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Two winter trials were conducted with 304 lactating range cows on dry grass to evaluate non-protein-nitrogen (NPN) in 30% protein supplements containing biuret (pure and feed grade), urea and extruded grain-urea. The NPN sources contributed one-half of the supplemental nitrogen with natural 15 and 30% protein supplements serving as negative and positive controls.
Winter weight loss of cows was greater (P~.02) on the negative than on the positive control in both trials. The apparent utilization of all NPN sources was low and the utilization of urea and extruded grain-urea was less than pure or feed grade biuret. Rumen biuretolytic activity was apparent within 6 days and reached and maintained a high level of activity 20 days after the initiation of feeding biuret, even with intermittent supplementation. Apparent value of NPN supplements was slightly improved with 40% dehydrated alfalfa but not with methionine- hydroxy-analogue (MHA). Palatability of supplements was lowered by urea and especially by MHA and extruded grain-urea.
In a third trial with yearling heifers fed prairie hay, gains were similar on natural protein and supplements containing urea or extruded grain-urea to provide one-half of the nitrogen. When the heifers were fed the same supplements but low quality winter harvested range grass, NPN utilization appeared to be low.