Short communication: Feed restriction around insemination did not alter birth sex ratio in lactating dairy cows.

Studies investigating variation in birth sex ratio indicate that the energy status of the dam prebreeding can influence the sex of the subsequent offspring. The Trivers-Willard hypothesis suggests that dams in good condition would produce more male offspring, whereas dams in poor condition (relative to dams in good condition) would produce more female offspring. However, results testing this hypothesis are associative in nature, with no data available from controlled experiments. Hence, this study tested the hypothesis that feed allowance around the time of conception alters birth sex ratio in lactating dairy cows. Cows (n=770 on 2 farms) were randomly allocated to 2 treatments before the seasonal breeding period and offered either unrestricted (UnRes; n=453, across 4 herd replicates) or restricted (Res; n=317, across 3 herd replicates) allowance of fresh pasture for the first 14 d of breeding. Restricted cows responded by reducing milk yields throughout the treatment period and took 3 to 4 wk to recover. The birth sex ratio of resultant offspring was evaluated for cows that conceived in the first 21 d of breeding (UnRes n=234; Res n=142). Birth sex ratio was not different between UnRes and Res treatments. Association analysis, within treatments, identified that cows had a greater odds of producing a male offspring if they had lower milk fat to protein ratio before conception, were in a lower body condition score precalving, and gained condition from calving to the breeding period. No relationship between body condition score prebreeding and birth sex ratio was identified. In the current study, no difference in the birth sex ratio was observed following a short-term feed restriction around the conception. However, alternative indirect measures of energy balance (e.g., milk fat:protein ratio) may be a useful tool when examining the relationships between energy balance in dairy cows and birth sex ratio.