The requirement of GnRH at the beginning of the five-day CO-Synch + controlled internal drug release protocol in beef heifers1

Abstract

The objective of this study was to determine if the omission of GnRH at controlled internal drug release device (CIDR) insertion would impact pregnancy rates to timed AI (TAI) in beef heifers enrolled in a 5-d CO-Synch + CIDR protocol that used 1 PGF2α dose given at CIDR removal. Yearling beef heifers in Ohio in 2 consecutive breeding seasons (2011, n = 151, and 2012, n = 143; Angus × Simmental), Utah (2012, n = 265; Angus × Hereford), Idaho (2012, n = 127; Charolais), and Wyoming (2012, n = 137; Angus) were enrolled in the 5-d CO-Synch + CIDR protocol. At CIDR insertion (d -5), heifers were randomly assigned either to receive 100 μg GnRH (GnRH+; n = 408) or not to receive GnRH (GnRH-; n = 415). At CIDR removal (d 0 of the experiment), 25 mg PGF2α was administered to all heifers. All heifers were inseminated by TAI and given 100 μg GnRH 72 h after PGF2α (d 3). In heifers at the Ohio locations (n = 294), presence of a corpus luteum (CL) at CIDR insertion (d -5) was determined via assessment of progesterone concentrations (2011) and ovarian ultrasonography (2012). Subsequently, in both years, ovarian ultrasound was conducted on d 0 to determine the presence of a new CL. In this same subgroup of heifers, blood samples for progesterone analysis were collected on d 3 to assess luteal regression. Pregnancy diagnosis was performed between 32 and 38 d after TAI. At CIDR withdrawal, presence of a new CL was greater (P < 0.05) in the GnRH+ (55.8%, 82/147) than GnRH- (26.5%, 39/147) treatment. Incidence of failed luteal regression did not differ between the GnRH+ (3.4%) and GnRH- (0.7%) treatments. Pregnancy rate to TAI did not differ between the GnRH+ (50.5%) and GnRH- (54.9%) treatments. In conclusion, although the incidence of a new CL at CIDR removal was increased in the GnRH+ treatment, omission of the initial GnRH treatment in the 5-d CO-Synch + CIDR protocol did not influence TAI pregnancy rate in yearling beef heifers. In addition, a single dose of PGF2α at CIDR removal was effective at inducing luteolysis in yearling beef heifers enrolled in the 5-d CO-Synch + CIDR protocol, regardless of whether or not the initial GnRH treatment was given.

Comparison of 4- versus 5-day Co-Synch + controlled internal drug release (CIDR) + timed artificial insemination protocols in dairy heifers

“…In contrast, another study in yearling beef heifers showed that omission of the initial GnRH treatment in the modified 5-day Co-Synch þ CIDR protocol did not influence TAI pregnancy rate [19]. In both studies, a single dose of PGF 2a at CIDR removal resulted in adequate luteolysis and pregnancy rates regardless of whether or not the heifers received GnRH at CIDR insertion [18,19].…”

“…Interestingly, a recent study in dairy and beef heifers showed that GnRH administration at the time of CIDR insertion was beneficial in beef heifers but not in dairy heifers to improve pregnancy when using the modified 5-day Co-Synch þ CIDR protocol [18]. In contrast, another study in yearling beef heifers showed that omission of the initial GnRH treatment in the modified 5-day Co-Synch þ CIDR protocol did not influence TAI pregnancy rate [19]. In both studies, a single dose of PGF 2a at CIDR removal resulted in adequate luteolysis and pregnancy rates regardless of whether or not the heifers received GnRH at CIDR insertion [18,19].…”

Effects of initial GnRH removal from 5-d CO-Synch + CIDR protocol on ovarian parameters and fixed timed artificial insemination pregnancy rates in beef cattle

“…However, in beef heifers, the same contrast of treatments has mixed results. Cruppe et al (2014) reported similar pregnancy rates in beef heifers, while Kasimanickam et al (2014) reported that elimination of G1 resulted in decreased pregnancy rates in beef heifers when compared to the standard 5dCO. The inconsistent results, in beef heifers, leave an unanswered question as to the requirement of G1 within the 5dCO.…”

“…One way to increase convenience was to consider decreasing number of trips through the chute by administration of two doses of PGF2α concurrently with CIDR removal. More recent work in heifers indicated that a single dose of PGF2α at CIDR removal was adequate to induce luteolysis in both diary (Rabaglino et al, 2010) and beef heifers (Cruppe et al, 2014;Kasimanickam et al, 2014 More recently discussion has focused on the requirement or need for GnRH on the front end on the 5dCO. The fact remains that any estrous synchronization protocol that uses GnRH on the onset of the protocol, such as the 5dCO, only 66 to 70% of beef cows have been shown to respond to the initial injection of GnRH (Geary et al, 2000).…”

“…The two separate, 25-mg doses of PGF2α are speculated to be required in beef cows to induce luteolysis of the 5-d old accessory corpus luteum (CL) created by the administration of G1 (Bridges et al, 2008;Kasimanickam et al, 2009). However, in heifers, a single, 25-mg dose of PGF2α has proven effective in engaging luteolysis of the 5-d accessory CL; and similar pregnancy rates were achieved upon FTAI when compared to heifers receiving two, separate, 25mg doses of PGF2α (Rabaglino et al, 2010;Cruppe et al, 2014;Kasimanickam et al, 2014). The difference in response rates of the 5-d accessory CL to a single, 25-mg dose of PGF2α between cows and heifers has been thought to be due to an increased mg/kg of body weight dosing in heifers compared to cows (Bridges et al, 2012).…”