A review of starch digestion in the lactating dairy cow and proposals for a mechanistic model: 1. Dietary starch characterisation and ruminal starch digestion

The objective of this paper is to review the literature concerning starch d igestion in the rumen of the lactating dairy cow and to propose the framewo rk for a mechanistic model representing this process that may be used as pa rt of a larger model of whole rumen function. The review defines starch as it occurs in the diet of the dairy cow, followed by a detailed description of the digestion of starch in the rumen. Emphasis is placed on the role of the microbial population in the degradation of starch and the significance of both amylolytic bacteria and protozoa. During the investigation, use is made of data gathered from both in vivo, in vitro and in situ studies conce rning mainly lactating daily cows. Regression relationships ale presented t o describe the significance of dietary starch intake, starch source and pro cessing method on digestion characteristics. The quantity of rumen escape s tarch is increased at high starch intakes. Maize and sorghum starches tend to be more slowly degraded within the lumen than other cereals and they als o benefit to a greater extent from physical and chemical processing techniq ues designed to increase ruminal and total tract starch digestion. Other fa ctors influencing the nature of starch digestion are also presented in orde r to allow the interpretation of experimental data and hence the developmen t of a conceptual model of starch digestion. The review subsequently examin es starch digestion in the rumen as it is represented in extant models of l uminant digestion. The essential elements of a rumen model that would accur ately account for the fate of dietary starch within the rumen are identifie d. Finally, a scheme representing starch digestion in the dairy cow that ma y be incorporated into a model of whole rumen function is presented.