The effects of the physical form of hay on digestion and retention time in sheep
Sheep with cannulas in the rumen, abomasum or terminal ileum were used to study the effects of the physical form of hay on digestion. The hay, perennial ryegrass with timothy, was chopped or was ground through 2, 5, 20 or 40 mm screens and fed twice daily at 800 g/d.Various indicators of mean retention time (MRT) were first investigated, oat husk stained with brilliant green or basic fuchsin showed reduced digestibility in nylon bags suspended in the rumen. The MRT of hay stained with basic fuchsin was 1.4 times that of hay marked with 103Ru-phenanthroline (103Ru-phen). Subsequently, MRT of particles and liquid were measured using 103Ru-phen and 51Cr-EDTA respectively.Apparent digestibility of DM was significantly decreased by reduction in dietary particle size. The rumen and whole gut MRT of particulate matter were reduced when the hay was finely ground but large intestine MRT was unaffected. Particle size in the abomasum was independent of the physical form of the diet, so coarse hay must stay longer in the rumen for breakdown to a size that permits onward passage. Relative to coarse hay, fine hay in nylon bags in the rumen showed greater solubility and rate of DM digestion, and when fed caused a sharper fall in rumen pH but reduced digestibility of hay contained in nylon bags in the rumen. Evidently, although the soluble component of a ground hay diet is rapidly digested overall microbial activity is reduced.Rumen retention time of liquid was significantly shorter with ground than with chopped hay, rumen volume being similar. Solids were retained longer than liquid in the rumen, but solids and liquid had similar retention times in the large intestine.It is concluded that the effects of grinding were largely due to their influence on the digestive function of the rumen.