Modelled changes in cow numbers, numbers of calves weaned, feed demand, numbers of cattle sold, cattle enterprise income, expenses, and COS are shown and discussed in the following subsections. Total annual cattle feed demand was fixed at 12 million MJ ME for all scenarios, achieved through adjustments in herd size.
3.1. Cattle Numbers
With a death rate of 2.2% for
Y2 to 10 cows and cull rate of 16% for
Y2 to 9 cows (with all
Y10 cows culled for age after weaning), the average replacement rate was 22
Y2 heifers per 100
Y2 to 10 cows. With increasing mature cow liveweight, their feed demand was greater for maintenance on a per cow basis and for the growth and maintenance of non-mature cattle. This necessitated a reduction in cow numbers in order to maintain the same total annual cattle feed demand (
Figure 3). For scenario A, numbers of
Y2 to 10 cows were reduced from 263 to 209 cows as mature cow liveweights increased from 450 to 600 kg. As
Y2 to 10 cows were bred with a constant calf weaning rate of 84%, total numbers of calves weaned also decreased from 221 to 176. Similar reductions in cattle numbers occurred for scenarios B and C. Therefore, previous suggestions that farming heavier cows with a fixed feed supply requires a smaller herd size weaning fewer calves [
11,
14] were supported in the current analysis. Proportions of
Y1 heifers,
Y1 steers, and
Y2 steers sold were maintained at constant rates relative to mature cow numbers, such as 20% of
Y1 heifers sold, and therefore total numbers of cattle in these stock classes decreased with decreasing herd size.
At a constant mature cow liveweight, total cattle numbers changed between scenarios A, B, and C, though with smaller changes than occurred within scenarios due to changes in mature cow liveweight (
Figure 3). With a mature cow liveweight of 450 kg, numbers of cows in
Y2 to 10 were 263, 260, and 258 for scenarios A, B, and C, respectively, with proportionate changes in the numbers of cattle in other stock classes. Similar changes occurred with a mature cow liveweight of 500 kg, though with smaller changes observed. Calf weaning weights were higher in scenarios B and C than in scenario A for those from 450 and 500 kg cows, increasing total feed demand per cow through increased demand for lactation and growing cattle (
Table 1,
Table 2 and
Table 3). Cow numbers were therefore lower in scenarios B and C in order to maintain total annual cattle feed demand at 12 million MJ ME. Conversely, with mature cow liveweights of 550 and 600 kg, there were smaller reductions in cattle numbers when moving from scenario A to B and then scenario B to C (
Figure 3). For example, with a mature cow liveweight of 550 kg, the numbers of cows in
Y2 to 10 were 225, 227, and 228 in scenarios A, B, and C, respectively. With mature cow liveweights of 550 and 600 kg, calf weaning weights were lower in scenarios B and C than in scenario A (
Table 1,
Table 2 and
Table 3). Therefore, feed demand per animal for lactation and growing cattle was lower in scenarios B and C with mature cow liveweights of 550 and 600 kg, thus cattle numbers were higher.
3.2. Feed Demand
Daily feed demand per cow for maintenance was 52, 56, 61, or 65 MJ ME with liveweights of 450, 500, 550, or 600 kg, respectively. With WWRs of 43% for all cows (scenario A), the proportion of total annual cattle feed demand accounted for by cow maintenance (replacements and
Y2 to 10 cows) decreased from 60% to 59% (7.16 to 7.08 million MJ ME) when mature cow liveweight increased from 450 to 600 kg (
Table 5). In the same scenario, the proportion of feed demand accounted for by reproduction and sold non-mature cattle increased from 21% to 22% (2.55 to 2.64 million MJ ME) with increasing mature cow liveweight from 450 to 600 kg. Heavier birth and weaning weights increased feed demand for gestation and lactation, respectively. Cattle sale timings were constant for all scenarios and cow liveweights, and therefore sold cattle from herds with heavier cows had greater feed demand for maintenance and growth until leaving the farm. This relatively small increase in the use of feed directly for production of heavier cows would occur if heavier cows were achieving the same WWRs as their lighter counterparts.
