Skin folds and Merino breeding. 2. The relative influence of the ram and the ewe on fertility and perinatal lamb mortality in flocks selected for and against skin fold

Abstract

Reciprocal matings were carried out between the Trangie Folds Plus and Folds Minus selection Aocks in March 1961 and 1962. Folds Minus rams were more fertile than Folds Plus rams as shown by ewes lambing per insemination (72 per cent versus 48 per cent, P&0.01) and by incidence of twinning per ewe lambing (57 per cent versus 34 per cent, P<0.05). Folds Plus and Folds Minus ewes showed a similar reproductive performance if mated to the same rams. 236 There was a 17 per cent mortality in single lambs born to Folds Plus ewes compared with 4 per cent (P<0.05) for single lambs born to Folds Minus ewes. This increased mortality was caused by dystocia in association with relative oversize of the lamb. A positive genetic correlation of 0.32 ¦ 0.21 between fold score and corrected birth weight was calculated using independent data. The mortality of twin lambs born to Folds Minus ewes was higher than for Folds Plus ewes (18 per cent versus 11 per cent). There was a maternal effect on the birth weight of twins, which were relatively much heavier from Folds Plus ewes. Approximately 80 per cent of the difference in net reproductive rate between the Folds Minus and Folds Plus flocks was explained on the basis of infertility of Folds Plus rams and excessively heavy birth weights of Folds Plus lambs.