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.

Abstract

The objectives of this study were to investigate whether estrous activity and its impact on pregnancy outcomes would differ in heifers subjected to a 5d 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 5d CO-Synch plus PRID protocol either with (GnRH; n=154) or without (NGnRH; n=154) an initial administration of 100μg of GnRH at the time of PRID insertion (Day 0). Heifers received a single administration of 500μg of cloprostenol (PGF) at the time of PRID removal (Day 5) and again 24h later (Day 6). Approximately 72h after PRID removal (Day 8), heifers were timed-inseminated (TAI) and concurrently 100μg 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 45d 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. 59.2h, respectively; P<0.01). Pregnancy per AI (P/AI) at 30d post-TAI tended to be greater for GnRH than NGnRH heifers (68 vs. 59%, respectively; P=0.1). However, P/AI at 45d post-TAI (65 vs. 57%, respectively) and pregnancy loss between 30 and 45d 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 30d post-TAI was linearly negative for GnRH heifers; for every 1h increase in the interval from PRID removal to the onset of estrus, the predicted probability of P/AI at 30d 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 30d post-TAI was not significant for NGnRH heifers. In addition, the interval from TAI to subsequent estrus, in non-pregnant heifers, was approximately 3d longer for the GnRH than the NGnRH group (20.7 vs. 17.5d, respectively). In summary, the initial GnRH treatment in a 5d 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 30d, but did not affect P/AI at 45d post-TAI.