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Steers Grazing Blue Grama Rangeland throughout the Growing Season. I. Dietary Composition, Intake, Digesta Kinetics and Ruminal Fermentation^1,2,

New Mexico State University, Las Cruces 88003-0009

Abstract

Four sampling periods on blue grama rangeland in northeastern New Mexico evaluated effects of advancing forage maturity and drought-induced dormancy on dietary nutrient and botanical composition, intake, digesta kinetics and ruminal fermentation in grazing beef steers. Six ruminally cannulated and three esophageally cannulated steers freely grazed a 12-ha pasture during the study. Sampling periods lasted 11 d and started June 2, 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). Forage availability was not limiting in any sampling period. Steers consumed a greater (P<.05) percentage in forbs and lower percentage of grasses in EGS and ESD than in LSD and LGS. Dietary in vitro organic matter digestibility was lower (P<.05) in ESD than in EGS, LSD and LGS. Dietary N content was higher (P<.05) in EGS and LGS than in ESD and LSD. Neutral detergent fiber content was lower (P<.05) in EGS than in other sampling periods, while dietary lignin contents were similar for all sampling periods. Voluntary organic matter intake was similar for all sampling periods; however, estimated gastrointestinal tract fill was greater (P<.05) in ESD and LSD than in EGS and LGS. Particulate passage rate was slower (P<.05) and total mean retention time longer (P<.05) in LSD than in other sampling periods. Rate and lag time of neutral detergent fiber digestion were not different among sampling periods. Ruminal pH was greater (P<.05) at 3 and 6 h after sunrise in ESD than in other sampling periods. Ruminal ammonia concentrations were lower (P<.05) in ESD and LSD than in EGS and LGS at 3 and 6 h after sunrise. Total volatile fatty acid concentrations were lower (P<.05) in ESD than in EGS and LSD at 3 h after sunrise and lower (P<10) than EGS and LGS at 9 h after sunrise. Molar proportions of acetate were greater (P<.05) at 3 h after sunrise in ESD and LSD than in EGS and LGS. Changes in digesta kinetics and ruminal fermentation patterns observed in this study appeared to be related to both forage maturity and dietary botanical composition.

¹ Journal article 1296 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.

⁴ Present address: Anim. Sci. Dept, Univ. of Nevada, Reno 89557.

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