Effect of mating rate on offspring sex ratio in Thoroughbred stallions.

Changes in factors like an increase in mating frequency, age, and subfertility have been associated with a female-biased sperm sex ratio and may result in a female-biased offspring sex ratio. In the New Zealand Thoroughbred industry, there is a preference for male foals due to a greater potential economic return from successful racing and breeding careers. If female-biased sperm sex ratios are linked to higher mating rates, this has economic implications and practical implications for stallion breeding management.

Within the industry, there are multiple commercial and economic drivers to mate large number of mares (31 - 114 mares within a season) within a heavily condensed breeding season (approximately 110 days). Therefore, stallions have a high daily reproductive rate (mating up to four mares per day), with mating concentrated within the narrow mare reproductive season and breeding window.

This study investigated the effect of average daily mating rate from 7 to 0 weeks before conception on the likelihood of conceiving a male foal using logistic regression and data derived from official breeding records (2005-2015) of New Zealand Thoroughbred stallions (n=220 stallions, 33704 observations). The 7-week window was included to capture the effect of oxidative stress from mating on stages of spermatogenesis. Other relevant variables such as sire, season, stud fee category, age, book size, and fertility were also investigated. Variables were tested using univariable analysis (significance obtained using Chi-square test) followed by a forward stepwise approach.

Only the increased mating rate (≥3 mares/day) at 4 weeks (28-35 days) prior to conception significantly reduced the likelihood of conceiving a male foal (Odds ratio = 0.93, CI:0.89-0.98, P<0.01). This coincides with the meiosis stage of spermatogenesis, where greater Y-chromosome susceptibility to stress-induced damage during early development has been reported in the literature. These findings are consistent with observations in previous studies and highlight the relationship between economic optimisation and biological pressures associated with intensive production practices.