Estrous activity and pregnancy outcomes in Holstein heifers subjected to a progesterone based 5-d CO-Synch protocol with or without administration of initial GnRH.

The objectives of this study were to investigate whether estrous activity and its impact on pregnancy outcomes would differ in heifers subjected to a 5 d CO-Synch plus progesterone releasing intravaginal device (PRID) protocol with or without an initial GnRH treatment. Holstein Heifers (n = 308) were fitted with a collar-mounted automated activity monitoring system approximately 1 week prior to the initiation of the synchronization protocol (Day -7). Heifers were assigned randomly to a 5 d CO-Synch plus PRID protocol either with (GnRH; n = 154) or without (NGnRH; n = 154) an initial administration of 100 μg of GnRH at the time of PRID insertion (Day 0). Heifers received a single administration of 500 μg of cloprostenol (PGF) at the time of PRID removal (Day 5) and again 24 h later (Day 6). Approximately 72 h after PRID removal (Day 8), heifers were timed-inseminated (TAI) and concurrently 100 μg of GnRH was administered to those not exhibiting estrus. All inseminations were done by one of two technicians using either sex-sorted (n = 252) or conventional (n = 56) frozen-thawed semen. Transrectal ultrasonography was done on Day 0 to determine ovarian cyclicity and normalcy of the reproductive tract and 30 and 45 d post-TAI to determine and confirm pregnancy, respectively. The percentage of heifers determined to be in estrus following PRID removal was greater in the GnRH than in the NGnRH group (94 vs. 82%, respectively; P < 0.01). The mean interval from PRID removal to the onset of estrus was shorter for GnRH- than NGnRH-treated heifers (50.8 vs. 59.2 h, respectively; P < 0.01). Pregnancy per AI (P/AI) at 30 d post-TAI tended to be greater for GnRH than NGnRH heifers (68 vs. 59%, respectively; P = 0.1). However, P/AI at 45 d post-TAI (65 vs. 57%, respectively) and pregnancy loss between 30 and 45 d post-TAI (6 vs. 4.5%, respectively) did not differ. The association between the interval from PRID removal to the onset of estrus and P/AI at 30 d post-TAI was linearly negative for GnRH heifers; for every 1 h increase in the interval from PRID removal to the onset of estrus, the predicted probability of P/AI at 30 d post-TAI tended (P = 0.08) to be reduced by 2.7%. The association between the interval from PRID removal to the onset of estrus and P/AI at 30 d post-TAI was not significant for NGnRH heifers. In addition, the interval from TAI to subsequent estrus, in non-pregnant heifers, was approximately 3 d longer for the GnRH than the NGnRH group (20.7 vs. 17.5 d, respectively). In summary, the initial GnRH treatment in a 5 d CO-Synch plus PRID protocol increased estrus expression and reduced the interval from PRID removal to the onset of estrus in Holstein heifers and tended to increase P/AI at 30 d, but did not affect P/AI at 45 d post-TAI.

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Declaration of competing interest The authors declare no conflicts of interest.