Sex ratio manipulation can sensibly enhance the effectiveness of selection and genetic improvement programs, through the differential increment of males or females born after AI. For a number of years, the time of insemination or mating during estrus has been believed to influence the sex ratio of offspring, with early insemination resulting in more females and late insemination, more males. Studies on sheep indicate a trend for more females to be produced from early inseminations and more males from late inseminations. Also Gutierrez-Adan et al. found a possible in vivo effect of oocyte maturational stage or aging on sex ratio in sheep. Berry et al. (2006) indicated a strong significant different between natural mating and AI on sex ratio and reported that AI increase the probability of a male calf in dairy and beef cattle. In sheep, there is not any study on relationship between sex ratio and type of mating. Therefore, the objective of present study was to determine whether natural mating or fixed timed artificial insemination in synchronized sheep influences the sex ratio of the offspring and fertility. A flock of two hundred Balochi ewes were randomly divided into two groups. In both group, lambing rate, Fecundity, Litter size and sex of lambs were recorded after parturition. Statistical analyzes of results was performed using the Genmod procedure of the SAS package and compared by chi-square analysis. As a result, sex ratio (female/male) was 61.37%/38.36%, 48.49%/51.51% in artificial insemination and natural mating groups respectively. The difference with two groups were statistically significant (P<0.05). However, Lambing rate decreased significantly in AI group (P<0.05). Furthermore, Fecundity was 54% and 44% in natural mating and AI, respectively and there is not statistically significant between two treatments (P>0.05). Therefore, Results of this study indicate that although artificial insemination decrease lambing rate but it can increase the probability of a female lamb in flock.

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