RUMINAL AND POSTRUMINAL UTILIZATION OF PROTEIN FROM FEED GRAINS BY STEERS

Studies were conducted to determine the suitability of diaminopimelic acid (DAP), lysine and lysine-leucine as bacterial markers, and to compare ruminal, postruminal and total tract protein utilization and bacterial protein synthesis of sorghum grain, corn and barley based diets by beef steers. Six abomasally fistulated steers were fed 81% grain diets with 10.7% crude protein (CP) in a replicated 3 x 3 Latin square design. Diaminopimelic acid, lysine and leucine were determined in bacteria isolated from abomasal digesta (two sampling times) and rumen contents, and the ratios were used to estimate percent abomasal bacterial protein. Diaminopimelic acid concentrations in ruminal and abomasal bacteria were positively correlated (r = .40), but correlations between sample sites for lysine and leucine were essentially zero. Correlations between abomasal sampling times for the three amino acid concentrations were positive (mean r = .44). Rankings of percent bacterial protein in abomasal digesta for dietary treatments were similar based on amino acid ratio methods (DAP vs lysine vs lysine-leucine) using bacteria of rumen contents. Digestibility and bacterial synthesis data were based on lysine-leucine ratios from bacteria of rumen contents and chromium oxide ratios (digesta flow marker). Ruminal digestibilities of organic matter (OM), corrected for bacterial OM synthesis in the rumen, tended to be lower (P .05) for the three diets (-38 and 72%, respectively). Apparent total tract CP digestibility was lower (P < .05) for the sorghum grain diet than for corn or barley diets (61 vs 66 and 68%, respectively). Ruminal digestibility of feed nitrogen was lower for the sorghum grain and corn diets than for the barley diet (27 and 40 vs 69%, respectively). Percent bacterial nitrogen in the abomasum was much higher (P < .05) for the barley diet compared to the sorghum grain and corn diets (72 vs 47 and 53%, respectively). Postruminal digestibility of bacterial nitrogen was lower (P < .05) for the sorghum grain and corn diets compared to the barley diet. It is postulated that the lower apparent digestibility of sorghum grain protein in the total tract is related to a lower extent of feed protein and OM digested in the rumen.