Plasma progesterone in cows with multiple ovulations.

Abstract

2 experiments were carried out in order to relate the number of corp ora lutea (CL) to plasma progesterone concentrations in maiden cows of beef breeds. In Experiment 1 plasma progesterone concentrations were determined in cows with multiple ovulations following treatment with melengestrol acetate (MGA) .5 mg/cow/day for 18 days followed by 1500-3000 IU pregnant mare serum gonadotropin (PMSG) on Days 15 20 25 and 30 after mating. On Day 15 120 ng progesterone/ml was recorded in 1 cow with 3 CL and 3.2 ng/ml in 1 cow with 9 CL. Evidently the lat ter were regressing. The cow which had 120 ng progesterone/ml on Day 15 had only 17 ng/ml on Day 20 and less than 5 ng/ml on Day 20. 7 out of 13 cows remained pregnant throughout the period of study. Those with 3-8 CL had considerably higher levels of progesterone than cows with 1 CL though there was no clear relation between number of CL and progesterone levels. Progesterone concentrations fluctuated considerably from 1 day to another. In Experiment 2 the individual values for plasma progesterone on Day 13 after estrus were related to the number of CL. Cows returning to estrus 11-22 days after mating had less than 1 ng progesterone/ml plasma on Day 16. A significant (P less than .01) regression of plasma progesterone on number of CL was observed for the data from cows which did not return to estrus by 22 days after mating. Each increment of 1 CL was associated with an additional 8 ng (+ or - 2.14 standard error) progesterone/ml plasma. There was a wide scatter of values. The results of progesterone assays for Day 16 after mating (not including cows which returned to estrus) were similar to those for Day 13 (regression= 9.2 + or -2.5). It was determined that cows with multiple CL had plasma concentrations considerably in excess of normal levels. Results for induced single and double ovulations in cows were compared with natural single and double ovulations in ewes in order to compare the mean plasma progesterone concentrations. In the ewes the mean progesterone (ng/ml) was 1.71 in those with 1 CL and 1.64 for those with 2 CL. In the cows the mean progesterone (ng/ml) was 6.8 in those with 1 CL and 20.1 for those with 2 CL.