Starch digestion by ruminants and its consequencies

Abstract

It is now well documented that the ruminal digestion of starch varies largely according to the feed and its technological treatment. The in sacco effective degradability (X, %) of starch allows to predict its pre-intestinal digestibility (Y, %) : Y = 0.483 X + 45.62. The variations of the digestion parameters of starch influence the microbial proteosynthesis, the fermentative profile and the ruminal ecosystem stability. The starch digestibility in small intestine is not complete and seems to reveal a limitation of the intestinal amylolytic capacity. The undegraded starch passing out of the small intestine is partly degraded by the microbes of the hing gut. The data of starch digestibility measured at different levels of the gut allow to estimate the quantity of volatile fatty acids and glucose potentially available from starch. When the diet leads to normal values of ruminal fermentation [ratio (acetate + butyrate) /propionate from 3 to 4] and for milk fat content (from 37 to 40 g/kg), the rate of starch degradation in the rumen does not seem to have any influence. However, when diets are rich in concentrate inducing dominant propionic ruminal fermentation , and/or low milk fat content, results between type of starch are differents. Slowly degradable starch appears, inturn, to have a less perturbative influence on the ruminal fermentations and the milk fat content. More researchs are needed to achieve a more complete knowledge on the nutritional and zootechnical consequences of the variations of the starch degradation rate in the reticulo-rumen.