80 Relationship between anogenital distance and reproductive efficiency of embryo recipient dairy heifers

Anogential distance (AGD) in females is defined as the distance between the anus and base of the clitoris. Results from previous research in multiple species indicate there is great variation in AGD among individuals and that a longer AGD may be associated with adverse reproductive outcomes. The objective of the present study was to test the hypothesis that longer AGD is associated with reduced reproductive efficiency in embryo transfer recipient heifers. Holstein heifers 16.4 ± 0.1 mo of age (n = 760) were synchronised using a modified 5-day CO-Synch + CIDR protocol as follows: Day −8 CIDR inserted, Day −3 CIDR removed and PGF_2α treatment (500 μg cloprostenol sodium), Day −2 PGF_2α treatment, Day 0 gonadotrophin-releasing hormone (GnRH; 100 μg gonadorelin acetate). An Estrotect patch was placed on all heifers on Day −3, and heifers were evaluated for signs of oestrus on Day 0. On Day 5, heifers were evaluated using ultrasonography to determine presence and size of the corpus luteum (CL). Heifers with a CL received an embryo 7 ± 1 days after GnRH administration, and pregnancy status was determined using ultrasonography 30 and 60 days after GnRH. The AGD was determined at random stages of the oestrous cycle (Day −8) using a caliper. Heifers were classified into short (n = 244), medium (n = 261), and long (n = 255) AGD based on tercile distribution. Potential differences in reproductive performance between AGD groups were analysed using generalised linear mixed models, and results are reported in Table 1. In addition, the correlation between AGD and heifer age was evaluated using Pearson correlation. Overall average AGD was 103.4 ± 0.4 mm (range: 62 to 135 mm). Average AGD was 92.2 ± 0.4 mm for the short group, 103.8 ± 0.2 mm for the medium group, and 114.6 ± 0.3 mm for the long group. There was a positive correlation between age and AGD (r = 0.21; P < 0.001) such that a greater age was associated with longer AGD. The percentage of heifers that expressed oestrus tended (P = 0.08) to be different between AGD groups. Conversely, utilisation rate (transferred/treated) was not different (P = 0.80) between heifers with short, medium, or long AGD. Pregnancy data were analysed only for recipients transferred with a fresh in vitro-produced embryo, and included embryo stage, embryo quality, and interval from GnRH to transfer as covariates. Pregnancies per embryo transfer (P/ET) at Day 30 and 60 were not different (P > 0.18) between heifers with short, medium, or long AGD. Similarly, pregnancy loss between Day 30 and 60 was not different (P = 0.15) between AGD groups. In conclusion, even though AGD varies greatly among heifers, efficiency of recipient utilisation and fertility is not associated with AGD.

Table 1.  Oestrous expression, utilisation rate, pregnancies per ET (P/ET), and pregnancy loss in recipients with short, medium and long anogenital distance (AGD)