Abstract
Sixty-six cycling Holstein heifers were given two injections of prostaglandin F2α (25mg intramuscular) at an 11-day interval. Average time of first observation of estrus differed little between July (55.7±5.3h) and December, 1980 (50.6±5.1h). Heifers were inseminated artifically at either 65 or 80h following the second injection of prostaglandin. Synchronized heifers inseminated at 65h had a higher conception rate in July (9/18; 50%) than in December (3/15; 20%). However, heifers inseminated at 80h had a similar conception rate in July (9/18; 50%) and in December (8/15; 53%).Progesterone in blood serum was measured prior to the second prostaglandin F2α injection before insemination and on day 21 postbreeding. Averages of progesterone for the 65-h group were 3.91±.48, .39±.07, and 3.80±.76ng/ml in July and 4.25±.44, .33±.08, and 3.91±.63ng/ml in December for the three collections. Averages for the 80-h group were 3.62±.28, 1.06±.56, and 3.60±.65ng/ml in July and 2.17±.35, .22±.07, and 2.77±.43ng/ml in December. An influence of season on response to prostaglandin and timed inseminations in dairy heifers may affect estrus behavior and conception rate.
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