Effects of forage particle size, level of feed intake and supplemental protein degradability on microbial protein synthesis and site of nutrient digestion in steers.

A 2(3) factorial arrangement of treatments was used to study main effects and interactions between particle size of prairie hay (chopped vs ground), two levels of feed intake (60 and 90% of ad libitum) and ruminal degradability of protein sources [dry corn gluten feed (DCGF) vs dry distillers grains (DDG)] on ruminal and total tract digestion in eight ruminal- and duodenal-cannulated steers. Steers were fed every 2 h to approach steady-state feeding conditions. Steers fed ground hay diets digested higher (P less than .05) percentages of total digestible organic matter (OM) and neutral detergent fiber (NDF) in the rumen and had lower (P less than .05) nonammonia-nonbacterial N (NANBN) flows to the duodenum than did those fed chopped hay, probably because greater surface area of ground hay allowed more extensive ruminal fermentation. Protein source X intake interactions were noted for ruminal OM and NDF digestion when expressed as percentages of total digestion. At low intakes, steers fed DCGF had higher (P less than .05) percentages of total digestible OM and NDF disappearing in the rumen than did those fed DDG. Steers fed DCGF had lower total N, NANBN and total amino acid (AA) flows at the duodenum than did those fed DDG, indicating that less DCGF protein escaped ruminal degradation. Steers fed DDG had greater (P less than .05) total tract NDF digestion, suggesting that escape protein from DDG may stimulate hindgut fermentation and thereby affect site and extent of nutrient digestion. Regression analysis indicated that extent of ruminal fermentation and efficiency of microbial growth in vivo are associated with ruminal rates of passage within individual animals. When steers were fed at high-intake levels (1.6% of body weight), ruminal dilution rates were not increased (P less than .05) due to forage particle size or level of intake treatments, accounting, in part, for the lack of expected treatment differences in efficiency of bacterial growth and duodenal N flow, and for the low number of interactions between main effects.