1. Introduction
Sheep have a seasonal reproductive cycle, becoming most active as photoperiod shortens and experience anestrus or low ovarian activity as photoperiod lengthens [
1,
2]. Further, the liveweight at which optimal fertility is achieved is influenced by both mating season and breed [
3,
4]. Adding to variability, breeding season length [
5,
6,
7] and ovulation rate are dependent on breed [
8,
9]. Mating season, liveweight and body condition score (BCS) have also been associated with variation in ovulation rate [
10]. Further, the response to photoperiod length is also influenced by nutrition [
11] and improving nutrition to increase liveweight or BCS prior to mating is known to improve reproductive performance [
4,
12]. However, a breed dependent upper liveweight and BCS bounds have also been identified, above which no reproductive advantage was achieved [
13,
14,
15].
The management of different sheep breeds provided with the same resources may result in different liveweight gain and productivity due to the different energy requirements of larger versus smaller animals [
16,
17]. This is reflected in the predictability of wool production and reproductive performance based on maternal liveweight and BCS, which varies by breed [
12,
18]. As such, guidelines for ewe management in Australia currently have two forms, those for Merino ewes and those for non-Merino ewes [
19,
20].
Computer modelling of whole sheep enterprises across different locations indicated the optimum lambing time was dictated by the average length of the pasture growing season and production output (wool and/or lamb target weight) [
21]. This was supported by a survey of sheep producers that identified over 53% of producers use feed availability as the primary determinant of lambing time [
22], and thus month of mating. This decision may also be influenced by environmental factors, such as grass seed burden, weather extremes and the marketability of lambs.
Management conditions and practices leading up to and at mating, including ewe breed, mating season, and management of liveweight and BCS may influence reproductive performance. While guidelines have been developed, the impact of mating conditions and practices in a commercial setting is relatively unknown. The objective of this study was to examine relationships between mating liveweight and BCS on pregnancy and fetal number at pregnancy scanning across different breeds and seasons of mating in sheep enterprises in southern Australia. It was hypothesized that the relationship between mating liveweight and BCS and pregnancy and fetal number would vary between ewe breed and mating season. The impact of other variables and practices were also explored.
4. Discussion
The dataset contained little variation in pregnancy rate (90.1%) and the number of multiple fetuses (53.5%), being similar to a previous pregnancy scanning report [
34]. This was reflected in the BN sensitivity analyses which indicated a low association between the variables explored, and that no standalone variable was responsible for driving pregnancy and/or number of fetuses. This highlighted the importance of considering all variables explored in this study at mating due to their interrelatedness. Although lowly associated, breed had the greatest influence on pregnancy outcome (1.35%) and fetal number (6.09%). This was expected as ovulation rates of nil, 1.62 and 2.0 in Border Leicester, between 0.2 and 1.4 in Merino and 1.0 to 1.73 in Merino cross Border Leicester ewes have previously been reported [
7,
8,
9,
35]. The influence of mating liveweight and BCS on pregnancy outcome varied by breed (
Table 3). Further, mating liveweight was the most influential variable on number of fetuses in wool-based breeds and the impact of mating BCS was, again, variable across all breeds explored. The low association is likely due to the favorable seasons experienced during the survey collection period. Similarly, the influence of season on pregnancy and fetal number outcomes across the breeds varied (
Table 3) and autumn was most often associated with low non-pregnant and high fetal number outcomes (
Table 4,
Table 5,
Table 6 and
Table 7). As such, the hypothesis, that the relationship between mating liveweight and BCS and pregnancy rate and fetal number vary with ewe breed and mating season, is supported.
The pregnancy rates of spring mated flocks (85%) across the survey, albeit lower than summer (90%) and autumn (93%) mated flocks, were greater than expected given the cyclic nature of the ovine reproductive cycle [
1]. The length of the breeding season differs by sheep breed with cross breeds intermediate of parental potential [
5,
6,
7,
36]. This was negated in the spring mated Composite ewe flock through both a highly fecund breed and use of subcutaneous melatonin implants to advance the onset of the normal breeding cycle [
23,
37]. Melatonin implants, however, were only used in one other Merino flock of the remaining 11 spring mated flocks surveyed.
Reports of ovulation and pregnancy rates outside of the breeding season are varied. The number of ovulating Merino ewes may be as low as 30% during spring in the absence of rams [
8], however pregnancy rates of 80% and approximately 90% have been achieved in the presence of rams [
38,
39]. The ‘ram effect’, whereby reproductive hormone production, luteinizing hormone and gonadotropin releasing hormone, are stimulated by male pheromones resulting in ovulation, may be responsible for out of season mating success [
40]. Twelve of the 58 ewe flocks captured (~21%) in this survey routinely practice spring mating, and must therefore, be a viable management practice regardless of the variation of reported ovulation and pregnancy rates in the literature. However, it must be noted that majority of flocks captured during spring were Merino and only two spring-mated Maternal and shedding ewe flocks were surveyed.
Further, producers ranked the season in terms of pasture availability during the mating period between ‘well below average’ to ‘well above average’, a total of five possible ranks. Over 65% of producers with spring mated flocks reported ‘above average’ to ‘well above average’ seasons. As observed previously [
9,
41], improved nutrition at mating may have also increased spring pregnancy rates above expected levels.
