129 THE RELATIONSHIP OF BODY CONDITION SCORE AT TIME OF AI AND SUBSEQUENT GENDER RATIO OF OFFSPRING

Abstract

The chance of a female calf being born is less than that of a male calf from lactating dairy cows (M:F ratio from two large data sets was 54:46; Ryan and Boland 1991 Theriogenology 36, 1-10, and Berry et al. 1995 J. Anim. Sci. 78, 76 abst). Data from this study were analyzed to determine if there was a relationship in body condition of the dam at time of artificial insemination and subsequent gender ratio of offspring from lactating Holstein dairy cows. Lactating dairy cows from n = 3 farms were timed-AI at approximately 8 h before or 16 h after a GnRH induced LH surge and were body condition scored (BCS; 1 to 5, with 1 representing least amount of condition) at time of AI. Previous data from our laboratory indicated that cows in the -8 and +16 h groups received AI approximately 36 and 12 h prior to ovulation, respectively, and the longer sperm incubated in vivo the greater the subsequent percentage of female calves were born. In the study, the aforementioned data were analyzed in SPSS for the relationship between gender ratio and BCS from n = 333 cows that carried calves to term utilizing correlation analysis, and for pregnancy losses in cows (n = 1501) that received AI and had been evaluated for BCS utilizing binary logistic regression analysis. Cows were classed into 3 body condition groups at time of AI: <2.5 (low), 2.5 to 3.5 (medium), and >3.5 (high) for analysis. There was a significant negative correlation in BCS with M:F gender ratio (P < 0.05). Cows with low, medium, or high BCS had 40:60 (n = 82), 52:48 (n = 141), and 56:44 (n = 110) M:F ratio, respectively. The AI approximately 36 h prior to ovulation tended (P = 0.10) to attenuate M:F ratio in the high BCS group. Cows in the high BCS group that received AI approximately 36 h prior to ovulation had 46:54 compared with 62:38 M:F ratio in cows receiving AI approximately 12 h prior to ovulation. Pregnancy loss between 28 and 56 d post-AI was not different (P > 0.10) between the three BCS groups suggesting that differences in gender ratio were not the result of sex-specific losses. In summary, BCS at time of AI altered subsequent gender ratio of offspring and was attenuated in the high BCS group by time of AI relative to ovulation. We speculate that differences in gender ratio in cows with different BCS may be due to alterations in the uterine and/or oviductal environment that in turn alters the ratio of X and Y chromosome-bearing sperm at time of fertilization.