The Effect of System Parameters on the Seasonality of Feed demand

In this research, the livestock system had a very large effect on the seasonality of feed demand. This is in contrast with Woodford (1997), who found that alternative venison production systems did not have a very large impact on the seasonality of feed requirements. From the results, it is apparent that, in relation to matching feed supply and demand, by tailoring the sheep farming systems, system feed demand can be adjusted to match a wide range of environmental conditions.

In relation to increased lambing percentages, it can be argued that the seasonality impacts are not intuitively obvious. Increased lambing percentages resulted in more twins, and in the absence of complementary strategies relating to pre-weaning growth rates, then it was actually post-weaning where the overall system demand increased. This contrasts with the conclusions of Brookes et al. (1998) who found that increased feed demand for high lambing ewes occurred mainly from late pregnancy until weaning. The difference in findings is due to the modelling method used. For

example, while the model used in the present study controlled ewe numbers so that total feed demand of the system remained constant, the model used by Brookes et al. (1998) kept the size of the ewe flock constant but adjusted the total feed energy demand of the system.

Assuming a fixed feed energy requirement, although lambing percentage increased post-weaning system feed demand, the size of this impact depended on the growth rate of lambs and on the weight at which lambs were slaughtered. For example, if lambs grew fast pre-weaning and/or were slaughtered at low carcass weights, then increased lambing percentage would have a much smaller impact on feed demand in the post-weaning period compared to if lambs grew slowly and/or lambs were killed at much heavier carcass weights. The effect of lambing percentage under different scenarios of lamb growth rate and target carcass weight is shown in Chapter 7.

In relation to carcass weight, the direction of shift in the seasonality of feed demand is intuitively obvious but the extent of the shift is remarkable. This has major implications for management of feed supply.

Lamb growth rate had a major impact on the seasonality of system feed demand. Accordingly, and depending on the availability and cost of pre-weaning versus post-weaning feed, and the price premiums for early production, then increasing lamb growth rates may still have a high impact on economic returns despite not having a large impact on feed conversion efficiency.

6.11.3

Stocking Rate

In order to keep system feed demand constant at 10,000,000 MJME, single-factor experimentation of system parameters forced ewe numbers at mating date to vary between 1,134 ewes and 1,507 ewes (Figure 6.17). This represents 33% change in ewe numbers. This compares to Cruickshank et al. (2009) who observed that production parameters such as scanning percentage, cull age, and hogget lambing had a large impact (21% reduction) on the number of ewes able to be carried. In relation to ewe size, in contrast with Rutherford et al. (2003), who reported that for the same amount of feed 14% more 60 kg ewes could be run in comparison to 70 kg ewes, in this study, 60 kg ewes resulted in an increase of just 6.7% in ewe numbers in comparison to 70 kg ewes (Table 6.4). In relation to ewe lambing percentage, the results observed in this research are consistent with that observed by Harrison et al. (2014).

From the results found in this study, a key insight is that unless attempts at increasing feed

conversion efficiency through improvements to livestock production parameters are accompanied by changes in farm feed supply, the size of the ewe flock may need to increase or decrease by a

considerable degree to compensate for the change in total feed demand. If this does not occur then the sensitivity of ewe numbers to changes in system parameters, especially to parameters such as

lamb carcass weight, implies that considerable feed shortages may occur and overall production targets may not be achieved.

A second key insight from Figure 6.17 is that each of the livestock parameters tested in this research had quite large differences in respect to their impact on ewe numbers, both in terms of the

magnitude of the impact and on the slope of the response curve. Implicit in this finding is that, by itself, stocking rate is an inadequate predictor of overall system feed conversion efficiency. For example, from the results of single factor experimentation presented in this chapter, feed conversion efficiency had an R2 _{value of just 0.39 in relation to its correlation with the number of ewes at mating}

date. Therefore, the conclusions found in this study build on from those of Woodford and Nicol (2005) who argue that comparative analysis of livestock systems cannot be based on the number of animals carried per hectare of land but must include allowance for production variables.

Notwithstanding, it is notable that extreme values of feed conversion efficiency correspond with extreme stocking rates and this issue is explored further in Chapter 7.

Figure 6.17 Relationship between feed conversion efficiency (g carcass/MJME) and ewe numbers at mating date 2.5 2.7 2.9 3.1 3.3 3.5 3.7 1,000 1,100 1,200 1,300 1,400 1,500 1,600 FC E (g carca ss /MJ ME )

Ewe Numbers at Mating Date

Lambing % Pre-Wng LWG Post-Wng LWG Ewe Size Carcass Weight Hogget Lambing Cull Age Ewe Mortality 170% 80 kg 50 kg 12 kg 24 kg 1% 120% 10.3 yr 5.3 yr 7% 100 g/day 300 g/day 450 g/day 110% 0%