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Effect of Dietary Crude Protein on the Dry Matter Digestibility of Wheat Straw Diets in Cattle¹

Washington State University, Pullman 99164

Abstract

Eight mature, pregnant cows were fed diets consisting of: (1) 63.4% mixed hay (MH) + 36.6% wheat straw (WS), 7.16% crude protein (CP); (2) 86.57% WS + 13.34% soybean meal supplement, 10.42% CP; (3) 71.34% WS + 28.57% MH, 5.29% CP, or (4) 75% WS + 25% urea supplement, 6.02% CP. Diets 1, 2 and 3 were fed at 80% and diet 4 at 100% of energy requirement in a 4 x 4 Latin square experiment. The diets supplied 121, 177, 90 and 102% of the recommended CP level, respectively. Apparent dry matter digestibilities (DMD) were determined from fecal grab samples using Cr₂O₃. Ruminal fluid, for determination of pH and NH₃-N was obtained at 0 and 4 h postfeeding on d 16 via stomach tube. Apparent DMD, mean ruminal pH and NH₃-N were: 49.5, 58.6, 44.4 and 48.1%; 7.17, 6.91, 7.28 and 7.08, and 7.55, 17.57, 4.92 and 6.25 mg/dl, for the four diets, respectively. All measurements differed (P<.05) except those of diets 1 and 4. In a second study, DMD was determined with eight 250 kg yearling steers fed 81.5% WS and 18.5% of supplements, with soybean meal varied to raise dietary CP to 11, 9.5, 8 or 6.5% in a 4 x 4 Latin square experiment. Ruminal samples obtained by stomach tube at 0, 2, 4 and 8 h postfeeding were analysed for pH and NH₃-N. Apparent DMD, mean ruminal pH and NH₃-N were: 48.5, 47.2, 45.4 and 42.1%; 6.7, 6.8, 6.9 and 7.0, and 6.9, 4.3, 1.9 and .7 mg/dl, respectively. All measurements differed (P<.05) from the 6.5% basal diet values. Dietary CP intakes. higher than required for nonlactating beef cows result in increased DMD and utilization of WS diets.

¹ Scientific Paper No. 6221. College of Agriculture Research Center, Washington State Univ. Research conducted under project 0414.

² Present address: Star Route, Kila, MT 59920.

³ Dept. of Anim. Sci.