Abstract

Forty-eight yearling Holstein-Friesian steers were assigned to three diets (Panicum virgatum (P) var. Cave-in-rock, Dactylis glomerata (D) var. Hallmark and Medicago sativa (M) var. Waterman Loomis 316) fed at two levels of intake (low (L) 65g DM kg-1 liveweight -0.75 day -1 or high (H) 90 g DM kg -1 liveweight -0.75 day -1) during a 6 month growth study. At the conclusion of the study animals were stunned, exsanguinated and promptly eviscerated and the rumen quickly emptied of its contents and subsequently weighed. Upon slaughter, rumen tissue samples were acquired from the ventral sac and placed in oxygenated M199 media. Mucosa and muscularis weights were determined along with their total 02 consumption, ouabain-senitive- (OSO2) and ouabain-insensitive-(OIO2) respiration. Tissue oxygen consumption was determined in vitro using a YSI Clark style electrode. Total, Mucosa and muscularis weights were greater (16.9-43.5%) with H and tended to be affected by diet (P<0.10). On a tissue basis, rumen muscle O2 consumption was not affected by treatment but mucosal OSO2 and percent inhibition of O2 consumption by ouabain were higher (6.9-94%) with Medicago. Total rumen O2, OSO2 and OIO2 consumption were higher (8.6-80.5%) in H (P<0.05) and diet affected OSO2. Rumen muscle total O2 consumption was greatest (3.51 ul 02/mg/hr) for D and altered by level of intake (P<0.03). Mucosal OSO2 and OIO2 were greater with H (P<0.04 and P<0.07, respectively). Total rumen O2 consumption is dominated by organ size rather than changes in weight specific O2 consumption. Therefore, at high forage intakes part of the decrease in the efficiency of ME utilisation is a consequence of increased heat production from the larger rumen.

KL, Isaacs, KP McNatty, and RP Littlejohn

Proceedings of the New Zealand Society of Animal Production, Volume 51, , 87-90, 1991