Lactation Studies and Lamb Survival in Merino Sheep Selected for High and Low Weaning Weight

Abstract

A series of experiments were conducted to examine the effects of selection based on weaning weight on lactation and lamb survival. The lactation performance of single rearing Merino ewes selected for high (W+) and low (W-) weaning weight for 12 generations and that of random bred control ewes (R) were examined over 2 years of study. The ewes were machine milked by the oxytocin method described by McCance (1959). Differences between selection lines were established in the milk yield, cumulative milk yield, shape of the lactation curve, and time of peak production. The W- ewes peaked the earliest (7-24% below R), while the W+ ewes peaked the latest (10% above R). Large between-animal variability in milk yield was recorded, especially in early lactation. Significant differences(P<0.05) between selection lines were found after Day 22 of lactation. Correlated responses in lactation performance associated with selection were attributed to both changes in ewe liveweight and lamb suckling stimuli. Ewe age had no effect on milk production. ... Observations on preweaning lamb survival in the selection flocks revealed survival of the w- lambs to be consistently low. Heaviest lamb losses in all flocks occurred at or within 7 days of birth. Differences between lines in the time of death were generally non-significant. The main cause of death in the W+, R, and R+ lambs was birth injury due to dystocia, while starvation was the main cause in the W- lambs. However, differences between lines in the cause of death were insignificant.