EFFECT OF SEQUENTIAL TREATMENT WITH PROSTAGLANDIN-F2-ALPHA AND OR OXYTOCIN ON ESTRUS AND PREGNANCY RATE OF LACTATING DAIRY-COWS/

A total of 335 lactating dairy cows was used to determine the effect o f oxytocin or PGF2a given 8 h after treatment with a luteolytic dosage of PGF2a on the percentage of cows exhibiting estrus within 7 d after treatment, and the pregnancy rate to a single insemination at this ti me. On the initial day of treatment (Day 0), cows with a palpable corp us luteum on the ovary were treated with 25 mg, im of PGF2a. At 8 h la ter, the cows were divided into 3 groups. Cows in Group 1 (n = 112) we re treated with oxytocin (0.33 IU/kg bwt im); cows in Group 2 (n = 112 ) were treated with 25 mg, im of PGF2a; and cows in Group 3 (n = 111) served as the untreated controls. Cows in all 3 groups were continuous ly observed for estrus visually or by way of an activated heatmount de tector within 7 d after treatment, and were inseminated within 12 h of the observed estrus. Plasma progesterone (P4) concentration was deter mined using radioimmunoassay on Day 0 and Day 2. Of the cows with P4 g reater than 1 ng/ml on Day 0, the percentage of cows observed in estru s within 7 d after treatment was 75, 89 and 72% for cows in Group 1, G roup 2 and Group 3, respectively. When all cows were evaluated, the pe rcentage of cows observed in estrus within 7 d after treatment was 60, 70 and 55% for cows in Group 1, Group 2 and Group 3, respectively. In both instances, the value for cows in Group 2 was significantly highe r than that for either cows in Group 1 or Group 3. The pregnancy rate for cows inseminated within 7 d was similar for cows in all 3 groups. The results of this study demonstrated that treatment of dairy cows wi th 2 luteolytic dosages of PGF2a at an 8-h interval resulted in more c ows being observed in estrus within 7 d than with 1 treatment with PGF 2a, or with oxytocin given at an 8-h interval after a luteolytic dosag e of PGF2a.