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In Vitro and in Vivo Ammonia Release from "Slow-Release" Urea Supplements¹

Oklahoma Agriculture Experiment Station^,5, Stillwater 74074

Abstract

The rate of release of ammonia-N(NH₃-N) from corn urea (CU) and molasses urea (MU), two gelatinized starch-urea products (GSUP) and a slow release liquid supplement (SLS) were studied in vitro and in vivo to determine if they gave a slow continual release of NH₃ over a 12- to 24-hr period. The rate of NH₃-N release in a buffered urease solution was the same for the GSUP and CU supplements. The SLS had an initial rate of NH₃-N release that was faster than MU; however, only 40% of the total N that was present was released as NH₃ from the SLS. Similar results were also found in in vitro rumen fluid incubations.

The supplements were fed with cottonseed hull diets to 12 (trial 1) and eight (trial 2) fistulated wethers. On sample days either 50% (trial 1) or 75% (trial 2) of the normal daily supplement was placed directly into the rumen at T₀.

Samples for rumial NH₃-N and pH in both trials and plasma urea-N (PUN) in trial 2 were collected at 0, .5, 1, 2, 4 and 8 hr after feeding. NH₃-N accumulation from GSUP-2 was lower (P<.01) than NH₃-N accumulation from either CU, GSUP-1 or MU. The SLS had the same initial rate of NH₃-N accumulation as MU; however, a major portion of the N appeared to be tightly bound, resulting in less total NH₃ accumulation. Ruminal pH and PUN were lower (P<.01) when the two liquid supplements were fed, suggesting that the lower rumen pH resulted in decreased NH₃ absorption.

¹ Journal Article No. 3563 of the Oklahoma Agriculture Experiment Station, Stillwater.

² Present address: Department of Animal Sciences, Washington State University, Pullman WA 99164.

³ Present address: Department of Animal Science, University of Tennessee, Knoxville, TN 37901.

⁴ Present address: Head, Department of Animal Science, University of Tennessee, Knoxville, TN 37901.

⁵ Department of Animal Sciences and Industry.