Short synchronization system for estrus cycles in dairy heifers: a preliminary report

In previous studies we demonstrated that the administration of a luteolytic dose of cloprostenol to dairy cows in luteal phase, followed by hCG plus estradiol benzoate (EB) 12 h later, led to successful timed AI 48 h after the initiation of treatment. This article reports two consecutive studies. In Study 1 we determined the pregnancy rate of dairy heifers in luteal phase (established by palpation per rectum) treated with cloprostenol followed by 250 IU of hCG plus 1 mg of EB 12 h later, and inseminated 48 h after cloprostenol injection. Study 2 was designed to evaluate the efficiency this synchronization protocol, irrespective of the estrus stage of the animals. In Study 1, 1272 Friesian heifers aged 14 to 16 months with a palpable corpus luteum received 500 mcg cloprostenol. Heifers were then synchronized either according to the hCG plus EB protocol (hCG-EB, n=637), or by a second dose of cloprostenol 11 d later (PG, n=636). Animals in this last group served as controls and were inseminated 72 and 96 h after the second cloprostenol injection. The pregnancy rate was significantly higher (P<0.0001) in the hCG-EB group (59.5%, 379/637) than in PG (44.8%, 285/636). In Study 2, 135 contemporary heifers (with no corresponding information on estrus stage) were subjected to the same protocol as those in the hCG-EB group of Study 1. These animals were classified in retrospect according to estrus stage established by plasma progesterone concentration. Pregnancy rates were 66.7% (24/36), 51% (25/49) and 58% (29/50) for animals in the follicular, early/late luteal, and mid-luteal phase, respectively. The total pregnancy rate was 57.8% (78/135). These findings indicate an improved pregnancy rate for heifers subjected to single insemination after cloprostenol/hCG/EB synchronization, compared to double insemination after synchronization by 2 cloprostenol injections 11 d apart. The cloprostenol/hCG/EB protocol did result in acceptable pregnancy rates after timed AI of dairy heifers regardless of their estrus cycle phase.