Comparison of Nitrogen-15 and Diaminopimelic Acid for Estimating Bacterial Protein Synthesis of Lactating Cows Fed Diets of Varying Protein Degradability

¹ Department of Animal Sciences, University of Arizona, Tucson 85721

Three lactating Holstein cows fitted with duodenal cannulae were fed diets containing cottonseed meal, corn gluten meal, or blood meal as protein supplements in a 3 x 3 Latin square experiment. Diets averaged 15% CP and were 60% concentrate, 31% corn silage, and 9% alfalfa hay. The flow marker was Cr₂O₃; the bacterial protein fraction of digesta CP was estimated by ¹⁵N (as ammonium sulfate) and diaminopimelic acid. The undegraded fraction of total feed protein entering the duodenum for respective diets was .52, .57, and .69. The ¹⁵N method was less variable than diaminopimelic acid. Based on ¹⁵N, percentage of bacterial of total protein differed among treatments (61.5, 59.4, and 55.0. respectively). Ten percent more protein entered the duodenum on blood meal than other diets, but differences were not significant. Protein sources were similar in microbial passage, but degraded protein was used most efficiently for microbial synthesis on blood meal.

Incorporation of ¹⁵N consumed into bacterial protein ranged from 50 to 83% with numerically highest values on blood meal, suggesting greater efficiency of ammonia capture. Recoveries of ¹⁵N for the 72 h as milk, feces and urine ranged from 54 to 78%.

Key Words: bacterial protein • diaminopimelic acid • protein degradability

Submitted on May 11, 1989
Accepted on September 11, 1989

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