Influence of sex ratio during multiple pregnancies on productive and reproductive parameters of lambs and ewes.

Because twin (and sometimes triplet) births commonly occur in sheep, interactions between foetuses of opposite sex may occur during life in utero and affect adult productive or reproductive performance. This was investigated by comparing growth, milk production and reproductive features of female lambs born as twins with a sister (FF) or a brother (FM) or born as triplets with two sisters (FFF), one brother and one sister (FFM) or two brothers (FMM). Birth weight, age and weight at puberty and milk production during the first lactation were similar between FF and FM lambs as well as between FFF, FFM and FMM lambs. Most of the reproductive parameters were unaffected by the sex ratio in utero. Ovarian development during the postnatal and prepubertal periods was identical between groups as demonstrated by the lack of differences in follicle-stimulating hormone concentrations and ovarian response to human chorionic gonadotropin. Adult ovulation rate and litter size at first lambing were also not related to sex ratio in utero. In contrast, embryonic mortality measured by the difference between ovulation rate and litter size was significantly (P < 0.05) increased in FMM lambs compared to FFF and FFM lambs. The same was observed for FF versus FM lambs. It is concluded that sex ratio in utero may have to be entered in programmes evaluating genetic merit for embryo survival in sheep.