PLASMA HORMONE LEVELS IN BEEF COWS WITH INDUCED MULTIPLE OVULATIONS

Abstract

Summary. Doses of 0, 1600 or 3200 i.u. PMSG were administered on Day 16 of an oestrous cycle to twenty-four beef heifers. Jugular blood was collected at 08.30 and 17.30 hours each day until oestrus and at 0, 3, 6, 9 and 12 hr after the beginning of oestrus. The plasma was analysed for total oestrogen, progesterone and LH concentrations. The heifers were killed 3 to 8 days after mating, and their ova and ovaries were examined to determine the incidence of fertilization, numbers of CL, and numbers of follicles greater than 10 mm diameter. The low dose of PMSG caused less variation in number of ovulations and fewer large follicles than the high dose, though the mean number of fertilized ova was similar. The hormonal patterns were related to the dose of PMSG. Plasma progesterone levels remained elevated for a longer period and decreased more rapidly in the animals given 3200 i.u. than in the others. Total oestrogen began to rise about 48 hr before oestrus in all groups and reached highest levels and remained elevated for longest periods in the animals given 3200 i.u. PMSG. The LH peaks occurred at 0 hr, 3 hr, or 6 hr after the beginning of oestrus in cows treated with 3200 i.u., 1600 i.u. or nil PMSG, respectively.