Data from both New Zealand and international beef cow studies indicate WWRs of heavier cows to be lower than WWRs of lighter cows (
Figure 1). When heavier cows were modelled with lower WWRs than lighter cows (scenarios B and C), the proportion of total annual cattle feed demand accounted for by cow maintenance increased with increased mature cow liveweight. For scenario B, the proportion of total annual beef feed demand accounted for by cow maintenance increased from 59% to 61% (7.03 to 7.26 million MJ ME) as mature cow liveweight increased from 450 to 600 kg (
Table 5). The proportion of total annual cattle feed demand accounted for by production (sold non-mature cattle and reproduction) decreased from 22% (2.67 million MJ ME) with a mature cow liveweight of 450 kg, to 20% (2.43 million MJ ME) with a mature cow liveweight of 600 kg (
Table 5). Proportions of feed demand accounted for by cow maintenance had similar increases with heavier cow liveweights in scenario C. The increased proportion of feed demand accounted for by cow maintenance with heavier mature cow liveweights was expected [
1,
9], occurring when WWRs were lower for heavier cows, which has previously been used as a measure of efficiency [
3,
10,
11,
12,
14,
15].
3.4. Economics
Prices were consistent for each group of sold cattle in this analysis on a per kg basis as sale timings were consistent across scenarios and mature cow liveweights. Therefore, prices per head were higher with heavier cattle weights. Changes in beef income with varied mature cow liveweight were driven by changes in both cattle numbers and liveweights. For scenario A, increased mature cow liveweight had proportionate increases in calf weaning weight and sale weights of all non-mature cattle (
Table 1). Beef income increased from NZD 203,000 to NZD 211,000 as mature cow liveweight increased from 450 to 600 kg (
Figure 4), indicating increased sale liveweights increased income by a relatively larger value than income reductions from fewer total cattle. Although it is not certain which of the modelled scenarios with varied pre-weaning growth rates is most likely occurring on New Zealand beef farms, pre-weaning growth rates of 1.00 kg/day are considered to be an industry target [
30,
34]. An average pre-weaning growth rate of 1.19 kg/day for calves from a herd of 600 kg cows (scenario A in
Table 1) is therefore not likely achieved in many beef herds feeding exclusively on pasture and in hill country conditions. Although mature cow liveweights have increased due to selection for higher potential growth rates [
1], high growth rates are likely limited by the environment in which they are farmed. In the North Island hill country system under study, the beef herd typically feeds on low quality-pasture in summer prior to weaning when other stock classes on farm are prioritised, such as growing lambs for slaughter. Pre-weaning growth rates in scenarios B and C had less variation between calves from cows with varying mature liveweights compared with scenario A, either with a small range from 0.95 to 1.07 kg/day or fixed for all calves at 1.00 kg/day (
Table 1,
Table 2 and
Table 3).
Relative to scenario A, scenarios B and C had smaller increases in calf weaning weights and cattle sale weights with increased mature cow liveweight (
Table 1,
Table 2 and
Table 3). Therefore, differences in sale weights of cattle from cows with different mature cow liveweights were smaller (compared with scenario A) and the lower numbers of heavier cows did reduce income (
Figure 4). Beef enterprise income decreased by NZD 7000 or NZD 15,000 as mature cow liveweight increased from 450 to 600 kg in scenario B or C, respectively. Scenario C had the most similar weaning weights for calves from cows of the varied mature liveweights (
Table 1), where the reduced cattle numbers with heavier cows had the biggest reduction in income.
Total annual beef feed demand was maintained at 12 million MJ ME for all modelled scenarios. Therefore, with stock units based on feed demand (
Section 2.5), expenses were the same for all scenarios at NZD 114,000 (
Figure 4). [
11] reasoned that larger beef cows would be more profitable than smaller cows if all cows were equally as efficient in converting feed to product and if expenses were constant on a per cow basis. The experimental data of beef cows WWRs (
Figure 1) combined with the greater feed demand for cow maintenance with heavier liveweights [
7] have indicated larger cows to be less efficient in conversion of feed to product. Economic analysis in New Zealand beef systems usually consider expenses on a per stock unit or hectare basis [
27], essentially on a per unit of feed consumed basis. This suggests that larger cows incur higher expenses per head. Therefore, the base assumptions of [
11] that informed their suggestion that a herd of larger cows would be more profitable may not be relevant to New Zealand beef production systems.
With expenses held constant on a total herd level, beef enterprise COS changed with the changes in income outlined above. COS ranged from NZD 401/ha to NZD 474/ha with varying mature cow liveweights in scenario C. The range in COS for scenarios A and B fell between the range in scenario C. Industry survey data suggest the median EBITR (earnings before interest, tax, and rent) to be slightly above NZD 450/ha for East Coast hill country sheep and beef farms in the 2017/2018 production year [
46]. The predicted beef enterprise COS in the current analysis were similar to industry survey values, suggesting the model outputs to be representative of North Island hill country beef production systems.
