A Lower Dietary n-6 and n-3 Ratio During Timed AI Enhances Luteal Development and Tends to Increase Female Offspring Sex Ratio and Calf Birth Weight in Lactating Dairy Cows

The objectives of this study were to evaluate the effects of different dietary omega-6 to omega-3 (n-6:n-3) fatty acid ratios during timed artificial insemination (TAI) on reproductive responses, luteal development, and offspring characteristics in lactating dairy cows. Holstein cows (n = 60) averaging 109 ± 10 days in milk (DIM) were randomly assigned to receive either a greater n-6:n-3 diet or a lower n-6:n-3 diet for 42 days. All cows were synchronized using a modified Ovsynch protocol. Although synchronized ovulation rates and pregnancy outcomes were not significantly affected by treatment, cows fed the lower n-6:n-3 ratio exhibited significantly greater corpus luteum (CL) volume and cross-sectional area on days 11 and 14 post-TAI, indicating enhanced luteal development. Pregnancy rates following the first AI and cumulative AI, as well as the number of services per conception, did not differ between treatments. However, cows fed the lower n-6:n-3 ratio tended to produce a greater proportion of female offspring (66.67% vs. 42.90%; p = 0.09). In addition, calves born to cows receiving the lower n-6:n-3 ratio had greater birth weights than calves born to cows fed the greater n-6:n-3 ratio diet. Additionally, the lower n-6:n-3 diet tended to increase milk yield and significantly increased lactose and solids-not-fat yields. In conclusion, a lower dietary n-6:n-3 ratio during a TAI program enhanced luteal development, tended to increase the proportion of female offspring and increased calf birth weight. These findings suggest that reducing the dietary n-6:n-3 ratio during the breeding period may enhance luteal development and may influence offspring sex ratio and calf birth weight in lactating dairy cows.