Intake and rumen kinetics in dairy cows fed grass, maize and alfalfa silage

Ad libitum intake of roughage in dairy cattle is limited by chemical and physical constraints. The physical constraints relate to limitations in degradation of feed and flow Of digesta through the rumen and other parts of the gastrointestinal tract. In this study, physical limitations were investigated by a combination of nylon bag incubation in the rumen, rumen evacuation and marker passage. Three lactating (rumen-fistulated) Holstein Friesian cows were fed twice daily grass (GS), maize (MS) and alfalfa (AS) silage (ad libitum) and mixed concentrates (7 kg day(-1)). Nylon bag incubation showed lowest degradation rates for MS and highest truly undegradable fractions for AS. Results of marker passage, indicated that GS has a lower passage rate of particles and a higher liquid flow than AS. This suggests that, in the rumen, particles are more separated from liquid in GS than in AS. Clearance rates showed the lowest values for GS. The combination of a larger rumen pool size for GS compared with ASI a lower GS intake and a lower GS clearance may reflect physical regulation. The lower passage rate of GS particles may be the reason.