Increased proportion of female lambs by feeding Border Leicester × Merino ewes a diet high in omega-6 fatty acids around mating

The proportion of female lambs was previously higher when ewes were offered a diet high in omega-6 fatty acids (n-6) based on oats and cottonseed meal (oats/CSM) around joining. The effect of the length of time of feeding the high n-6 diet 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 Border Leicester × Merino ewes were fed a diet high in n-6 either pre-conception only or both pre- and post-conception. Ewes were fed a low n-6 diet based on silage or a high n-6 diet based on oats/CSM for 40 days pre-mating until the end of mating (pre-mating) or 40 days pre-mating and 17 days post-mating. Liveweight, fat score, plasma fatty acid status and lamb sex was determined. Plasma n-6 concentration was higher (P < 0.001) and the time to oestrus shorter (P < 0.001) when ewes received oats/CSM compared with silage. The proportion of female lambs was higher when ewes were fed a high n-6 compared with low n-6 diet pre-mating only (54.1% vs 44.4%, P ≤ 0.049) and both pre- and post-mating (57.2% vs 43.4%, P ≤ 0.018). The interaction between diet and length of time of feeding was not significant (P ≤ 0.537). The mechanism resulting in the diet high in omega-6 influencing the sex ratio of lambs appears to operate before or around conception but is yet to be determined.