Selected hormonal changes with summer fescue toxicosis.

The effects of fescue endophyte content (low, 16 or high, 44% of tillers examined) and of N fertilization rate (low, 134 kg N.ha-1.yr-1 or high, 336 kg N.ha-1.yr-1) upon serum prolactin (PRL) in Angus steers were examined. Jugular blood samples for serum PRL determination were taken before (basal) and after thyrotropin releasing hormone (TRH) administration (stimulated). Areas under both the basal and stimulated PRL curves were calculated. While areas under the PRL curves varied with length of photoperiod, high endophyte content resulted in a consistent PRL suppression during 1984. During four time periods in 1984 (May to October), areas under the PRL curves [basal and(or) TRH stimulated] were suppressed (P less than .05) with high endophyte on three dates. Although basal areas under the PRL curves in 1983 were nonsignificantly suppressed with high endophyte, there was a suppression (P less than .05) post-TRH in October with high endophyte. There was no effect of N on PRL areas in either year. No relationship was found to exist between basal PRL areas and average daily gains as computed to encompass a period 2 wk before and after a blood collection date. Mean basal growth hormone (GH) concentration as determined from one bleeding date were elevated (P less than .05) in steers on high compared with low endophyte (7.9 and 6.2 ng/ml +/- 1.1 overall SE, respectively). There was no effect of treatment on TRH-stimulated serum GH values. Mean basal serum insulin values ranged from 13.2 to 17.5 microU/ml (+/- 1.2 overall SE) and were not affected by treatment.(ABSTRACT TRUNCATED AT 250 WORDS)