For the summer-dry farm avoiding the discount from selling light store lambs might be worth the extra feed cost of running much heavier ewes and their replacements.
The proviso is that the heavy ewes are more able to wean heavy lambs with a high proportion of these being killable at weaning and the remainder sold as desirable heavy store lambs.
These heavier ewes might be seen as efficient because they make the most of the season where growth is present and help avoid the season where feed is scarce.
At the extreme ends of the debate on efficiency you might have a high stocking rate of small to modest-sized hill country ewes with a high lambing percentage and growing out the lambs progressively over summer at one end, and at the other end, a flock of very heavy ewes mated to a large terminal sire killing 70% at weaning. Which is the most efficient?
Looking for an answer without including the context is not going to provide an answer of great value.
The answers lie within a whole farm system context – define the optimum system for your property first, then source the appropriate mix of enterprises and genetics plus your chosen management strategies.
This isn’t something that can be easily done with a pen and paper.
Bio-economic modelling tools such as Farmax give today’s farmers the ability to readily investigate and analyse whole-offarm options in a way that was previously extraordinarily difficult to do.
For summer-dry farms the heavier ewes are able to more than offset their higher feed costs once the NLB gets up above 185%.
The impact of NLB is summarised in Table-1.pdf, which highlights the point where there was no economic gain from further increases in NLB.
For practical purposes NLB can be regarded as being the same as scanning percentage on commercial farms. In these examples it is assumed the EWT is the same across all scenarios (65kg).
The summer-dry optimum is quite different to the summer-not-dry scenario.
In this case it is not the impact of lamb survival that is the major influence, but the influence of changing birth rank and lighter individual lamb weights at weaning as the percentage of twins and triplets changes as NLB rises.
In the summer-dry model, light lambs at weaning are sold as stores and their economic value is reduced.
Optimum ewe liveweight
The modelling included variations in feed value at different times of the year. For a summer-dry farm this meant summer feed had a higher value than on a summer-not-dry property.
This also meant larger ewes with their higher maintenance requirements had a higher economic cost on a summer-dry farm.
When assessing the optimum ewe liveweight, the method used chose 65kg as our NZ standard ewe.
For ewes heavier than this their additional annual feed cost was deducted from the value of their lambs at weaning, and on smaller ewes the annual feed “saved” was added to the value of their lambs at weaning.
The predicted lamb weaning weight at various EWT and NLB combinations was forecast using Farmax.
Differences in optimum adult ewe liveweight
For summer-not-dry farms there are fewer store lamb sales and summer feed is available to finish lambs, which in turn reduces the benefits of higher weaning weight – the optimum favours a fertile lighter-weight ewe.
To match a 55kg ewe producing an NLB of 180% and a weaning of 135% at 25.8kg, an 80kg ewe would need to achieve an NLB of 210%, a weaning of 154%, and a 38kg average lamb weaning weight.
The extra feed costs for the heavier sheep are significant and to be economically efficient requires a lot of extra kg of lamb weaned.
For summer-dry farms the result is quite different. The heavier ewes are able to more than offset their higher feed costs once NLB gets up above 185%, with the gains coming from a combination of higher average weights of lambs weaned and fewer lambs sold as stores.
Below an NLB of 185%, the lighter ewes produce a higher net lamb weaning value than both the average and heavy ewes albeit with a reliance on store sales.
The message from this is big sheep must be very fertile. There are significant differences between heavy and light ewes in the proportion of lambs sold as stores (in the summer-dry model all lambs less than 30kg at weaning are sold as stores) with the lighter ewes being much more dependent on the store market.
Whether this dependence is positive, negative or neutral depends on farmers’ perceptions as well as any structural changes in the demand for store stock.
So back to our starting question – what is the most efficient ewe and how do we measure it? The answer, well it depends.
The answer you need is what is the most efficient for you – which might be different for your neighbour.
• Bruce McCorkindale is a farming and agribusiness consultant with AbacusBio.
