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Post-shearing growth in lambs ... dispelling the myths

We've all heard it many times, and it’s hard to argue against it; when a few weeks after the carry-over lambs are shorn, they look full, they appear well conditioned, and "doing well". But is this really the case, or is there more to it than the eye sees?

To understand what actually happens, let’s consider the mechanisms that are put into place by sheep after they are shorn.

Of course, shearing means removing the fleece, and the main function of the fleece in woolled sheep is to keep the animal in a ‘thermo-neutral’ condition. In other words, the fleece is a highly effective regulator of body temperature, both during winter and summer.

When the fleece is removed, irrespective of the time of year, the animal ‘feels’ some degree of cold, and as an immediate consequence the body kick starts a compensatory mechanism to generate more heat to cope with the effects of exposure. This mechanism requires energy, and the animal responds by shifting a substantial proportion of the energy that was previously being used to ‘grow’, to processes that are devoted to keep the sheep ‘warm’.

With this energy devoted to regulate body temperature, it is therefore not available to be put towards growth. The net result is that lambs that have been shorn actually grow slower than unshorn lambs, and this effect can last up to 2 months after shearing.

In the case where shorn lambs are put onto fresh lucerne after shearing, a further complicating factor may also be at play. Green, leafy lucerne (pre-flowering) has high concentrations of protein. While protein is essential for muscle growth, when levels become excessive sheep must initiate mechanisms to detoxify the excess ammonia that is produced when they digest these very high protein feeds; and that process also requires energy. Thus shorn lambs on fresh lucerne pastures are undoubtedly suffering a ‘double whammy’.

So why is it that the lambs generally "look better" after they are shorn? Well, to cope with the extra energy requirement to regulate body temperature after shearing, the lambs are able to increase their intake by some margin. However, this takes a couple of weeks to be fully achieved, and by then, with full (over-full) stomachs, the lambs "look great". A classic deception of the eye; that if they were weighed and compared to unshorn sheep a couple of months after shearing, the deception would be exposed!

So why shear carry-over lambs? That question comes back to a flock management decision. If the lambs have longer skins than the market requires, or there is a risk of seed contamination, flystrike, or lice prblems, then the management imperative is clear and shearing is justified. Otherwise, the cost of shearing, and the hidden cost of a growth rate reduction, and less efficient conversion of feed to bodyweight for up to 2 months after shearing make a strong economic argument against shearing ‘out of season’.

Shorn carry-over lambs may appear to be well conditioned but are they really doing as well as they look?

Written by Sean Miller

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