A series of experiments were conducted to ascertain the effects of sex conditions, slaughter weight, environment and sire performance on the growth and carcass composition of Welsh Mountain lambs. Entire lambs reached target slaughter weight 28.5 days earlier than castrates. Killing our percentage was higher in castrates regardless of rearing environment and they yielded significantly greater carcass weights than entires. Estimated lean and fat percentages showed entire animals to have greater lean and less fat than castrates. Increases in slaughter weight resulted in significant increases in fat percentage with a corresponding decrease in lean percentage. Progeny testing of high and low performance tested rams was carried out to assess their influence on liveweight growth and carcass composition of entire Welsh Mountain lambs. Ram lambs were reared on the ffridd. High progeny reached slaughter weight on average 30 days earlier than the progeny of low rams. Killing out percentage and cold carcass weights were significantly higher in low performance progeny. However, high progeny had significantly greater lean and less fat percentage in their carcasses. Ewe lambs reared on the open mountain also showed that progeny of high rams grew at a faster rate than progeny sired by low rams. When ram lambs were individually penned indoors it was found that the progeny of high sires had greater liveweight gain than low progeny whilst feed intake was similar for both groups, as a result feed conversion efficiency was better in progeny of high performance rams. Wool samples taken from ewe lambs were compared but no consistent differences could be detected in any wool characteristics. The results suggest that Welsh Mountain lambs if reared as entires could be taken to heavier slaughter weights than is normal, with no adverse effect on carcass composition. Progeny sired by rams who perform well on performance test grew faster and at the same slaughter weights produce leaner carcasses than progeny sired by low performance sires.

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