Abstract
The objective of this study was to evaluate pregnancy rates in lactating Holstein cows treated with an Ovsynch protocol (GnRH–PGF 2α–GnRH) or a progesterone-based timed AI (TAI) protocol, and to determine the factors that may influence pregnancy rate following protocol treatment. In experiment 1, lactating Holstein cows were randomly assigned to three treatments: (1) an injection of GnRH (Day 0), an injection of PGF 2α on Day 7, a second injection of GnRH on Day 9, and TAI 16 h after the second GnRH injection (GPG group, n=34); (2) insertion of a CIDR intravaginal progesterone (1.9 g) device combined with a capsule containing 10 mg estradiol benzoate (Day 0), an injection of PGF 2α and removal of the device on Day 7, an injection of GnRH on Day 9, and TAI 16 h after the GnRH injection (CPG group, n=34); (3) an injection of PGF 2α after confirming the presence of CL by ultrasonographical observation and artificial insemination at estrus (AIE) (P group, n=75). The pregnancy rate after TAI following the CPG protocol (41.2%) was higher ( P<0.05) than that after TAI following the GPG protocol (20.6%) and that after AIE (20.0%). In experiment 2, lactating Holstein cows were randomly assigned to two treatments: a GPG group ( n=31) and a CPG group ( n=31). The GPG and CPG protocols were identical to those used in experiment 1. The proportion of cows with premature estrus prior to injection of PGF 2α and with incomplete luteal regression tended ( P=0.056) to be or were greater ( P<0.05) in the GPG group (4/31, 8/31) than in the CPG group (0/31, 2/31), respectively. Average diameters of dominant follicles (1.5±0.1 mm versus 1.4±0.1 mm) on Day 7 and preovulatory follicles (1.8±0.1 mm versus 1.6±0.1 mm) on Day 9, and the proportion of cows with synchronized ovulation by 40 h after the second GnRH injection were not different (81.5% versus 87.1%, P>0.05) between groups, respectively. We conclude that the pregnancy rate after TAI following the CPG protocol was higher than that after TAI following the GPG protocol, probably due to a decreased incidence of premature estrus and incomplete luteal regression.
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