Estrus synchronization in beef heifers with progestin-based protocols. I. Differences in response based on pubertal status at the initiation of treatment.

摘要

Two progestin-based protocols for oestrus synchronization in replacement beef heifers were compared on the basis of oestrous response, interval to and synchrony of oestrus, and pregnancy rate. The objective was to determine, whether addition of GnRH to a melengestrol acetate (MGA)-prostaglandin F2 alpha (PGF2 alpha ) oestrus synchronization protocol would improve synchrony of oestrus without compromising fertility in yearling beef heifers. Heifers at two locations (Location 1, n=60 and Location 2, n=64) were assigned randomly to one of two treatments by breed and pubertal status. Heifers were defined as, pubertal when concentrations of progesterone in serum were elevated (>=1 ng/ml) in either one of two samples obtained 10 and 1 day prior to treatment initiation. Prior to MGA administration, 18/60 (30%) and 36/64 (56%) of the heifers at Locations 1 and 2, respectively, were pubertal. Heifers in both treatments were fed MGA (0.5 mg/head/day in 1.8 kg/head/day supplement) for 14 days followed by 25 mg of PGF2 alpha i.m. (MGA-PGF2 alpha ) 19 days after MGA withdrawal (Day 33 of treatment). One-half of the heifers at each location received 100 micro g of GnRH i.m. 12 days after MGA withdrawal (Day 26 of treatment; MGAReg. Select). The control group received only MGA-PGF2 alpha . Heifers were observed for signs of behavioural oestrus continuously during daylight hours for 7 days beginning on the day PGF2 alpha was administered. Heifers were inseminated 12 h after observed oestrus. There was a treatment by location by pubertal status interaction (P<0.05) for interval to oestrus. Compared to the respective control treatment at each location, prepubertal heifers assigned to the MGAReg. Select protocol at Location 1 had longer intervals to oestrus, whereas at Location 2, prepubertal heifers assigned to the MGA-PGF2 alpha protocol had longer intervals to oestrus. The higher number of pubertal heifers at Location 2 was associated with a reduced variance in the interval to oestrus among MGAReg. Select treated heifers. Total oestrous response and synchronized conception rates were similar between treatments at both locations. These data suggest that addition of GnRH to the MGA-PGF2 alpha protocol may improve synchrony of oestrus, however, the degree of synchrony may be influenced by pubertal status of heifers at the time treatments are imposed. Further studies are needed to define production systems in which the MGAReg. Select protocol is warranted for use in beef heifers.