Control of oestrus and ovulation in beef cattle in central Australia

Abstract

Hormonal treatments were used to control the time of oestrus and ovulation in cattle in central Australia and to observe the efficacy of several luteolytic regimes under these conditions. All animals received intravaginal progesterone (PRID) for 7-14 days with 10 mg oestradiol benzoate (ODB) in a gelatin capsule attached to each PRID and 750 i.u. pregnant mare serum gonadotrophin (PMSG) by intramuscular injection 24 h before PRID removal. In addition half the animals received 25 mg dinoprost (PGF2ß) at either 0 or 24 h before PRID removal. Differences in luteolytic treatments had no significant effects on calving rates to a single fixed-time insemination of acyclic animals (52%) or cyclic animals treated with PRID for 14 days (48%). In contrast the calving rate of cyclic animals treated with PRID for 7 days was depressed in all groups except that which received PGF2, 24 h before PRID removal (18 v. 56%, P<0.025). Independent of treatment effects lactating cows had a higher calving rate than non-lacting animals (50 v. 35%, 0.05 <P<0.10). These results indicate the importance of using effective luteolytic treatments in cyclic animals where short PRID treatments are used. The most appropriate regimes for use in extensive environments would appear to be based on a 14-day PRID treatment which eliminates the need for an injection of PGF2ß before PRID removal while still producing acceptable fertility.