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Nitrogen Sources for Growing Cattle Fed Barley and either Wheat Straw or Dehydrated Alfalfa^1,2,3,

University of Saskatchewan⁵ , Saskatoon, Canada

Abstract

Diets composed of barley grain and wheat straw (35.7 to 40.5%) containing no protein supplement, 2.15% urea or 19% soybean meal and a diet composed of 34% barley and 60% alfalfa meal were compared in a 98-day feeding study with 295-kg yearling steers. Crude protein contents of the diets were: unsupplemented control, 7.7%; urea, 14.4%; soybean meal, 14.2%; alfalfa meal, 14.3%. One-hundred and seventy-six Hereford steers were randomly allotted to 16 pens (four pens per replicate). Digestibility was measured in eight steers per treatment. Jugular blood and ruminal fluid were collected from 12 steers per treatment at 35 and 49 days of the trial. Digestibilities of organic matter, crude protein and acid detergent fiber were increased (P<.05) by urea and soybean meal, but not by alfalfa meal. Ruminal pH values were lower (P<.05) for steers fed the alfalfa-supplemented diet than those fed straw. The proportion of ruminal propionate was highest (P<.05) and that of butyrate lowest (P<.05) in steers fed the unsupplemented diet. Plasma urea N, ruminal ammonia and free amino acid concentrations were elevated (P<.05) in steers in the three N-supplemented groups and were higher (P<.05) in steers fed urea or soybean meal than in those fed alfalfa meal. Urea had no significant effect on feed efficiency or average daily gain, whereas steers fed soybean meal or alfalfa meal gained 8 and 24% more (P<.05), respectively, than steers fed the unsupplemented diet. Steers fed alfalfa meal utilized feed 8% more (P<.05) efficiently than those fed straw diets. Performance of feed-lot steers fed a diet containing 40% straw with a suboptimal level of crude protein was improved by supplementation with soybean meal but not by urea. However, gains were highest and feed was utilized most efficiently when alfalfa meal replaced both soybean meal and straw in the diet.

¹ Journal Paper No. 2383 of the Florida Agr. Exp. Sta.

² The authors gratefully acknowledge the skilled technical assistance of Harold Ruecker, Graham Steacy, Anne Harrop, Ken Bassendowski and Susan Scribner.

³ This study was supported by grants from the Natural Sciences and Engineering Research Council of Canada and the Saskatchewan Horned Cattle Trust Fund.

⁴ Present address: Univ. of Florida, Agricultural Research Center, Ona 33865.

⁵ Dept. of Anim. and Poul. Sci.