An increased intake of rumen undegradable protein (RUP) or n-3 fatty acids (FA) around mating is associated with improved reproductive efficiency in ewes. The simultaneous effects of RUP and n-3 FA on the sex ratio of lambs has not previously been reported. The aim of the present study was to determine whether the proportion of female lambs was higher when Kurdish × Romanov ewes were fed a diet high in n-3 FA and RUP together around mating. Experimental diets were supplemented with 5% (DM basis) calcium salts of saturated FA (SFA), 5% calcium salts of fish oil (as n-3 FA source; n-3FO) or a combination of 5% calcium salts of fish oil and 5% fish meal (as a RUP source; n-3FO+RUP). The proportion of total n-3 polyunsaturated FA (PUFA) in plasma was higher and the n-6 to n-3 ratio was lower at the day of CIDR removal in ewes fed the n-3FO+RUP diet (P < 0.05) compared with the other diets. The concentration of oestradiol, glucose and urea nitrogen (UN) in plasma at the day of oestrus (P < 0.05) was also lower when ewes were fed the n-3FO+RUP diet compared with the other diets. The proportion of female lambs was higher when ewes were fed the n-3FO+RUP diet around mating (P < 0.05) and was greater than an expected 50:50 ratio (72% females, P = 0.028). These results indicated that feeding ewes a diet high in n-3 FA and RUP content around mating, rather than n-3 FA alone, can skew the sex ratio of lambs toward females. The effect of the interaction between dietary FA sources with other nutrients on the sex ratio of lambs should be monitored in future research.
