Physiological Limitations to Using Concentrates to Increase the Digestible Energy Intake of Ruminants given Roughage Based Diets

The low intake and digestibility of low-quality roughages such as mature forages or crop residues often results in a digestible energy intake insufficient for a desired level of production. One alternative to increase digestible energy intake is to provide supplementary concentrates of high digestibility. ‘ Associative effects on forage fibre digestion may occur when roughage and concentrate are given together to ruminants. Positive associative effects may occur if the forage is deficient in a specific nutrient (e.g. fermentable N or S) and the concentrate supplement provides this nutrient in excess of the requirements for the fermentation of the concentrate in the rumen. More often negative associative effects are observed when the rumen fermentation of readily fermentable carbohydrates (RFC) in the concentrate reduces -the rate of rumen digestion of the forage cell wall constituents (CWC). This often causes reduced intake of the roughage i.e. substitution of concentrate for roughage occurs.