Impact of mature size on efficiency
Report for AHDB, HCC and QMS by AbacusBio Limited and Scottish Rural University College, 31 March 2019
Breeding flocks and herds represent the backbone of lamb and beef production in Great Britain. It is long been recognised that the profitability of such enterprises is related to the productivity of the breeding population. Enterprise efficiency however relies on understanding the cost base as well as the output potential. It is for this reason that AHDB, HCC and QMS have identified assessing efficiency of breeding enterprises as an important area for their levy payers. Mature size is a hotly debated topic, affecting both the cost of maintaining the breeding population but also the growth potential of the offspring. Therefore it is a key driver of system efficiency. Clear guidance on what the optimum mature size is and how this relates to different production systems is lacking as is guidance on how producers should achieve it.
To determine the implications of differences in breeding female mature weight, the effect of changing cow and ewe breeding female mature weight on other biological traits in the farm system was evaluated. The costs and revenues on farm were then modelled.
Costs and revenue per animal were calculated (breeding female, replacement female, or slaughtered progeny), and scaled to reflect a herd or flock of 100 breeding females. The age distribution of breeding females, the replacement rate, and the slaughter offspring produced are representative of typical beef and sheep farms in the UK. Costs were subtracted from revenues across the entire herd or flock at each breeding female mature weight, to identify changes in margin over feed across a range of breeding female mature weights.
With increasing breeding female mature weight, cull cow revenue increases, while cow maintenance costs, replacement costs, and progeny feed costs all increase. Breeding females heavier than 700kg require higher quality and higher cost feed, which accounts for maintenance and replacement costs increasing at a faster rate above 700kg. Increasing breeding female mature weight up to 725kg significantly increases prime carcase revenue (when progeny are slaughtered on an age constant basis). After 725kg, prime carcase revenue plateaus (to 785kg) and then decreases slightly, due to lower fertility (less progeny being slaughtered) and because heifers and bulls are also being penalised for weighing over 440kg.
The model suggests that the optimum breeding female mature weight for a typical beef production system is 680kg, which for a 100-breeding female herd is associated with a total margin over feed of £25,281. The optimum breeding female mature weight was not sensitive to weight constant versus age constant slaughter practices.
For phenotypic correlations, between breeding female mature weight and progeny carcase weight, from 0.90 to 0.20 there is some sensitivity for the optimum breeding female mature weight, which alternates between 680kg and 695kg over this range. The optimum breeding female mature weight is sensitive to the phenotypic correlation between breeding female mature weight and progeny carcase weight, albeit in a narrow range when considering realistic phenotypic correlations (0.60 to 0.90 and 680kg to 695kg).
The optimum breeding female mature weight is not sensitive to changes in the penalty structure, when that change represents carcase weight over 440kg paid for (but the -£0.40 per kg of carcase weight still applies).
As feed prices decrease, the margin over feed increases at every breeding female mature weight. However, the optimum breeding female mature weight was not sensitive to changes in feed costs, remaining at 680kg regardless of feed costs. The additional increase in overall cow feed cost is not great enough to offset the additional revenue from the heavier progeny carcase, and for this reason the optimum does not change.
With increasing breeding female mature weight, cull ewe revenue increases, while ewe maintenance costs and replacement costs increase. Breeding females heavier than 75kg require higher quality and higher cost feed, which accounts for maintenance and replacement costs increasing at a faster rate above 75kg. Total progeny feed costs increase over the range of ewe mature weights; this is because per animal feed savings (weight constant slaughter) are slightly less than the increase in total feed demand resulting from increases in the number of lambs available for slaughter, as ewe mature weight increases. Progeny revenue increases significantly from 45kg to 55kg mature weight. This increase is associated with increased ovulation rates and increased number of lambs born, and thus increased lambs slaughtered per 100 ewes.
The model indicates that the optimum breeding female mature weight for a typical sheep production system is 55kg , although there is very little difference in margin over feed for breeding female mature weights of 55kg to 65kg. The optimum breeding female mature weight for a 100-breeding female flock is associated with a total margin over feed of £8,060.
The “flatness” of this optimum breeding female mature weight line is the result of several elements. Firstly, there is very little difference in progeny carcase value across the range of ages, when slaughter is at a constant carcase weight; approximately £1/ carcase. When combined with a lower (compared to beef) phenotypic correlation between breeding female mature weight and progeny carcase weight, little value is gained by increasing ewe mature weight in terms of progeny carcase value. Secondly, the reproductive rate effect manifests primarily over the range of 45kg to 55kg. Thus, little value comes from increases in fertility associated with breeding female mature weight above 55kg. Cost and revenue more-or-less increase in proportion as breeding female mature weight increases and, as such, the optimum could be said to sit between 55kg and 65kg.
For phenotypic correlations from 0.90 to 0.10 there is some sensitivity for the optimum breeding female mature weight, which decreases from 57kg at 0.90, to 55kg at 0.20, and then to 51kg at 0.10. The optimum breeding female mature weight is sensitive to the phenotypic correlation between breeding female mature weight and progeny carcase weight, albeit in a narrow range when considering realistic phenotypic correlations.
For grass-based systems, the optimum remains at 55kg with increases and decreases in feed price. For mixed diets systems, the optimum is 51kg under normal pricing, but increases to 55kg when prices are 20% lower, and decreases to 50kg when prices are 20% higher. In a situation where feed is limiting (i.e. feed costs are higher, because they have to be purchased in), e.g. hill farming, the optimum appears to be closer to 50kg.
The optimum breeding female mature weight was sensitive to changes in feed costs. Similar to beef, the rate of change in overall ewe feed cost with the introduction of a mixed diet is only slightly greater than the rate of change in a grass-based diet, as mature weight increases. However, the additional increase in overall ewe feed cost is greater than the additional revenue from the heavier progeny carcase, and for this reason the optimum does change i.e. this “flatness” (driven by little variation in revenue from progeny carcases) means that the optimum is sensitive to feed costs, when average feed costs are high.