092 Influence of GnRH supplementation at CIDR removal on estrus expression and interval to estrus in beef cattle

Previous research has indicated that small doses of GnRH after CIDR removal increased circulating estradiol concentrations. Thus, our objective was to determine if a single dose of GnRH (5 or 10 µg) at CIDR removal would impact estrus expression and/or interval to estrus (IE). Beef cows/heifers (n = 1620) were synchronized using the 7-d CIDR protocol and randomly assigned to one of three treatments (0, 5, or 10 µg of a GnRH analog at CIDR removal). Animals were observed for estrus and inseminated following estrus detection. Interval to estrus was calculated for each animal that exhibited estrus (Interval1). Animals that did not exhibit estrus (DSS) were given 2 cc of GnRH at AI, and 120 h was recorded as their IE (Interval2). Interval to estrus was analyzed using the MIXED procedure of SAS. Expression of estrus and pregnancy success were analyzed using the GLIMMIX procedure of SAS. There was an effect of age (P < 0.01) and a treatment by age interaction (P = 0.05) on Interval1. Heifers had a shorter IE than cows (P < 0.01; 50 ± 1.1 vs. 54 ± 1.1 h, respectively). Furthermore, heifers given 5 µg of GnRH tended to have a shorter IE (P = 0.07; 47 ± 1.4 h) compared to 0 µg (50 ± 1.5 h) and had a shorter IE compared to 10 µg (P < 0.01; 53 ± 1.2 h). Among cows, there were no differences between treatments on IE (P > 0.34). When DSS animals were included in the analysis, there was no treatment by time interaction (P = 0.49). This is likely due to treatment (P < 0.01), not age (P = 0.96) or treatment by age (P = 0.74), influencing estrus expression, with 5 µg tending to induce estrus in more animals (P = 0.10; 79 ± 4%) compared to 0 µg (74 ± 5%) and 10 µg inducing estrus in fewer animals compared to 0 and 5 µg (P < 0.04; 68 ± 6%). Estrus (P < 0.01) and age (P < 0.01) influenced pregnancy success with heifers having greater success (49 ± 5% vs. 38 ± 4%) and animals exhibiting estrus having greater success compared to DSS animals (57 ± 4% vs. 32 ± 4%). There was no difference in pregnancy success between treatments among cows that exhibited estrus (P > 0.30); however, among DSS cows, 0 µg had increased pregnancy success (P ≤ 0.05; 41 ± 6%) compared to 5 and 10 µg. In summary, 5 µg of GnRH at CIDR removal tended to decrease IE and increase estrus expression among heifers but not cows, and both 5 and 10 µg of GnRH at CIDR removal decreased pregnancy success among DSS cows.