INFLUENCE OF YEAST CULTURE SUPPLEMENTATION AND ADVANCING SEASON ON STEERS GRAZING MIXED GRASS PRAIRIE IN THE NORTHERN GREAT-PLAINS .1. 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 y east 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). Steer s grazed from late June to early November 1991 on mixed-grass prairie. Experimental periods consisted of 10 d for adaptation and a 13-d coll ection 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 wa s greater (P < .10) by YC-supplemented steers during June. Fecal outpu t 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 i n June, at 8, 48, and 72 h in July, and at 8 h in August for the YC tr eatment group. Supplemental YC did not affect (P > .10) rate or lag ti me 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, an d at 0, 4, 8, and 12 h in August for YC-supplemented steers. Steers su pplemented with YC had an increased extent (P < .10) of ISCPD in July and an increase in CP degradation rate in June; however, total CP degr adation was greater (P < .10) during June for control than for YC stee rs. The proportion of rapidly degraded forage protein increased (P < . 10) from June to October. In summary, these data indicate that YC incr eased intake during June, in situ NDF, and CP degradation at various t imes measured and consistently increased (P < .10) IVOMD and dietary s oluble N. Increased in situ CP degradation associated with YC suppleme ntation generally corresponded to sampling periods when forage CP was higher.