Experimentally measured digestibility coefficients are used to predict intestinal digestibilities and faecal losses based on summaries of data in the literature (Sniffen et ah, 1992; Knowlton et al., 1998). The total NDF escaping the rumen is assumed to have an intestinal digestibility of only 20% because the small intestine lacks the enzymes to digest cellulose and hemicellulose. Intestinal digestibilities for protein fractions and amino acids that escape the rumen are 100% for protein fractions A, B1 and B2; 80% for protein fraction B3; and 0% for protein fraction C. Intestinal starch digestibility depends on type of grain, degree of processing, and level of intake above maintenance (Sniffen et al., 1992; Knowlton et ah, 1998). Ranges are provided for the user to adjust intestinal starch digestion for specific situations, based on observation of the faeces, and in adjusting inputs to account for differences between actual and predicted performance. For most conditions, the ranges suggested are based on studies with growing beef steers and lac-tating dairy cows consuming feed at two to three times maintenance level of intake
(Sniffen et al., 1992): whole maize, 30-50%; cracked maize, 50-70%; dry rolled maize, 70-80%; maize meal, 80-90%; whole high moisture maize, 80-90%; high moisture ground maize, 85-95%; steam flaked maize, 92-97%; dry rolled sorghum, 60-70%; dry ground sorghum, 70-80%; and steam flaked sorghum, 90-95%. Based on recent studies and a review of the literature (Knowlton et al., 1998), modifications of the above for high-producing dairy cows (above 45 kg milk) fed maize are: whole maize, 30-40%; cracked maize, 40-60%; maize meal, 70-90%; and rolled high moisture maize, 75-85%. Although little escapes digestion in the rumen, starch from processed small grains (wheat, barley, oats) has a high intestinal digestibility (over 90%).
A more mechanistic approach is needed that incorporates the integration of digestion and passage to predict intestinal digestion. However, the accuracy of prediction of intestinal pool sizes digested depends on the accuracy of prediction of ruminal flows, and therefore has second priority to prediction of ruminal fermentation, particularly since, with most feeds, over 75% of total tract digestion occurs in the rumen.
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