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 10µg) at CIDR removal would impact estrus expression and/or interval to estrus (IE). Beef cows/heifers (n = 1620) were synchronized using the 7-day CIDR protocol, and randomly assigned to one of three treatments (0, 5, or 10µg 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 2cc of GnRH at AI, and 120hr was recorded as their IE (Interval2). Interval to estrus was analyzed using the MIXED procedure of SAS. Expression of estrus and pregnancy success was 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±1.1vs. 54±1.1 h, respectively). Furthermore, heifers given 5µg of GnRH tended to have a shorter IE (P = 0.07; 47±1.4hr) compared to 0µg (50±1.5 h) and had a shorter IE compared to 10µg (P< 0.01; 53±1.2hr). 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 5µg tending to induce estrus in more animals (P = 0.10; 79±4%) compared to 0µg (74±5%), and 10µg inducing estrus in fewer animals compared to 0 and 5µg (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, 0µg had increased pregnancy success (P ≤ 0.05; 41±6%) compared to 5 and 10µg. In summary, 5µg of GnRH at CIDR removal tended to decrease IE and increase estrus expression among heifers but not cows, and both 5 and 10µg of GnRH at CIDR removal decreased pregnancy success among DSS cows.