18 September 1998

Selection boosts lamb returns

ENSURE best returns from finished lambs by regular selection to avoid over-finish, says MLC sheep scientist Jenny Anderson.

Careful, regular lamb selection by weighing and condition scoring is essential to ensure individual lambs from the flock are matched to selected target weight and finish.

Liveweight of growing lambs is a fair guide to fatness but has little value because variation in liveweight of adult sheep is determined by breed, environment and management conditions. Breed liveweights – the average of mature ram and ewe weights – are a more reliable guide to fatness prediction. As a general guide, lambs that have grown well without a check will produce adult liveweight.

To calculate potential slaughter weight of crossbred sheep, add the average weight of sire breed to average weight of dam breed, then divide by two to give a potential lamb adult weight.

As an example, for a lamb sired by a Suffolk ram (91kg) out of a Mule ewe (80kg), add the two figures to give 171, then divide by two to give 85.5kg adult liveweight. Therefore slaughter at roughly 43kg (85.5kg/2 = 42.75). This will give a carcass of 3H specification.

As leaner lambs are required, a 5% reduction (2.13kg) will give 3L – slaughter weight 40.6kg – and a 10% cut (4.275kg) in slaughter weight will produce fat class 2 – slaughter weight 38.5kg. Similarly, ewe lambs require less time to finish than wether lambs so slaughtering at 10% less weight will allow them to finish on target. &#42