Abstract
This study aimed to evaluate the effect of GnRH administered 12 days after FTAI on pregnancy rate and progesterone levels in crossbred beef cows. Fifty-nine non-pregnant crossbred cows (Bos taurus indicus x Bos taurus taurus) previously selected by gynecologic examination with a body condition score (BCS) of 3.0 (scale of 1 - 5). Cows were randomly assigned to two treatments: TEB (n = 30), on day 0, an intravaginal progesterone releasing device (Primer®) was inserted and cows received i.m. 2.0 mg of estardiol benzoate (EB, Estrogin®); on day 8, the PRIMER was removed and cows received i.m. 300 IU of eCG (Novormon®) and 0.15 mg of PGF2α (Prolise®); on day 9, cows received i.m. 1 mg of EB. FTAI was carried out 48-56 hours after PRIMER removal; and TEBGnRH12 (n = 29): similar protocol to TEB, but with administration of 25 µg GnRH (Lecirelin, Gestran Plus®) on the 12th day after AI. On the 35th day after AI, diagnosis of pregnancy was performed by transrectal ultrasound. The pregnancy rate was analyzed by logistic regression. The results showed that 53.33% of the cows (16/30) in TEB and 37.93% (11/29) in TEBGnRH12 became pregnant after the first service. No significant difference was found among the pregnancy rate of animals in the treatments (P> 0.05). There was no difference between the P4 concentrations (TEB = 3.88 ng/mL and TEBGnRH12 = 3.12 ng/mL) among the heifers in the treatments (P> 0.05). The administration of GnRH analogs 12 days after TAI did not influence pregnancy rate or the progesterone concentration.
Highlights
Baruselli et al (2004), Kasimanickam et al (2006) and Smith et al (2008) showed that hormone protocols that eliminate the need for estrus detection, favoring fixed-time artificial insemination (FTAI), are more attractive, they present pregnancy rates ranging between 25 and 67%
Nascimento (2005) compared the pregnancy rate after synchronization of ovulation and estrus among three hormonal protocols and obtained satisfactory results, similar to those achieved by AI with estrus detection, with fixed-time artificial insemination using a combination of the hormones estradiol benzoate, progesterone, prostaglandin and eCG
It has been suggested that the administration of GnRH analogs or after AI (11 to 14 after estrus), period coinciding with the largest diameter of the dominant follicle in the first follicular wave, the initiation of pregnancy recognition and the onset of the luteolytic mechanism, can act in the ovary, enhancing the formation of an accessory corpus luteum and thereby increasing the concentration of P4 serum, which contributes to embryo survival (Peters et al, 2000, Borges et al, 2001)
Summary
Synchronization and induction of estrus are important tools in beef cow production systems, as they allow most of the herd to return to cyclicity and/or to begin a new gestation in a shorter period of time (Lucy et al 2001 Borges, 2008). Baruselli et al (2004), Kasimanickam et al (2006) and Smith et al (2008) showed that hormone protocols that eliminate the need for estrus detection, favoring fixed-time artificial insemination (FTAI), are more attractive, they present pregnancy rates ranging between 25 and 67%. Baruselli et al (2004), Kasimanickam et al (2006) and Smith et al (2008) showed that hormone protocols that eliminate the need for estrus detection, favoring fixed-time artificial insemination (FTAI), are more attractive, they present pregnancy rates ranging between 25 and 67%. This variation occurs because of the percentage of cows that initiate estrous cycles in the postpartum period, the environment influence and nutritional condition of the animals and the hormone combination that was used. It has been suggested that the administration of GnRH analogs or after AI (11 to 14 after estrus), period coinciding with the largest diameter of the dominant follicle in the first follicular wave, the initiation of pregnancy recognition and the onset of the luteolytic mechanism, can act in the ovary, enhancing the formation of an accessory corpus luteum and thereby increasing the concentration of P4 serum, which contributes to embryo survival (Peters et al, 2000, Borges et al, 2001).
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