Breed and environmental influences on weaner lamb production in Zimbabwe.

Performance of the Dorper and Mutton Merino breeds and their crosses was assessed from 1,305 ewe and 1,453 lamb records from six lambing seasons. Dorper and Mutton Merino were mated in all combinations under an accelerated lambing system and crossbred females were mated to Suffolk rams. The Dorper and Mutton Merino breeds were not significantly different for fertility, pre-weaning lamb growth and survival. Suffolk sired lambs from crossbred ewes grew rapidly and were heaviest at birth and at weaning. They also survived better than lambs from the other breeding groups and this contributed to the higher weaning rate (P less than 0.05) in crossbred ewes. Weight of weaned lambs per ewe exposed to the ram per season was 12% higher (P less than 0.05) in the Dorper than the Mutton Merino ewes. However lifetime production was 28% higher (P less than 0.01) in Mutton Merino ewes due to significantly superior longevity. Crossbred ewes were highly productive. Individual heterosis was not significant but estimates were positive for most traits. Growth rate and survival of lambs increased as age of dam increased to 48 and 40 months respectively. Lambs born to previously non-lactating ewes were heavier and survived better than lambs born to ewes lactating in the previous season. Lambs born and reared as singles were 25% heavier at birth (P less than 0.001) and 52% heavier at weaning (P less than 0.001) than those born and reared as twins. Male lambs grew faster and were 7% heavier at weaning than females (P less than 0.001).