A model to describe ruminal metabolism and intestinal absorption of long chain fatty acids

A sub-model developed within the structure of the Cornell-Penn-Miner Dairy System (CPM-Dairy) predicts ruminal metabolism and intestinal absorption of fat in dairy cows. Data from 36 dietary treatments in eight published experiments that reported intakes and flows (g per day) of long chain fatty acids (LCFA) to the duodenum and feces were used to estimate rate constants and develop equations in the model. Similar data on 36 diets from another eight published papers were used to validate the model. Fat from each feed ingredient is specified in terms of its content of the major lipid components: glycerol, pigment and 10 LCFA (C12:0, C14:0, C16:0, C16:1, C18:0, C18: 1c, C18: It, C18:2, C18:3 and 'COther'). Dietary fat is assumed to exist mainly in the form of triglycerides. In the rumen, fat is either lipolysed (defined as enzymatic cleavage of ester linkages and dissociation of salts of fatty acids) or escapes the rumen to the small intestine in the non-lipolysed form. Estimated rates of lipolysis are presented for fats from 27 different feeds. The lipolysis rates for fat from corn silage, alfalfa silage, ground corn, alfalfa hay, ground-roasted soybeans, hydrogenated tallow and calcium salts of palm fatty acid distillate (Ca-PFAD) were 500, 500, 309, 65, 35, 18 and 6.3%/h, respectively. Ca-PFAD was the only ingredient that showed appreciable protection against lipolysis. Lipolysed LCFA undergo biohydrogenation in a stepwise process: C18:3 --> C18:2 --> C18:1t --> C18:0; C18:1c --> C18:0; C16:1 --> C16:0. At each step, the specific LCFA either undergoes biohydrogenation or passes out of the rumen. The estimated mean biohydrogenation rates for C16:1, C18: 1c, C18:1t, C18:2 and C18:3 were 39.3, 27.4, 22.8, 87.6 and 244%/h, respectively. In addition, within the rumen, small amounts of C16:0, C16:1, C18:0 and COther LCFA are produced de novo from dietary carbohydrate. Specific intestinal digestion coefficients are used to describe absorption of each of the 10 major LCFA in both the lipolysed and non-lipolysed forms. Digestion coefficients for most LCFA were generally in the range 0.7-0.9. For total LCFA and for the majority of individual LCFA, the model showed good concordance between measured and predicted duodenal flows and also for measured and predicted intestinal absorption. The quantity of specific LCFA absorbed from the intestines can influence milk fat percentage and fertility. This model allows nutritionists to more accurately formulate dairy cattle rations in terms of fat, and to examine productivity in terms of individual LCFA. (C) 2004 Elsevier B.V. All rights reserved.