Effect of breed of dam, type of birth and sex of lamb on efficiency of conversion of food to lamb and wool in Merino, Poll Dorset × Merino and Border Leicester × Merino ewes

Abstract

The effects of breed of dam (South Australian Merino (M), Border Leicester x Merino (BL x M) and Poll Dorset x Merino (PD x M)), type of birth (single and twins) and sex of lamb (wether and ewe) on the efficiency of the ewe (organic matter intake per unit (liveweight)0.75 and clean wool weight per unit intake) and of the ewe-lamb(s)) unit (liveweight and carcass weight per unit intake) were examined in two pen experiments (PE1, PE2) and a grazing experiment (GE). Each single-born lamb in PE1 and PE2 was slaughtered at weights =32 and =35 kg, respectively, and twin-born lambs when their combined weight doubled that of singles. In GE all lambs were slaughtered on the same day at 35.5 kg mean liveweight. Ewes rearing twins ate 19% more (P < 0.05) organic matter per unit (liveweight)0.75 during lactation than those rearing singles, the other effects of breed and sex were not significant during either late pregnancy or lactation. BL x M ewes grew 32% less (P < 0.001) clean wool per unit intake than did M ewes in PE, whereas the breed difference in GE was not significant, PD x M ewes grew 39% less wool than M ewes in GE (P < 0.001). Ewes rearing twins were 23% less efficient in converting food to wool than those rearing singles in PE, (P < 0.01), the corresponding difference in GE was not significant. There were no significant differences between breeds of dam for efficiency of the ewe-lamb unit when lambs were compared at the same age. However, when lambs were grown to the same liveweight the M ewe-lamb unit produced 14% (P < 0.001) and 8% (P < 0.05) more liveweight and carcass weight per unit intake than the BL x M in PE,. These results include adjustment for the effects of type of birth and sex. In all experiments ewes with twin lambs were about 45-55% and 35-45% more efficient than those with singles on a liveweight and carcass weight basis, respectively (P < 0.001), the effects were independent of whether the lambs were at the same age or the same liveweight. We conclude that efficiency of conversion of food to carcass weight on a flock basis will be largely dependent on variation in reproduction rate and to a lesser extent on breed of dam, and on breed of dam only when lambs are slaughtered at the same liveweight. In contrast, the effects of breed of dam on efficiency of food conversion to wool fibre were large.