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Influence of Protein Supplementation on Forage Intake, in Situ Forage Disappearance, Ruminal Fermentation and Digesta Passage Rates in Steers Grazing Dormant Blue Grama Rangeland^1,2,

New Mexico State University, Las Cruces 88003-0009

Abstract

Ten steers (avg wt 454 kg) cannulated at the rumen and cecum and six steers (avg wt 365 kg) cannulated at the esophagus grazed a 173-ha pasture of dormant rangeland in two trials. Supplemented (S) steers were fed a 45.5% CP (OM basis) supplement, whereas control (C) steers received no supplement. Cobalt-EDTA and Cr-EDTA were used to estimate fluid passage from the rumen and cecum, respectively. Ytterbium-labeled esophageal forage (ruminal) and a DyC1₃ solution (cecal) were used to estimate particulate passage. Nylon bags containing esophageal masticate were incubated ruminally, and ruminal fluid from each steer was used to estimate in vitro OM disappearance (IVOMD). No treatment x trial interactions (P > .05) were noted for ruminal data. Ruminal NH₃ concentration was greater (P < .08) in S than in C steers at –3, 0, 1 and 4 h after supplementation. Total concentration of ruminal VFA was greater (P < .07) at 1 h after supplementation in S than in C steers. In situ rate of NDF disappearance was not affected (P > .15) by supplementation; however, NDF disappearance was greater (P < .05) for S than for C steers at 4, 8, 12, 18 and 36 h. Forage IVOMD was increased (P < .06) by supplementation. Cecal passage rates did not differ (P > .10) between C and S steers. Ruminal fluid (P < .11) and particulate (P < .06) passage rates were greater and retention times less (P < .05) in S than in C steers. Forage OM intake (11.6 vs 9.3 g/kg BW) also tended (P < .12) to be greater in S than in C steers. Protein supplementation of free-ranging steers altered ruminal NH₃ and VFA but did not affect cecal digesta kinetics.

¹ Journal article 1350 of the New Mexico Agric. Exp. Sta., Las Cruces 88003-0009.

² Dept. of Anim. and Range Sci. The authors express appreciation to M. E. Branine for assistance in data collection. Gratitude is also expressed to Paula Ross, Dale Schickendanz and Boo Reynolds for clerical assistance in preparing the manuscript.

³ Present address: Dept. of Anim. Sci., Univ. of Missouri, Columbia.

⁴ Present address: Clayton Livestock Res. Center, Clayton, NM.

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