Nutrient intake and digestion by cattle grazing midgrass prairie rangeland and Plains bluestem pastur

Scope and method of study. Two consecutive trials (1990 and 1991) were conducted to evaluate midgrass prairie rangeland and Plains bluestem (Bothriochloa ischaemum var. Plains) pasture as complementary forages. Cattle cannulated at the esophagus, rumen and duodenum were used to evaluate diet quality, nutrient intake, nutrient digestion, and microbial protein production. Six ruminally- and duodenally- and four esophageally-cannulated cattle grazed each study site during both years. Samples were collected in mid-May, late-June, mid-August, and mid-October. Because of winter kill of Plains bluestem this site was not grazed during May of 1990.;Findings and conclusions. Nitrogen in esophageal masticate collected from cattle grazing prairie during both years was low in June and August because of a mid-summer dormancy. Nitrogen in esophageal masticate collected from cattle grazing Plains bluestem increased in August 1990 and decreased in August 1991 due to differences in fertilizer application and forb production on the site. In vitro organic matter disappearance from prairie and Plains bluestem masticate followed a pattern similar to nitrogen. The in vitro organic matter disappearance from Plains bluestem masticate was consistently greater than from prairie masticate. In situ digestion of masticate suggested that the prairie diets were nitrogen deficient in June, August, and October. Plains bluestem diets were nitrogen deficient only during 1991, when nitrogen concentration in the diet followed a pattern similar to prairie. Organic matter intake was similar between forage types except during October when intake was higher for cattle grazing prairie. Because organic matter intake was usually similar, cattle grazing Plains bluestem almost always consumed more digestible organic matter and nitrogen. During all months, the non-ammonia nitrogen supply at the duodenum was disproportionately high in relation to digestible organic matter intake. This finding suggests that energy intake may limit performance in cattle grazing either forage. Neutral and acid detergent fiber digestion varied little over the entire grazing season and appeared insignificant in regulating intake. Based on these data, the use of these forages as complements should ideally use prairie during the spring and fall and use Plains bluestem from June through August. This schedule would allow the prairie to rest during the summer when the nutritional value of Plains bluestem is highest. Additionally, these data indicate that cattle grazing either forage should benefit from a moderate amount of a supplement high in energy and moderate in protein content.