Effects of Tallow Supplementation and Protein Withdrawal on Ruminal Fermentation, Microbial Synthesis and Site of Digestion

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Abstract
The effects of tallow supplementation [0% (NT) vs 7.5% (T)] and crude protein level [8.5% (LP) vs 12.0% (HP)] on ruminal fermentation, microbial protein (MCP) synthesis, digesta passage and site of digestion were estimated using yearling Angus × Simmental steers (390 kg) fitted with ruminal and T-type duodenal cannulae. Chromium-EDTA and ytterbium (Yb) chloride were used as markers of the liquid and solid phases. Passage and site of digestion data were estimated from the concentrations of Yb in the duodenal digesta and feces. Dry matter (DM) intakes were 6.8, 6.5, 6.3 and 6.6 kg/d for the NT-LP, NT-HP, T-LP and T-HP diets, respectively. Ruminal ammonia concentrations (mg/100 ml) for the NT-LP, NT-HP, T-LP and T-HP diets were 1.22, 4.75, 1.05 and 3.41, respectively. Tallow decreased (P<.05) acetate (mol/100 mol), increased (P<.01) propionate (mol/100 mol) and decreased the total volatile fatty acid concentration. Tallow depressed apparent ruminal DM and organic matter (OM) digestibilities only on the HP diet. High protein increased ruminal DM, OM and fiber digestibilities. Tallow and LP tended to shift the site of OM digestion to the lower tract. The liquid and solid dilution rates for the NT-LP, NT-HP, T-LP and T-HP diets were 9.53, 3.37; 5.63, 3.28; 6.66, 5.10 and 6.79, 5.34%/h, respectively. Nitrogen (N) intake (g/d), duodenal non-ammonia N flow (g/d), non-degraded N flow (g/d), microbial N flow (g/d) and efficiency of MCP synthesis (g MCP/100 g of OM digested in rumen) for the NT-LP, NT-HP, T-LP and T-HP diets were 101, 130, 77, 53, 10.4; 127, 134, 59, 75, 10.6; 81, 108, 60, 48, 11.3; and 132, 146, 84, 62, 12.0, respectively. A higher percentage of the N intake escaped ruminal degradation on the LP diets. Tallow increased the efficiency of MCP synthesis, but an increase in the solid dilution rate resulted in decreases in microbial synthesis and ruminal OM digestion on T diets. Copyright © 1987. American Society of Animal Science . Copyright 1987 by American Society of Animal Science.