DIETARY AND FACTORS AFFECTING RUMEN CAPACITY IN DAIRY-COWS

The capacity of the rumen to hold and turnover feed can limit intake, especially when the demand for energy is high (early lactation) and the energy density of the feed is low. Two phenomena are important in this respect; rumen fill and rumen clearance rate. Rumen fill is mainly determined by the physiological state of the animal and the concentrate proportion in the diet. It varies between approximately 14 and 29 g dry matter/kg body weight. Rate of clearance from the rumen varies between 5 and 8.2% h-1 and depends more on the cell wall content of the roughage and the body weight of the animal than on the physiological state and the concentrate to roughage ratio. Rumen fill and rumen clearance rate appear to vary independently. The rumen clearance capacity (rumen fill x clearance rate) varies considerably (from less than 300 to over 1000 g/h), and is like rumen fill more related to physiological state of the animal and concentrate proportion in the diet than to roughage characteristics. However, with a fixed concentrate proportion in the diet, a lower cell wall content of the roughage and thus a higher rate of clearance, results in a higher rumen clearance capacity (gh-1) and allows a higher milk production. Rate of degradation decreases with an increase in cell wall content of the roughage. Despite an increase in rate of passage of particles, an increase in cell wall content of the roughage still results in a decrease in rumen clearance rate.