THE EFFECTS OF ROASTING TEMPERATURE APPLIED TO WHOLE SOYBEANS ON SITEOF DIGESTION BY STEERS .2. PROTEIN AND AMINO-ACID DIGESTION

A 4 x 5 Youden square design was used to determine the effect of roast ing temperature of whole soybeans on escape of CP from the rumen and d isappearance of N from the small intestine in steers. Four steers (ave rage BW 373 kg +/- 30.7) cannulated at the rumen, duodenum, and ileum were fed each of five diets (1.8% of BW) over five periods. The basal diet contained corn silage (50% of diet DM), alfalfa hay (24%), corn-u rea premix (6%), corn starch grits (16.6%), and soybean oil (3.4%). So ybeans (16% of diet DM), either raw or heated to an exit temperature o f 141, 149, or 157 degrees C in a commercial roaster, replaced the soy bean oil and most of the corn starch grits in the soybean-containing d iets. Ruminal ammonia N was lower (P<.05) for the basal diet than for the soybean-containing diets. Roasting temperature of whole soybeans h ad no significant impact on ruminal ammonia N. Total N reaching the du odenum was greater (P < .05) for steers fed the soybean diets than for steers fed the basal diet. Non-bacterial N (dietary N) at the duodenu m was increased (P <.05) by feeding soybeans. Soybean N reaching the d uodenum decreased with increased roasting temperature. In-creased roas ting temperature of whole soybeans seemed to make the soybeans more br ittle, subsequently increasing degradation of CP. in the rumen. Howeve r, disappearance of soybean N in the small intestine, as a proportion of the soybean N entering the small intestine, increased with increase d roasting temperature. Apparent total tract N digestibility was incre ased (P <.05) by feeding soybeans. Flow to the duodenum and small inte stinal digestibility of total, essential, and nonessential amino acids increased(P < .05) when soybeans were fed and when roasted vs raw soy beans were fed. Under normal roasting conditions, it seemed that littl e potential for heat damage to the soybean protein existed. Instead, u ndesirable effects of heating on handling characteristics of the soybe ans were reached before the point at which loss of nutritive value occ urred. Feeding steers diets containing roasted whole soybeans increase d the N and amino acids available to steers over steers fed diets cont aining raw soybeans due to increased ruminal soybean nitrogen escape a nd increased small intestinal digestibility.