Relationship between in vitro gas production profiles of forages and in vivo rumen fermentation patterns in beef steers fed those forages

The gas production (GP) technique simulates fermentation in the rumen and could be used to predict the pattern of rumen fermentation. Only, a limited number of studies have actually related GP profiles of feeds, and the terminal short chain fatty acids (SCFA) produced, to in vivo rumen fermentation data of the same feeds. Nine rumen-cannulated beef steers were fed eight forage diets (i.e. grass (G), high digestibility grass silage (HDS), low digestibility grass silage (LDS), high dry matter grass silage (HDMS), low dry matter grass silage (LDMS), maize silage (MS), whole-crop wheat (WCW) and hay (H)). Diets were offered at maintenance energy level, twice daily at 09:00 and 16:00 h, in a partially balanced change over design, with each experimental period lasting 16 days. Samples of rumen fluid were taken at - 1, 1, 2, 4 and 6 h after the morning feeding, and 1, 2, 4, 7, 11, 14 and 16 h after the evening feeding, for analysis of pH, ammonia and SCFA. The GP profiles of the individual forages were estimated using an automated pressure transducer technique. Post-termination, SCFA profiles of the incubation media were determined using gas chromatography. The GP profiles were fitted to the modified Michaelis-Menten model and from these data, the total volume of gas produced (TGV) and the maximum rate of GP (Rm gas) were calculated. Significant differences were observed between the individual SCFA concentrations and proportions determined in vitro and in vivo. When stepwise multiple linear regression analysis was applied, significant relationships were observed between in vivo pH, and the in vitro concentrations of SCFA, TGV and Rm, gas. Similar relationships were also derived for in vivo total SCFA and molar proportions of individual SCFA. Results confirm that GP can be used as a dynamic estimate of feed fermentability to predict the pattern of rumen fermentation. (C) 2002 Elsevier Science B.V. All rights reserved.