Flock Structure

IT’S WORTHWHILE trying to minimise the number of older ewes leaving prime lamb flocks, according to Holmes and Sackett farm consultant Sandy McEachern.

Jill Griffiths
Flock Structure

Speaking during a recent Making More from Sheep (MMfS) webinar, Sandy detailed his recent analysis of the impacts of flock structure on farm profitability, which he says comes down well in favour of trying to keep older ewes in the flock.

"The age of your ewes affects profitability in three ways - reproduction rates, survival of breeding ewes, and the weight of lambs at sale," Sandy said.

In his analysis of these factors and subsequent modelling of a simple system to tease out optimal flock structure, Sandy found conclusive evidence to support efforts to keep older ewes (Figure1).

"Reproduction rates in ewes increase up to when they are about seven years old and then decline, but the decline is relatively slow," he said.

"Survival of breeding ewes increases up to five years of age and declines beyond that.

"The weight of lambs at sale increases up until the breeding ewes are three years old, due to increased birthweight and improving lactation.

"So, there is a penalty in terms of ewe survival, reproductive rates and lamb weight up until the ewes reach five years of age.

"The average age of breeding ewes is a key determinant of the proportion of ewes lambing that are ewe lambs. The lower you drive down the proportion of ewes lambing that are ewe lambs, the better your production.

"The more older ewes that leave the flock, the more production drops overall. That's because it puts more reliance on ewe lambs which have lower production.

"Simply put, it's more profitable to keep an older age group of ewes than it is to join ewe lambs. You can't match production per hectare by pushing the ewe lambs to produce more."


The system Sandy modelled assumed a mortality rate of 4-5%. The numbers show that if an additional 5% of ewes are culled for any reason, flock production drops by 2.5%. If 10% are culled, production drops by 7.5% (Figure 2).

"The data says to keep the old ewes as long as you can. I'd expect the increases to become smaller as you go to eight, nine and 10 year-olds but not much," Sandy said.

This approach to flock management has implications for decisions around culling, and Sandy recommends a conservative approach to culling older ewes.

"For every older ewe that I pull out of the flock, I have to put in a ewe lamb for the next year," he said. "The loss of production kicks in that year, because the older ewe that I culled would have had a higher productivity than the ewe lamb that I replaced her with."

This has obvious implications for decisions about culling, and Sandy recommend thinking carefully before deciding to cull a ewe; if the loss of production that caused the ewe to be culled wasn't high, it won't be enough to offset the loss of production from taking her out of the flock. He recommends only culling on factors that affect ewe and lamb survival.

"Udder imperfection may be a reason to cull. Survey work shows approximately 6% of ewes have malfunctioning udders which in turn causes an increase in death rates in lambs and drops weaning weight by about 10%. It can be a serious source of lost production in the flock.

"Failure to rear is a more important consideration than failure to conceive, but I still wouldn't cull on the first year an older ewe failed. 

"For ewe lambs, failing to conceive is a reason to cull if that ewe lamb can be sold as a lamb.

"None of the data says that you shouldn't breed ewe lambs," Sandy said. "You can't afford not to. Even if you only get 50-70% lambs, the marginal cost benefit is actually very high. The question is about how many of them you need to keep. The more older ewes you keep, the fewer ewe lambs you need to keep, and that's better for your overall productivity."


Sandy said the only time it pays to shift focus to younger ewes is if you are trying to rapidly improve the flock's genetics and are beginning from a low base.

"In that case, it is worthwhile copping the lower productivity from breeding ewe lambs while you improve your flock genetics," he said.

 "You can make a tactical decision to sell older ewes off because of mutton prices, but you are better off setting your course and sticking to it, rather than second guessing what may or may not happen."


Sandy McEachern, Holmes and Sackett

02 6925 1758

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