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 digestion in the rumen of the lactating dairy cow and to propose the framework for a mechanistic model representing this process that may be used as part 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 concerning mainly lactating daily cows. Regression relationships ale presented to describe the significance of dietary starch intake, starch source and processing method on digestion characteristics. The quantity of rumen escape starch is increased at high starch intakes. Maize and sorghum starches tend to be more slowly degraded within the lumen than other cereals and they also benefit to a greater extent from physical and chemical processing techniques designed to increase ruminal and total tract starch digestion. Other factors influencing the nature of starch digestion are also presented in order to allow the interpretation of experimental data and hence the development of a conceptual model of starch digestion. The review subsequently examines starch digestion in the rumen as it is represented in extant models of luminant digestion. The essential elements of a rumen model that would accurately account for the fate of dietary starch within the rumen are identified. Finally, a scheme representing starch digestion in the dairy cow that may be incorporated into a model of whole rumen function is presented.