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

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 (5 μg or 10 μg) 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 10 μg 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 100 μg of GnRH at the time of AI and their interval to estrus was designated at 120 h (INTERVAL 2). There was a treatment by age interaction ( P = 0.05) on INTERVAL 1. Heifers treated with 5 μg of GnRH tended to have a shorter interval to estrus (P = 0.07; 47 ± 1.4 h) compared to 0 μg (50 ± 1.5 h) and did have a shorter interval compared to 10 μg (P < 0.01; 52 ± 1.5 h). There were no differences between treatments in interval to estrus among cows (P > 0.34). When animals that did not exhibit estrus by 120 h 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, 5 μg of GnRH at CIDR removal tended to decrease the interval to estrus and increased expression of estrus among heifers but not cows, and 10 μg of GnRH at CIDR removal did not improve estrus expression and lengthened the interval to estrus in comparison to the control.

Introduction

Estradiol plays a critical role in fertility among cattle, specifically it has been reported that preovulatory estradiol regulates uterine pH [1], sperm transport [2], and endometrial genes that support processes which set up the uterine environment for implantation [3], all of these favorably impact embryo development and survival. Estradiol is also critical within the ovary as it is associated with activation, growth, divergence, ovulation, and eventual regression/atresia of follicles [4]. Classically, estradiol from the follicle is believed to be produced via the two-cell two-gonadotropin theory [5]. A 5 μg dose of GnRH stimulates an LH pulse similar to a physiological pulse [6], and multiple small doses (5 μg) of GnRH administered systemically following CIDR removal increased circulating concentrations of estradiol [7]; the primary signal to the hypothalamus to initiate estrus behavior [8,9].

Cows and heifers that exhibit estrus prior to fixed-time AI had increased concentrations of estradiol [1] and increased conception rates compared to animals that did not exhibit estrus [10]. Variations in expression of estrus, interval to estrus, and/or pregnancy success have been hindrances to the successful use of protocols to synchronize estrus [11,12]. As a result, investigation in ways to manipulate these protocols to favorably impact expression of estrus, interval to estrus, and/or conception rates presents an imperative area of research. The hypothesis was that animals that received a single small dose of GnRH (5 μg or 10 μg) at CIDR removal would have increased expression of estrus contributing to increased pregnancy success. Therefore, the objective of this study was to determine if a single small dose of GnRH (5 μg or 10 μg) at CIDR removal would impact expression of estrus, interval to estrus, or pregnancy success.

Section snippets

Materials and methods

All procedures were approved by the South Dakota State University Institutional Animal Care and Use Committee.

Interval to estrus (INTERVAL 1)

There was an effect of age (P < 0.0001) on interval to estrus, heifers had a shorter interval to estrus compared to cows (50 h ± 1.1 h vs 54 ± 1 h, respectively). Furthermore, a weak tendency for an effect of treatment (P = 0.11), and a treatment by age interaction (P = 0.05; Fig. 1), on interval to estrus were detected. Heifers treated with 5 μg of GnRH at CIDR removal tended to have a shorter interval to estrus (P = 0.07; 47 ± 1.4 h) compared to 0 μg (50 ± 1.5 h) and did have a shorter

Discussion

Animals that exhibited estrus prior to fixed-time AI had increased conception rates compared to animals that did not exhibit estrus [10]. Variations in expression of estrus, interval to estrus, and/or pregnancy success have been a hindrance to the successful use of protocols to synchronize estrus/ovulation [11,12]. In the current study, heifers had a shorter interval to estrus compared to cows. When utilizing the 7-day CO-Synch + CIDR protocol, at time of CIDR removal and prostaglandin F

Competing interest statement

The authors declare no conflict of interest. This project was supported by South Dakota State University Agricultural Experiment Station HATCH funds.

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