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Feeding Value of Rations Containing Nonprotein Nitrogen or natural Protein and of Ammoniated Straw for Beef Cattle^{1 ,2 ,3,}

University of Saskatchewan, Saskatoon, Canada S7N 0WO

Abstract

Two trials were conducted to evaluate the feeding value of barley-urea (dry extrusion product) and molasses-urea (liquid mixture) when compared to two natural proteins (either dehydrated alfalfa or rapeseed meal) in growing rations for steers. Steers were fed 4 kg of concentrate per day and wheat straw ad libitum. Wheat straw treated with 3.5% (W/W) anhydrous ammonia was fed with a soybean meal-barley concentrate in a fifth treatment. Six 200-kg steers were used in Experiment 1 to evaluate the five concentrate mixtures and untreated wheat straw fed alone in a 6 x 6 Latin square. Total feed intakes were similar for all concentrate plus straw rations and about twice as high as for straw fed alone. Apparent digestion coefficients for dry matter, crude protein and gross energy in the total ration were higher (P<.01) with concentrates and were not affected by the nitrogen source. Crude fiber digestion was lower (P<.01) when concentrates were offered except when ammoniated straw was fed. Concentrates had no effect on the molar proportion of propionic acid in rumen fluid, though proportions of acetic acid were lower and levels of butyric acid were higher. Total volatile fatty acid concentrations were higher (P<.05) with alfalfa than with either ammoniated straw or straw alone. Ruminal ammonia and dehydrogenase activity were higher (P<.05) when concentrates were included in the rations. Plasma urea concentrations were measured 0, .5, 1, 2, 4 and 7 hr after feeding and were consistently higher when either molasses-urea or barley-urea were fed.

The feedlot performance of 216-kg steers fed the four concentrates (4 kg/day) with untreated wheat straw ad libitum or NH₃-straw ad libitum plus concentrate was measured for 169 days in Experiment 2. Straw intakes were similar for all treatments and were not improved when ammoniated straw was fed. Daily gains were lowest (P<.05) with molasses-urea and averaged about .56 kg with the other concentrates. There were no signs of greater compensatory growth by steers on the molasses-urea ration during the. subsequent 130-day finishing period.

¹ This research was supported by grants from the National Research Council of Canada, North East Alfalfa Dehydrators, Tisdale, Saskatchewan and the Saskatchewan Horned Cattle Trust Fund.

² Thanks are extended to Cargill Grain Co. (Ltd.), Winnipeg and Western Cooperative Fertilizers, Calgary, for providing the nonprotein nitrogen compounds.

³ The technical assistance of Susan Scribner, D. Hopkins and K. Bassendowski is gratefully acknowledged.

⁴ Department of Animal and Poultry Science.