Site of protein digestion in steers: predictability

The nature and predictability of protein digestion in steers were examined in a series of experiments. Bypass and intestinal digestion of supplemental proteins were measured with Angus steers (203 to 254 kg) fitted with dual reentrant intestinal cannulas. Daily feed intakes were 1.8 to 1.9% of body weight. Two trials were conducted with soybean meal, cottonseed meal, a hardened casein, meat meal and two different sources of dehydrated alfalfa meal fed with an 80% concentrate diet. In the third trial, digestion of soybean meal and cottonseed meal were measured with a 60% prairie hay diet. Estimates of ruminal escape or bypass were 43, 50, 36, 76, 57, 62, 24 and 43%, respectively. Results suggest that roughage level alters bypass. A standard reference system for predicting bypass based on measurements of N solubility and in situ disappearance rates was compared with measured bypass. The correlation between observed bypass to the small intestine and bypass predicted by this system was high (r = .91, P less than .01). Combining pepsin solubility, an indicator of true intestinal digestibility, with bypass estimated by the standard reference system, allowed us to predict N disappearance from the small intestine. Predicted and measured N disappearance of fed N from the small intestine were very close.