Influence of yeast culture supplementation and advancing season on steers grazing mixed-grass prairie in the northern Great Plains: I. Dietary composition, intake, and in situ nutrient disappearance.

Twelve beef steers (368 +/- 25.3 kg) and four beef heifers (559 +/- 79.5 kg) fitted with ruminal cannulas were used to evaluate effects of yeast culture (YC) and advancing season on dietary chemical composition, intake, and in situ CP and NDF degradation. Treatments were control and YC supplementation (28.4 g.steer-1.d-1 dosed ruminally). Steers grazed from late June to early November 1991 on mixed-grass prairie. Experimental periods consisted of 10 d for adaptation and a 13-d collection phase. Forage samples collected from YC-supplemented steers had greater (P < .10) soluble N and in vitro OM disappearance than forage from controls throughout the grazing season. Organic matter intake was greater (P < .10) by YC-supplemented steers during June. Fecal output and OM intake increased (P < .10) from June to September. Increases (P < .10) in in situ NDF disappearance were noted at 8, 16, and 24 h in June, at 8, 48, and 72 h in July, and at 8 h in August for the YC treatment group. Supplemental YC did not affect (P > .10) rate or lag time of NDF disappearance. In situ CP disappearance (ISCPD) was greater (P < .10) at all times except 48 h in June, at 48 and 72 h in July, and at 0, 4, 8, and 12 h in August for YC-supplemented steers. Steers supplemented with YC had an increased extent (P < .10) of ISCPD in July and an increase in CP degradation rate in June; however, total CP degradation was greater (P < .10) during June for control than for YC steers.(ABSTRACT TRUNCATED AT 250 WORDS)