Factors Affecting Diet Digestibility in Dairy Cattle

Feces typically comprises the greatest loss of ingested energy in ruminants, and the variation in converting digestible energy to metabolizable energy is usually small. Diet digestibility is determined by intrinsic properties of interand intramolecular structure of plant cell walls, which establish the upper potential for the rate and extent of cell wall digestion in ruminants (Mertens, 1993). Typically digestibility for feed evaluation in ruminants is estimated with sheep fed at maintenance level of feeding. It can be considered as an intrinsic digestibility of the feedstuff, although only 80-90% of potentially digestible NDF (pdNDF) is digested at maintenance level of intake (Huhtanen et al., 2006). The intrinsic rate and extent of digestion of cell walls set the upper limit of digestion in ruminants, but both the animal and diet factors can have a dramatic effect on the extent to which the potential digestibility is attained. It is wellknown that digestibility of dairy cow diets is reduced with increasing intake (Tyrrel and Moe, 1975), but the discounts for the feeding level effects on digestibility are variable in feed evaluation systems. The rate of decline has been shown to be related to diet digestibility at maintenance intake (NRC, 2001). The changes in digestibility observed with increased intake have been associated with reduced digesta retention time in the rumen. Further, both negative and positive associative effects between dietary components on digestion can occur, especially at high levels of feeding. The objective of this paper is to analyze the effects of some intrinsic and extrinsic factors influencing digestibility of mixed dairy cow diets, and to compare different models in predicting discounts in digestibility with increasing intake. The analysis is based on a dataset from North European feeding trials, in which diet digestibility was determined in dairy cows.