Lamb production from May or August lambing by Merino and Comeback ewes was compared from 1982 to 1985 in a factorial design experiment at Hamilton, Western Victoria. Groups of 50 ewes were stocked at 500 kg live weight/ha or 12.4 Merinos/ha and 10.8 Comebacks/ha. Natural fertility levels were similar for all treatments with an average of 5% ewes not lambing. Comeback ewes had a higher lambing rate (lambs born/ewes joined) at August compared with May lambing (1.28 v 1.19 sed 0.052) whereas the Merinos had similar lambing rates (1.03 v 1.07 sed 0.02), indicating that the Western Victorian Merinos were relatively insensitive to seasonal effects (photoperiod) on fecundity. Lamb mortality was higher at August compared with May lambing (18.5 v 14.5% sed 1.8%) and was associated with higher chill factors in August due to cooler, wetter weather. The effects of lamb mortality lead to similar weaning rates (lambs weaned/ewes joined) for Comebacks at May and August lambing and lower weaning rates for Merinos at August compared with May lambing. Lamb weaning weights were similar for both lambing times, except in 1985 when severe internal parasite infections before weaning in groups lambing in May restricted lamb growth. The lower stocking rate (ewes/ha) balanced the higher per head production for Comeback ewes and lead to no significant differences between breeds or lambing times in weight of lamb weaned per ha.
Proceedings of the New Zealand Society of Animal Production, Volume 49, , 225-228, 1989
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