Relationships between patterns of rumen fermentation measured in sheep and in situ degradability and the in vitro gas production profile of the diet

Abstract

The relationship between the pattern of rumen fermentation in vivo and the gas production (GP) profile of different diets, and the in situ degradability of the component feeds was investigated. Three mature wethers were fed three diets in a randomised block design. Each diet consisted of hay and micronised maize. The relative proportions of hay:maize (dry matter (DM) basis) was 80:20, 50:50 and 20:80. Samples of rumen fluid were collected over 24 h and analysed for pH, short chain fatty acids (SCFAs) and protozoal population. In situ degradability of hay and maize organic matter (OM) was estimated. The GP profile of each diet was estimated with an automated pressure transducer using rumen fluid collected from the animal being fed that diet. The SCFA profile of the incubation medium was estimated after 72 h, and from these data the amount of ATP produced in vitro was predicted. The proportion of maize in the diet had no effect on the effective degradability of maize or the mean total SCFA concentration in the rumen. However, increasing the proportion of maize in the diet reduced hay OM effective degradability and molar proportion of acetate in the rumen. The opposite effect was observed with propionate, and the proportion of butyrate was not affected. The ratio of non-glucogenic:glucogenic SCFA (NGR) decreased as the proportion of maize increased. Differences between in vitro and in vivo measurements were significant for total SCFA concentration and acetate proportion. The time to maximum gas production rate and predicted ATP production were related to in vivo mean total SCFA concentration ( R 2=0.81), mean molar proportion of acetate ( R 2=0.59), propionate ( R 2=0.63) and butyrate ( R 2=0.86), NGR ( R 2=0.61) and the effective OM degradability of hay ( R 2=0.57). Results suggest that the GP technique predicts dynamic parameters of rumen fermentation, particularly the SCFA profile. This would reduce the need to use surgically modified animals when determining the effect of diet on the pattern of rumen fermentation.