ENDOCRINE PROFILES AND FUNCTIONAL CHARACTERISTICS OF CORPORA LUTEA FOLLOWING ONSET OF POSTPARTUM OVARIAN ACTIVITY IN BEEF COWS

Abstract

Three experiments were conducted to evaluate endocrine profiles and to determine the morphological and functional characteristics of corpora lutea (CL) following the onset of postpartum ovarian activity in beef cows, suckled by a single calf once daily. In exp. 1, blood samples were collected from 12 cows at 6-h intervals beginning 25 days postpartum until ovariectomy which was carried out on each of two cows on days 25, 27, 29, 31, 33, 35 after parturition. Ovarian structures were examined grossly and histologically. In exp. 2, blood samples were collected from eight cows at 6-h intervals for 18 days beginning 25 days postpartum and at less frequent intervals thereafter. Laparotomies were carried out on day 36 after calving, the ovaries were observed, CL were sampled and the residual tissue was marked with charcoal. A second laparotomy was performed on day 50. Luteal tissue samples were processed and examined by light and electron microscopy. Luteal cell types were evaluated, percent of area covered by large cells was determined and average luteal cell diameter was calculated. In exp. 3, seven cows were bled daily from parturition until day 25 postpartum. Serum from all experiments was assayed for progesterone (P4), FSH, LH and a prostaglandin metabolite (PGFM). The data showed that serum PGFM levels declined from a peak at calving to basal levels by 10 days postpartum which was well before the first ovulation. In all instances the observed peaks of serum LH and serum FSH preceded the first rise in P4 which, in eight of nine cases, was due to a functional CL. These CL were functional for periods of time ranging from 3 to 12 days. The regression of short-lived CL appeared abnormal compared to the longer-lived CL in which regression was characterized by a rapid decline in P4 and elevated blood PGFM. These data show clearly that the first increase in P4 is preceded by a typical LH surge, followed by ovulation and CL formation which has a variable life-span. Key words: Beef cows, postpartum, anestrus, corpus luteum.