Supplemental protein sources for steers fed corn-based diets. I. Ruminal characteristics and intestinal amino acid flows
1995
Ludden, P.A. | Cecava, M.J.
Five cannulated Holstein steers (302 +/- 23 kg) were used in a 4 X 4 Latin square design experiment with extra observations to evaluate the effect of supplemental protein source on postruminal flow and intestinal disappearance of N and amino acids (AA). Diets were formulated to contain 12.5% CP using cracked corn (70%), ground corn cobs (15%), and supplement (15%). Except for an all-urea supplemented control diet, 40% of the total dietary CP was supplied by soybean meal (SBM), a high ruminal escape soybean meal (SoyPLUS; SP), or a corn gluten meal/blood meal combination (CB; 50:50 on a CP basis). The steers were fed twice daily and DMI was restricted to 2.0% of initial BW. Supplementation with CB increased (P < .10) non-microbial N flow at the duodenum but tended to decrease microbial N flow such that no differences (P > .10) in total N flows at the duodenum were detected among treatments. The efficiency of microbial protein synthesis was not affected by treatment but was greatest for urea and tended to decrease when SP and CB were fed (21.8, 20.5, 19.5, and 15.7 g of N/kg of OM truly digested for urea, SBM, SP, and CB, respectively). A possible shortage of ruminally degradable N, as evidenced by low ruminal NH3 N concentrations (3.6, 4.2, 3.9, and 2.1 mg/dL for urea, SBM, SP, and CB respectively), or other factors, may have limited microbial protein synthesis. However, microbial N flows averaged 60.0% of duodenal N flow across treatments. Duodenal flow of essential, nonessential, and total AA were similar (P > .10) among treatments and averaged 293, 361, and 653 g/d, respectively). Lysine and methionine flows did not differ (P > .10) and averaged 41.6 and 13.3 g/d, respectively. The source of supplemental protein had no impact (P > .10) on small intestinal disappearance of AA or total tract N disappearance. Feeding proteins resistant to ruminal degradation may have limited microbial protein production, resulting in a shift in the proportion of metabolizable protein arriving at the small intestine from dietary and microbial origins. These results suggest that corn-based diets may be limiting in ruminally degradable N, especially when high ruminal escape protein sources are fed as supplemental CP.
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