The pregnancy rates for autumn-mated Maternal and Merino ewes in the current survey are very similar to previous reports [
3,
38,
39,
42]. Shedding breeds, such as Dorper and Damara breeds, are recognized as being non-seasonal breeders [
43], which explains these results. Shedding ewes performed similarly between spring and autumn mating seasons in terms of pregnancy rate. Autumn litter size, however, was higher than spring, indicating a seasonal influence on ovulation rate in shedding breeds. Nutrition and/or genetic composition may have also influenced ovulation rate.
Pregnancy and fetal number outcomes were predicted to peak in the BN analysis at 60–69.5 kg in Composite, Maternal and Merino ewes. At liveweights lower than 60 kg and greater than 69.9 kg, pregnancy and fetal number decreased in Composite, Maternal and Merino ewes. The percentage of non-pregnant shedding ewes was least at a mating liveweight of 90–99.5 kg. Fetal number in shedding ewes, however, was greatest at a mating liveweight of 70–79.5 kg. The selection for greater fleece weight, due to the positive genetic correlation between fleece weight and bodyweight, has resulted in heavier sheep that have higher feed requirements [
17,
44]. However, breeding objectives for sheep producers are becoming increasingly complex [
44,
45] and the selection for other traits may have obscured the genetic relationship between liveweight and reproductive potential [
46]. Increasing mating liveweight and BCS is known to increase conception rates and fetal number [
9,
10,
12,
47,
48,
49]. The Lifetime Wool guidelines recommend a mating BCS of 3, or approximately 90% of standard reference weight, at mating in Merino ewes [
20,
50]. These guidelines have also improved both ewe reproductive and progeny performance [
51], and have been extended to Maternal ewes with a recommendation of BCS 3.8–4.2 at mating [
19]. Attaining the liveweight and BCS profiles is the focus of these guidelines to optimize productivity and profitability.
The influence of mating liveweight and BCS on conception and ovulation rate varies by breed. No improvement in conception or ovulation rate was achieved above a mating liveweight or BCS threshold in Romney, Composite and Cheviot ewes [
13,
14], however ovulation rate increased with mating liveweight and BCS in Beulah ewes [
49]. A review of the relationship between BCS and reproductive outcomes suggested a curvilinear relationship existed between BCS and productive performance [
52], as expressed in Malpura ewes, having significantly greater conception rates at mating BCS 3–3.5 compared to BCS 2 and BCS 4 [
15]. A linear relationship between mating BCS and pregnancy scanned fetal number, however, was observed in both Merino and Maternal ewes in southern Australia [
4]. The flocks studied were managed to have a minimum mating BCS of 2.5 and 60% of flocks were in BCS 3 or more [
4]. A full range (1–5) of mating BCS was not captured and likely responsible for the linearity of the observed relationship between mating BCS and fetal number. Further, flocks of similar genetic background may respond linearly to improvements in mating BCS [
53]. A curvilinear relationship was present in the current study across all breeds for pregnancy and fetal number outcomes with mating liveweight and was apparent when mating BCS was ≥4. This may be explained by the curvilinear relationship suggested between liveweight and ovulation rate [
48], whereby leptin may be responsible at the follicular level for reduced ovulation rate [
54]. This threshold of BCS 4 is at odds with current recommendations for Merino ewes of BCS 3 at mating [
20,
50], however aligns well with Maternal ewe recommendations [
19]. These recommendations were based on simulated productivity and economic evaluation of liveweight profile across the production year. The greater predicted mating liveweight range in shedding breeds is likely a consequence of the low number of flocks captured. Further research into the reproductive response and management of Dorper ewes, a shedding breed, in Australia by industry has previously been recommended [
55] and is still required.
The predicted impact of ram percentage on pregnancy rate and number of fetuses in the BN analysis was unexpected and was also curvilinear. Guidelines for producers recommend a ram percentage for mature ewes of 1.0–1.5% plus one additional ram [
56,
57]. However, the optimal ram percentage, in terms of low non-pregnant rates and high fetal number outcomes, occurred at 1.8–2.3% for Maternals, and at 1.3–1.75% in Composite and Merino breeds, which are greater than the current industry guidelines. Too few rams (less than 1%) can result in lower conception and ovulation rates, possibly due to crowding of the ram by ewes in estrus [
58]. Increasing the ram to ewe ratio increased the number of times a ewe was mated, pregnancy rate and the number of twin lambs born [
59,
60,
61]. Reasons for sub-optimal performance at high ram percentage may involve mating behaviors or nutrition. Rams may display dominance behaviors if in too close proximity, which may reduce subordinate ram performance [
62]. Ewes are also reported to seek out and preference more active rams [
63], which, in combination with ram dominance behaviors, may justify the lower conception and number of fetuses observed at greater ram percentages. Ram reproductive potential is also influenced by available nutrition, influencing the number of spermatozoa produced by the ram [
64], however overnutrition may also reduce reproductive potential [
65]. In the current study there was insufficient data collected from flocks experiencing below and well below average seasonal conditions to draw conclusions regarding ram nutrition. No further ram information was collected during the survey, however individual ram, age, libido and breed may also have had an effect [
61,
66].
This survey had several limitations. Capturing data across two (or multiple) years would allow the consistency of these effects to be explored as reproductive performance in the current year may be influenced by performance in the previous year [
67]. Further, a relatively small number of shedding breed ewe flocks were captured. The relatively small number of flocks that received hormone treatment meant separating this data was not possible and as a result, may have unduly influenced the spring mating system, especially for the Composite breed. The use of high ram percentages may reflect low flock fertility, but greater investigation is required to fully understand the relationship. Finally, the BN analysis is specific to the survey data collected, and thus caution should be taken when extrapolating findings to a wider context.