EVALUATION OF TIMED INSEMINATION DURING SUMMER HEAT-STRESS IN LACTATING DAIRY-CATTLE

We wished to compare the effect of summer heat stress on pregnancy rat e in cows that were inseminated at a set interval associated with a sy nchronized ovulation vs whose inseminated upon routine estrus detectio n. The study was carried out on a commercial dairy farm in Florida fro m May to September 1995. Lactating dairy cows were given PGF(2 alpha) (25 mg im) at 30 + 3 d postpartum and randomly assigned to be insemina ted at a set time (Timed group) or when estrus was detected (Control g roup). Cows in the Timed group were synchronized by sequential adminis tration of Buserelin (8 mu g im) on Day 0 at 1600 h, PGF(2 alpha) (25 mg im) on Day 7 at 1600 h and Buserelin (8 mu g im) on Day 9 at 1600 h . They were inseminated on Day 10 between 0800 and 0900 h (Day 9 + 16 h). Cows in the Control group were given PGF(2 alpha) at 57 + 3 d post partum and inseminated when detected in estrus. Estrus detection or in semination rate for control insemination cows was 18.1 +/- 2.5% versus 100% for time inseminated cows (P < 0.01). Mean interval from PGF(2 a lpha) to insemination was shorter for time inseminated cows (3 +/- 2.1 d < 35.5 +/- 1.9 d; P < 0.01). Pregnancy rate was greater for time in seminated cows (13.9 +/- 2.6 > 4.8 +/- 2.5%; P < 0.01) as was overall pregnancy rate by 120 d postpartum (27.0 +/- 3.6 > 16.5 +/- 3.5%; P < 0.05). Number of days open for cows conceiving by 120 d postpartum was less for time inseminated cows (77.6 +/- 3.8 < 90.0 +/- 4.2 d; P < 0. 05), as was interval to first service (58.7 +/- 2.1 < 91.0 +/- 1.9 d; P < 0.01). Services per conception were greater for time inseminated c ows (1.63 +/- 0.10 > 1.27 +/- 0.11, P < 0.05). The timed insemination program did improve group reproductive performance. However, the timed insemination program will not protect the embryo from temperature-ind uced embryonic mortality, but management limitations induced by heat s tress on estrus detection are eliminated. An economical evaluation of the timed insemination program indicates an increase in net revenue pe r cow with implementation tof timed insemination for first service dur ing the summer months. (C) 1998 by Elsevier Science Inc.