Method of stimulating ovulation rate in Merino ewes may affect conception but not embryo survival

Efficiency in breeding flocks can be improved by increasing twinning rate. While this can be achieved by several methods that stimulate ovulation rate, there is considerable variation in the number of extra lambs produced by alternative treatments. Variations in ovulation rate following stimulation treatments are well documented, but the subsequent processes of conception and embryo survival leading to extra lambs are not. Differences in lambs produced by four alternative stimulation treatments examined in the following experiments were not significantly affected by embryo survival. The stimulation treatments examined were: improved nutrition for several weeks pre-mating (N); lupins (L) as stubble or standing crop, or fed as grain; grazed tagasaste forage (T); Fecundin immunisation (F). Ovulation rates were measured using laparoscopy and viability of embryos monitored using ultrasound. Ovulation rates in treated groups were increased by up to 0.66 (L), 0.20 (T), 0.10 (N) and 0.36 (F) ova per ewe. Survival of twin ova in pregnant ewes ranged from 81 to 93% across treatments and flocks, but did not differ significantly between treatments within flocks. Proportions of ewes pregnant in the first cycle of mating were depressed by 8-11% in F and L treatments. Conception rates were higher (by 7-16%) in twin rather than single ovulating ewes, and, in one flock, higher (by 15%) following bilateral rather than unilateral twin-ovulations. All stimulation treatments increased numbers of lambs conceived, lupins being the most efficient. Differences between treatments in lambs conceived were dependent on ovulation and conception rates but not embryo survival. Rates of embryo survival that were similar between treatments, but differed between flocks, indicated failure due to maternal factors rather than the embryos themselves. The mechanism(s) responsible could not be determined in these experiments, but genetic differences between flocks are suggested.