FERTILITY OF ESTRUS SYNCHRONIZED DAIRY HEIFERS TREATED WITH CAP ALONE OR IN COMBINATION WITH ESTRADIOL BENZOATE. HCG OR GnRH

Abstract

The possibility that improvement in conception rates in chlormadinone-acetate (CAP)-treated heifers might be obtained by application of estradiol benzoate (EB) human chorionic gonadotropin (HCG) or GnRH shortly before the 1st insemination was investigated. Over a 3-year period a total of 206 German red pied heifers were used. 1/2 were used to determine the effect of the additional hormones on the conception rate. CAP was given orally in doses of 10 mg/day for 18 days. In Experiment 1 42 heifers were given CAP only and 39 were additionally given EB 5 mg im 24 hours before insemination. In Experiment 2 38 heifers were given CAP alone and 35 were also given 1000 IU HCG iv 6 hours before the 1st insemination. In Experiment 3 22 heifers were given only CAP and 30 were also given 1.5 mg of GnRH iv 5 hours before the 1st insemination. All of the inseminations were done with frozen semen after withdrawal of CAP therapy. In some animals 2nd inseminations were given. Pregnancies were confirmed by rectal exploration at 6-7 weeks after the last insemination. In Experiment 1 with EB heifers showed improved estrous symptoms but the pregnancy rate was the same as controls. In Experiment 2 with additional HCG treatment the conception rate was 20% lower than with only CAP treatmen t (p less than .05). In Experiment 3 with additional GnRH therapy the p regnancy rate was 26.9% higher than with only CAP therapy (p less than .05). In all 3 experiments 80% of all animals failing to conceive showed a regular cycle in the 1st posttreatment estrus. Only those which in addition had received EB showed subsequent irregular prolonged estrous cycles (p less than .05). Pregnancy rates in the experimental heifers were lower than usually obtained in nonsynchronized ones. Other investigators have reported better results. Postestrial bleeding did not influence pregnancy rates. It is concluded that after the CAP therapy estrus and time of ovulation had occurred over more than 3 days. Therefore no optimal service time could be determined. In heifers treated with only CAP the conception rate ranged from 33.3 to 36.4% when these heifers were artifically inseminated without regard to estrus on Days 3 and 4 following the cessation of CAP feeding. The only improvement of the pregnancy rate was obtained by injection of 1.5 mg GnRH 5 hours before insemination in CAP-synchronized heifers. The major causes of low fertility are thought to take place at the time of ovulation and fertilization.