Short-term treatment with deslorelin implants to improve the bucks’ ability to stimulate cyclic activity during the late non-breeding season

Highlights

• Treatment with deslorelin during 5 d induced an increase in testosterone in bucks.

• Testosterone concentration increased in all bucks joined with anestrous does.

• Response of does to male effect with treated or untreated bucks was similar.

Abstract

The sudden introduction of males to anestrous females stimulates synchronous ovulations. The effectiveness of males to induce this response is androgen dependent. The aim of the present study was to compare the reproductive response of anestrous does to the introduction of teaser bucks treated with or without deslorelin implants, which continuously release a GnRH agonist. While 5 bucks were acutely treated with a single subcutaneous implant containing 7.4 mg of deslorelin (Suprelorin, Virbac) (group Des), other 5 bucks remained as controls (group Con). Each buck was placed with one group of 8 adult females. Deslorelin implants were inserted on Day -6 and from Day 0 to Day 9, bucks were joined with females from 07:00 to 08:00 and from 19:00 to 20:00 every day. Testosterone concentration was measured from blood samples collected twice a day (12 h apart, am/pm) on Days -11, -10, -9 and from Day -6 to Day -1, and collected immediately after buck stimulation from Day 0 to Day 9. Does that came into estrus and that showed short estrous cycles was registered from Day 0 to Day 9. Transrectal ultrasound examination for recording cyclicity on basis of presence of corpus luteum was done between Days 22 and 27, whereas ultrasonographic pregnancy diagnosis was carried out on Day 49. Testosterone concentration was greater in Des than Con bucks before joining them with females (36.0 ± 7.6 nmol/L vs 15.0 ± 6.8 nmol/L; P = 0.04), but the difference disappeared when all the experiment was analyzed together. Non significant differences were observed in reproductive response of does stimulated by treated or control males. Therefore, we concluded that both treated and control bucks were fully capable of inducing reproductive activity in anestrous does.

Introduction

Reproductive seasonality is a major limitation for goat production in temperate zones, as it limits the kidding period, and thus, the period in which milk and/or meat are offered (Chemineau et al., 2008). The male effect has been used to induce out-of-season ovulations, and thus, parturitions (Chemineau, 1987; Chemineau et al., 2006; Delgadillo et al., 2009). The sudden introduction of males to anestrous females stimulates a rapid increase in LH pulse frequency that ends in an LH peak, followed by ovulations (Delgadillo et al., 2009). This technique is widely used among goat farmers in extensive systems considering that it requires minimal human labor and economic costs (Rivas-Muñoz et al., 2007).

The percentage of females that respond to the male effect is related to male’ sexual behavior (Véliz et al., 2002). In this sense, males that display high levels of sexual behavior induce ovulation in a greater proportion of females than males with low levels of sexual behavior (Martínez-Alfaro et al., 2014). For example, increase of bucks nutritional plane (Walkden-Brown et al., 1993) or application of photostimulation (Delgadillo et al., 2002) enhances bucks’ sexual activity, increasing bucks’ ability to induce out-of-season ovulations. In the same direction, at least in certain conditions females would not respond if bucks are not prestimulated with light regimes (Flores et al., 2000; Delgadillo et al., 2002). However, light treatments cannot be easily applied in extensive grazing systems, in which animals remain continuously grazing outside, in big paddocks, with low human intervention.

As both androgenized buck wethers and does are as effective to induce estrus as intact bucks (Mellado and Hernández, 1996), it was proposed that the effectiveness of males as teasers is androgen dependent. Therefore, it is important to develop practical strategies to increase bucks’ testosterone concentration before joining bucks and does to induce ovulations and pregnancies in more anestrous females. In rams, a practical alternative is to administer two doses of eCG, inducing sustained increase of testosterone, and therefore, induce estrus and pregnancies in a greater proportion of females (Ungerfeld et al., 2014). Ungerfeld et al. (2014) hypothesized that testosterone concentrations should remain elevated for several days before joining rams with ewes to increase the percentage of females responding . However, as the production of eCG is under debate due to ethical concerns, is important to develop other alternative treatments. The application of implants containing GnRH agonists is used as a pharmacological contraception technique, as the continuous release of the analogue desensitizes the pituitary GnRH receptors (Lincoln et al., 1986). However, during the immediate period after inserting these implants there is a short-term flare up period, during which GnRH agonists stimulates males’ reproductive activity and thus, testosterone concentration increases (Junaidi et al., 2007). Giriboni et al. (2020) observed elevated testosterone concentrations for several weeks after insertion of deslorelin implants in bucks. Therefore, the insertion of deslorelin implants during a short period before joining bucks with anestrous does may be an alternative to maximize the response of anestrous does. Considering all this information, the aim of this study was to compare the reproductive response of anestrous does to the introduction of teaser bucks treated with or without deslorelin implants.