Influence of declining level of feed intake and varying the proportion of starch in the concentrate on rumen ingesta quantity, composition and kinetics of ingesta turnover in dairy cows

Abstract

Five dairy cows fitted with large-diameter rumen cannulae were fed diets of two-thirds pelleted concentrate that contained 5 proportions of starch (8, 14, 20, 26 or 32%). Measurements were made over one complete lactation to determine the effects of intake level and proportion of starch in the concentrate on rumen content of soluble and dry matter (DM) components as well as the kinetics of rumen neutral detergent (ND) fibre digestion and passage. Intake declined from 19.44 to 4.86 kg organic matter (OM) per day at equal intervals. Rumen content of total ingesta, non-DM ingesta and all measured components of DM ingesta declined ( P <0.05) as level of intake declined, although the form of the reductions differed. Rumen pool sizes of ammonia and all volatile fatty acids (VFA's) either did ( P <0.05), or tended to ( P <0.10), decline as level of intake declined. Rumen passage rate of ND fibre declined linearly ( P <0.05), but tended ( P <0.10) to decline at an increasing rate as level of intake declined. Rumen rate of ND fibre digestion increased linearly ( P <0.05), but tended ( P <0.10) to increase at a decreasing rate as intake level declined. Rate of digestion of ND fibre in the rumen was closely related to rumen pH and rumen bacterial OM (RBOM) as a proportion of rumen ND fibre (RNDF); rate of digestion of ND fibre increasing as pH and RBOM/RNDF increased.