Shade-stimulated growth and nitrogen uptake by pasture grasses in a subtropical environment

Abstract

Fibrous feeds constrain voluntary feed intake by ruminants through a complex of interacting animal and plant factors. The animal factors are briefly assessed indicating some control over digesta load in the rumen and that only small particles (<1 mm screen size) achieve ready passage from the rumen. There is evidence that retention of small particles may be prolonged because their movement from the rumen to reticulum may be restricted by their buoyancy and entrapment within the digesta raft in the rumen. Ingestive chewing and rumination effort varies with feed type, is greater for stem than leaf and increases with feed maturity. It is still not clear from animal studies whether passage rate and VFI are limited more by the time to break large particles to small particles or by the hindrance of small particle passage from the rumen. Digestion in rumen decreases the physical grinding energy needed to comminute feed, but it is not readily shown that this makes a difference to efficiency of animal rumination. Anatomical characteristics of leaves and stems of tropical and temperate grasses and legumes are analysed to assess their influence on the ease and pattern of breakdown, and the characteristics of the resultant fibre particles. Epidermal and vascular structures, and microbial digestion, determine the initial breakdown of organs which is fast in leaves of legumes and high quality temperate grasses, but slow in tropical grass leaves. All stems require a high chewing effort to facilitate swallowing of feed and passage of particles from the rumen. Some tissue components directly enter the 'fines' fraction, the rest (vascular strands in isolated or composite form) become the large particle fraction. Rumination is essential to degrade these to small particles. It is hypothesised that inaccessibility of cell walls for microbial attack limits digestive weakening in both large and small particles. A conceptual framework summarizing the breakdown pattern for different feed groups is presented. Potential future opportunities for overcoming these intake constraints associated with modification of rumen microbes and plant anatomical structure are discussed.