Using lamb eight-week weights and ewe body condition scores to guide management decisions

A pilot project on sheep Key Performance Indicators (KPIs) commissioned by EBLEX has found a significant positive relationship between ewe body condition score (BCS) at lambing and lamb weaning weights.

The relationship predicts that for every one unit increase in ewe BCS at lambing, weaning weight would increase by about 1.5kg per lamb (up to BCS 3.5). This shows the benefit of targeting an optimum BCS at lambing of 3 for lowland ewes, 2.5 for upland ewes and 2 for hill ewes, with much thinner and fatter ewes likely to suffer from other issues, such as low lamb birth weights or difficulty lambing respectively.

However, there was no significant effect on lamb weaning weight of either ewe BCS at eight weeks post-lambing or the change in BCS from lambing.

Ewes with greater fat reserves are better placed to meet the milk production demands of lambs when diet is deficient by mobilising body fat. Thinner ewes are likely to struggle when the energy is deficient, because they have less body fat to mobilise. Good nutrition post-lambing will also help minimise BCS loss and is important for ewes to meet peak yield at three to four weeks post-lambing.

The rate a ewe mobilises body fat to meet requirements during lactation can affect her performance in the next tupping season. Ewes which are thin at weaning will require additional time and supplementary feed to achieve the optimum BCS for tupping.

Weighing lambs at eight weeks of age provides a good indication of how much milk ewes have been producing. However, lambs’ access to feed and forage to help with rumen development will also have an effect on growth rates in those crucial first weeks.

The weight of lambs at eight weeks and current ewe body condition, along with information on grazing and feeds available, can be used to decide when lambs should be weaned and whether to wean some groups early. This can optimise lamb performance and allow ewes to gain condition prior to tupping. Weighing lambs at about eight weeks of age can give an indication of:

· Ewe’s milk yield

· Lamb’s forage requirements

· The health status of the group

· Weaning date (also based on ewe condition)

· Lamb growth rates achieved

To calculate the daily liveweight gain of lambs, take the weight at eight weeks (for individuals or groups), deduct the birth weight(s) and divide this difference by the number of days. For example: 20kg at eight weeks minus 4kg birth weight, gives a 16kg difference to divide by 56 days = 285g/day.

Where there is a large variation between groups in their lamb growth rates at eight weeks, managing separate groups differently may be considered. Options may include weaning early, supplementary feeding or preferential grazing.

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