Evaluation of methods for estimating starch digestibility and digestion kinetics in ruminants

The objective of the paper is to review literature concerning the methods used to estimate starch digestibility in different segments of the digestive tract of ruminants and the kinetics of starch digestion. Starch digestibility and site of starch digestion have a profound effect on the amount and profile of substrates absorbed from the digestive tract. The prediction of starch digestibility in the rumen is laborious and also technically difficult because of unrepresentative sampling of digesta from duodenal cannulae. The double-marker technique does not solve the problems related to unrepresentative digesta sampling because any of the particulate markers are not intimately associated with particles rich in starch. Furthermore, physical separation of particles rich in starch and fibre, which is a requisite in applying multiple marker techniques, would be difficult. However, despite problems in the determination of starch flow, the double-marker technique is likely to improve the accuracy of the flow measurements of other components. Mechanistic dynamic models describing digestion and absorption of substrates require reliable estimates of the digestion kinetics of feed components. The in situ technique has been extensively evaluated and used in estimating the digestion rate of starch. Initial particle loss, often described as soluble starch, is the major problem of the technique. Various assumptions have been used for the behaviour of this fraction in the kinetic models. It is also possible that the secondary particle loss during the incubation would overestimate the rate of digestion. Both the patterns of duodenal marker excretion curves and duodenal starch flow strongly suggest that the passage of particles rich in starch do not follow the first-order passage kinetics. This means that the models used to estimate ruminal starch digestibility from the kinetic parameters are incomplete. The published data suggest that the in situ technique underestimates ruminal starch digestibility for slowly degradable starch sources such as maize and overestimates it for rapidly degradable starch sources such as barley. An advantage of the in vitro methods is that the estimates of starch disappearance are not biased by particle loss. In vitro gas production technique may also be used for estimation of the rate of starch digestion for cereal grains by applying the curve subtraction method; i.e. gas production from the cell wall fraction is subtracted from the total gas production curve. Starch represents the major proportion of cell solubles in cereal grains and even a greater proportion of gas production potential. A requisite of the in vitro techniques in estimating the rate of starch digestion is that the system itself is not limiting.