Role of particle size and forage quality in digestion and passage by cattle and sheep.

Diminution of forage particles includes mastication, chewing, and digestion. In rumen of cattle and sheep fed all forage diets, particle size can range from 200 to over 1200 microns. Particle size reduction to about less than 1200 microns must occur before passage. Dietary particle size may influence rumen particle size, but mastication and rumination minimizes differences among diets. Ruminants expend considerable effort to move digesta. Density, cell wall percentage, osmotic pressure, and pH may affect propulsion. Dense particles may sink to the bottom and resist escape. Cell wall may reduce digestion and passage. Osmotic pressure or pH may affect digestive efficiency and rhythm of intestinal tract muscles. Chewing, exercise, physiological functions, and body size may also affect the reduction of forage particle size. More effort is necessary to chew high than low fiber diets. Young cattle (less than 225 kg) lack rumination capability and body size to process forage particles efficiently. Exercised sheep (26,400 kg-m/d) ate less forage and ruminated less than controls. Other body functions, such as lactation, appear to influence chewing patterns and rumination. These relationships are poorly understood at best and need additional intensive examination.