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Effects of Incremental Urea Supplementation on Ruminal Ammonia Concentration and Bacterial Protein Formation^{1 ,2,}

Texas A & M University³, College Station 77843

Abstract

Increments of 0, 1, 2, 3, 4.5 and 6.5% crude protein equivalent (CPE) were added as urea to a basal diet containing 75% corn and 20% cottonseed hulls (8.3% protein from all natural sources) and fed to two nonlactating Holstein cows. Ruminal ammonia nitrogen (N) levels increased (P<.05) with each increment of urea and ranged from 1.3 (basal) to 28.9 mg/100 ml (basal + 6.5% CPE). Ruminal diaminopimelic acid (DAP) concentrations showed a negative quadratic relationship with incremental urea addition, increasing (P<.05) until ruminal ammonia-N reached 8.5 mg/100 ml (basal + 2% CPE). Ruminal DAP concentrations plateaued and then declined (P<.05) at the highest level of dietary urea and ruminal ammonia. Estimates of bacterial crude protein (BCP) synthesis rate (grams per day) and yields (grams /100 g fermented dry matter) followed trends similar to DAP but generally were not statistically significant because of variability observed in ruminal volume, dry matter digestibility and turnover rates. Estimates of BCP synthesis rates and yields were also made from incorporation and turnover of ³⁵S in ruminal bacteria. Patterns of BCP synthesis and yield were similar to those based on the DAP marker but tended to plateau at the next higher urea (basal + 3% CPE) and ammonia (13.8 mg/100 ml) levels. More confidence was placed in data based on DAP; therefore, results were interpreted to mean that 8.5 mg/100 ml was approximately the optimal concentration of ammonia-N under the conditions of this experiment.

¹ Technical Article 15484 of the Texas Agr. Exp. Sta., College Station 77843.

² The authors wish to acknowledge the excellent technical assistance of Miss Chien Kang and Mr. Mike Craig.

³ Dept. of Anim. Sci.

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