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Steers Grazing Blue Grama Rangeland throughout the Growing Season. II. Site and Extent of Digestion and Microbial Protein Synthesis^1,2,

New Mexico State University, Las Cruces 88003-0009

Abstract

Effects of advancing forage maturity and drought-induced summer dormancy on site and extent of digestion and microbial protein synthesis in beef steers grazing native blue grama rangeland were evaluated in four sampling periods. Five steers (avg initial wt 227 kg) fitted with ruminal, duodenal and ileal cannulae and three steers cannulated at the esophagus freely grazed a 12-ha study pasture. Sampling periods lasted 11 d and started June 2, which was during the early growing season (EGS); June 22, during early summer dormancy (ESD); July 21, during late summer dormancy (LSD); and August 25, 1985, during the late growing season (LGS). Dietary N content was lower (P<.05) in ESD and LSD than in EGS and LGS. Neutral detergent fiber (NDF) content was lower (P<.05) in EGS than in other sampling periods. Ruminal organic matter (OM) digestion was lower (P<.05) in ESD than in EGS, probably because of increased dietary NDF and lower N content. Ruminal OM digestion was greater (P<.05) in LSD and LGS than in ESD because of increased fiber digestion. Neutral detergent fiber and acid detergent fiber (ADF) digestion occurring in the rumen was greater (P<.05) in LSD and LGS than in EGS and ESD. Organic matter digestion in the small intestine and OM, NDF and ADF digestion in the hindgut were similar for all sampling periods. Over 90% of the fiber digestion occurred ruminally. Nitrogen intake was lower (P<.05) in ESD and LSD than in EGS and LGS; while N flow to the duodenum increased (P<.05) from EGS to ESD, decreased from ESD to LSD and increased from LSD to LGS. Apparent ruminal N digestion was negative in ESD and LSD and near zero for EGS and LGS. Efficiency of microbial protein synthesis was lower (P<.05) in LSD than in other trials. Results indicate that decreased dietary N content resulting from drought-induced summer dormancy does not markedly alter site and extent of OM and fiber digestion; however, efficiency of microbial protein synthesis may be depressed in the later stages of summer dormancy.

¹ Journal article 1297 of the New Mexico Agric. Exp. Sta., Las Cruces.

² Dept. of Anim. and Range Sci.

³ Present address: Dept. of Anim. Sci., Univ. of Illinois, Urbana 61801.