As mature cow liveweight and cattle sale weights increased in scenario A, beef enterprise income and COS rose (
Figure 4). COS increased from NZD 421/ha to NZD 460/ha as mature cow liveweight increased from 450 to 600 kg, an increase of 9%. However, as scenario A is unlikely to be representative of pre-weaning calf growth rates occurring in beef production systems in New Zealand hill country, the reductions in beef enterprise COS in scenarios B and C of 7% and 15% as cow liveweight increased from 450 to 600 kg, respectively, were likely more relevant for New Zealand farmers. Across the beef enterprise equivalent area of the farm, 40% of 530 ha, these reductions in COS were NZD 7000 and NZD 15,000 for scenarios B and C, respectively.
Changes in COS with varying mature cow liveweights were not linear for any of the modelled scenarios (
Figure 4). As shown in
Table 1,
Table 2 and
Table 3, differences in weaning weights were not constant between calves from cows with liveweights differing by 50 kg. For example, in scenario B, calves were 11 kg heavier at weaning as mature cow liveweight increased from 450 to 500 kg, then 6 and 8 kg heavier as mature cow liveweight increased by further 50 kg increments to 550 and 600 kg. As differences in weaning weights caused changes in total feed demand for lactation and for growing cattle, changes in COS between modelled herds with cows of varying mature liveweights were not expected to be linear.
Considering the size of COS reductions between scenarios B and C, it is important to consider which would be most realistic for the North Island hill country beef herds modelled. Although pre-weaning growth rates of 1.00 kg/day are considered an industry target for these production systems [
30,
34], the trend towards heavier mature cow liveweights has occurred with selection for increased weaning weight, yearling weight, and final sale weight. All scenarios had higher post-weaning growth rates for offspring of heavier cows (
Table 1,
Table 2 and
Table 3), and the higher pre-weaning growth rates of calves from heavier cows in scenario B mean that it is likely that the closest of the modelled scenarios to growth rates occurring on these commercial beef farms. In contrast, the WWR of 48% for 450 kg cows in scenario C may overestimate the performance of lighter cows.
Scenario B predicted reductions in COS of 7% for an increased mature cow liveweight from 450 to 600 kg (
Figure 4B). For a New Zealand North Island hill country farmer focusing on increasing beef cattle growth rates, the potential 1% reduction in COS for a mature cow liveweight increase from 550 to 600 kg would likely not change their breeding priorities towards lighter cattle. Greenhouse gas emissions intensity (kg CO2e emitted per unit of product) is closely related to kgs of pasture eaten per unit of product [
47]. Greenhouse gas emissions intensity is lower with a higher proportion of feed used for production rather than maintenance [
48], which the results of the current analysis for scenarios B and C suggest would occur with lighter cows. Beef breeding herds are generally farmed on hill country in New Zealand and heavier cows may cause more damage to soil and pasture during wet conditions [
34]. Therefore, these factors also suggest heavier cows to be less efficient and appropriate for producers in the pastoral hill country system modelled. Beef cattle weighing more than 600 kg are farmed in beef production systems internationally, with similar concerns around the production efficiency of increasing mature cow liveweights [
15,
49]. However, the results of the current analysis combined with likely negative impacts on soils and pasture suggest that the optimum liveweight of beef cows for New Zealand hill country farms will not be as heavy. Further, heifers should achieve 60% of mature liveweight at 15 months of age for successful breeding [
34] which may be more difficult for those with heavier mature liveweights in a seasonal, pasture-based New Zealand beef production system.
Factors other than mature cow liveweight can also influence WWRs and efficiencies in beef production [
15], such as the milk production potential of different breeds [
13]. Law et al. [
13] identified lighter New Zealand beef cows to be more efficient. Law et al. [
13] also identified cows of breeds with higher milk production potential to be more efficient as they mobilised more of their fat reserves during lactation. Differences in WWRs for a given cow liveweight may be achieved between different farms and the indirect economic impacts of farming heavier cows may be overcome with effective management. Therefore, although published data indicate heavier cows to have lower WWRs than lighter cows (
Figure 1), this may not always occur for comparisons between breeds, farms, or individual cows. Sufficiently fed heavier cows in hill country conditions may produce heavy weaner calves with an advantage in sooner reaching target slaughter weights before their second winter. These calves would be more attractive to farmers finishing beef cattle on easy contour land, thus heavier cows may be suited to hill country farms selling a high proportion of weaner calves to another farmer to finish. Heavier weaner calves could also be achieved through breeding of older cows with terminal sire breed bulls to produce heavier, faster-growing crossbred calves. This was not included in the current analysis.