FSH in the Ewe: Effects of Season, Live Weight and Plane of Nutrition on Plasma FSH and Ovulation Rate

Abstract

The concentrations of FSH (ng NIH-FSH-S6/ml) were measured by specific radioimmunoassay in jugular vein plasmas of 3 groups of ewes fed complete rations of alfalfa hay providing either 1/3 of a maintenance (M) requirement (1/3M, n=16), M (n=12) or 2M (n=9). The ewes were bled on different, known days of their estrous cycle and records of live weight (LW) and ovulation rate (OR) at the time of blood sampling were recorded. After 7 weeks the mean LW of the 3 groups (1/3M, 35 kg; M, 42 kg; 2M, 49 kg) reflected the differential nutrition. The OR (1/3M, 0.94; M, 1.17; 2M, 1.89) was highest (P<0.05) in the 2M group, but there were no significant differences in FSH concentration (1/3M, 79 ng/ml; M, 67 ng/ml; 2M, 67 ng/ml). The major sources of variation in FSH level were attributed to animals (75 percent), day (16 percent) and hour (9 percent) of blood sampling. There was no correlation of either LW or OR with FSH concentration. In a second experiment ewes were fed on similar planes of nutrition (1/3M, n=3; M, n=10; 2M, n=3) for 6 weeks. On Days 10–12 of the estrous cycle the metabolic clearance rate (MCR) (0.605 ml/min/kg) in 7 ewes with one ovulation was not significantly different (P>0.05) than the MCR of FSH (0.621 ml/min/kg) in 8 ewes with 2 ovulations. The FSH response to GnRH (2.5 μg/h for 4.5 h), measured 2 days after the MCR studies, appeared to be inversely related to number of corpora lutea (0.05<P<0.10). The peripheral concentration of FSH (72.7 ng/ml) in 37 ewes during the breeding season was higher (P<0.05) than in 13 lactating(58.4 ng/ml) or 45 nonlactating (59.4 ng/ml) anestrous ewes.