Influence of GnRH supplementation at CIDR removal on estrus expression and interval to estrus in beef cattle.
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abstract
Previous research has indicated that multiple small doses of GnRH following CIDR removal increased circulating concentrations of estradiol. Therefore, our objective was to determine if a single small dose of GnRH (5g or 10g) at CIDR removal would impact expression of estrus and/or interval to estrus. Beef cows and heifers (n=1620; n=1057 cows, n=563 heifers) were synchronized using the 7-day CO-Synch+CIDR protocol, and randomly assigned to one of three treatments (0, 5, or 10g of a GnRH analog at CIDR removal). Animals were inseminated following detection in estrus. Interval to estrus was calculated for each animal that exhibited estrus (INTERVAL 1). Animals that did not exhibit estrus were administered 100g of GnRH at the time of AI and their interval to estrus was designated at 120h (INTERVAL 2). There was a treatment by age interaction (P=0.05) on INTERVAL 1. Heifers treated with 5g of GnRH tended to have a shorter interval to estrus (P=0.07; 471.4h) compared to 0g (501.5h) and did have a shorter interval compared to 10g(P<0.01; 521.5h). There were no differences between treatments in interval to estrus among cows (P>0.34). When animals that did not exhibit estrus by 120h were included in the analysis there was no treatment by age interaction (P=0.49). This is likely due to the fact that treatment (P<0.01), but not age (P=0.96) or treatment by age (P=0.74) influenced expression of estrus. In addition, there tended to be a treatment by estrus interaction (P=0.08) on pregnancy success. There was no difference in pregnancy success between treatments among animals that exhibited estrus (P>0.30). In summary, 5g of GnRH at CIDR removal tended to decrease the interval to estrus and increased expression of estrus among heifers but not cows, and 10g of GnRH at CIDR removal did not improve estrus expression and lengthened the interval to estrus in comparison to the control.
