve in better body condition, and it is more cost effective to maintain body condition grazing cool-season or stockpiled forage than using harvested forage. Extending the grazing season with stockpiled forage and crop residues is a successful way to reduce winter feed costs; however, these feed supplies are often exhausted in early winter, causing the bulk of harvested feed and feed costs to occur in late winter and early spring, when spring-calving cows have the greatest nutrient requirements and, therefore, need for supplemental feed. Fall-calving systems have the advantage that cows can be allowed to lose some body condition after the breeding season in late winter and early spring to reduce feed costs, knowing that summer pasture will enable cows to regain body condition prior to calving. Thus, better body condition at breeding, along with any benefit of photoperiod, results in improved (or, at least, more cost-effective) pregnancy percentages in fall-calving cows. In northern climates, late-spring calving is more advantageous than early-spring calving due to reduced feed costs when cows are able to maintain body condition while grazing pasture during early lactation, which could increase pregnancy percentages. Late-spring calving in southern climates may not be as advantageous due to the decline in forage nutritive value during early lactation (depending upon the forage species being grazed) and the negative effects of heat stress on reproduction [66,67]. Likewise, fall calving systems may not be as advantageous as late-spring calving systems in northern climates due to harsher winter weather resulting in reduced calf growth and increased winter feed costs; however, only one study has made this comparison [3,4].

4.5. Limitations and Scope

One of the limitations of this study is the observational nature of the data collection, in contrast with a prospective study where cows are randomly assigned to calving season and period. Cows in Season 1 and Season 2 could have some inherent differences, as they are essentially separate herds, and cows could have adapted to the calving season. Additionally, the calving period in which a cow calves is determined by one of two factors: (1) lack of fertility in an earlier calving period in a previous year, shifting her to the later calving period or (2) the cow being born to a dam calving in the later calving period, which could have been due to reduced fertility of the dam in the past. Calving timing and offspring performance can be the results of past events of the dam, as females that are born early in the calving season, themselves, are more likely to calve early in their first calving season as 2-yr-old cows and wean heavier calves [68]. Thus, the observational nature of this study could bias the results, as female breeding lines with later calving periods could result in breeding lines with lower fertility.

Additionally, the results of this study are applicable to cow-calf operations in the central latitudes of the USA, where winter and summer weather is not extreme. The forage base was tall fescue with a bimodal growth curve, providing highly nutritious forage in both spring and fall; therefore, the results may not be applicable to other forage species.

Model assumptions were based on equations to predict the energy requirements of the mean beef cow; however, there is considerable variability among individual cows in a herd. Furthermore, the feed costs in the economic analysis were based on estimated feed requirements from the CVDS model. Thus, model outputs and economic costs could be biased toward the center of the population.

5. Conclusions

In forage systems with a bimodal growth pattern, such as tall fescue, fall-calving cows have an advantage in reproductive performance, as the greater forage nutritive value in summer increases the body condition score at the time of calving. Additionally, the shorter days during the breeding season shorten postpartum intervals and increase fertility in fall-calving cows. In contrast, feed costs for spring-calving cows are lower, and spring-born calves have greater forage nutritive value during late lactation, leading to more cost-effective gains. However, spring-born calves wean at lighter weights and are generally sold during periods of lower calf prices. The tradeoff between lower calf revenue and feed costs for spring calving compared with greater revenue and costs associated with fall calving results in a similar net income.

The calving period affects the alignment of cow nutrient requirements with forage nutritive value and the length of time to rebreed but is highly dependent upon the calving season. Reproductive performance declined in later calving periods during spring calving but not during fall calving. Calf weaning weight declined less in later periods during fall calving, but calf revenue declined to a greater extent. Replacement female costs were lower in later fall calving periods compared to later spring calving periods due to improved reproductive performance. Thus, net returns did not decline in later calving periods for fall-calving cows, as it did for spring calving cows.

The interactions among calving season and calving period add complexity in terms of identifying the optimal calving season, warranting further study. At the latitude of the current study, using a forage species with a bimodal growth curve, fall calving resulted in improved cow reproductive performance, calf growth, and calf revenue but increased feed costs and no difference in net returns.