Influence of alkaloid concentration of tall fescue straw on the nutrition, physiology, and subsequent performance of beef steers.

Two experiments were conducted to evaluate digestion, performance, and physiological responses to Acremonium coenophialum-infected tall fescue straw offered to vary ergovaline concentrations. In Exp. 1, 16 Hereford x Angus ruminally cannulated steers (370 +/- 12 kg BW, mean +/- SE) were blocked by age and BW and, within block, randomly assigned to one of four treatments containing graded concentrations of ergovaline in the diet: 1) 0 ppb, 2) 158 ppb, 3) 317 ppb, and 4) 475 ppb. Alkaloid concentrations were produced using various mixtures of two varieties of tall fescue straw that had similar genetic and phenotypic characteristics but differed in degree of endophyte incidence and associated concentrations of alkaloids. In the 36-d digestion study, feed intake, apparent DMD, and total tract NDF digestion were not influenced by alkaloid concentration (P > .10). Differences were observed with digesta kinetics; specifically indigestible ADF (IADF) fill and outflow decreased linearly with increasing alkaloid concentration (P < .10). These differences, however, may be attributed to differences in IADF concentration of the diets. Physiological variables (heart rates, respiration rates, and rectal, ear, and tailhead skin surface temperatures) were not influenced by alkaloid concentration (P > .10). Concentration of serum prolactin measured weekly tended (P < .10) to decrease with increasing alkaloid concentration of the diet. At the conclusion of the study, each steer was injected with 100 micrograms of thyrotropin-releasing hormone (TRH). All steers responded to the TRH challenge (P < .10); however, responses did not differ (P > .10) between alkaloid concentrations of the basal diet (P > .10).(ABSTRACT TRUNCATED AT 250 WORDS)