Modeling glucose metabolism in the dairy cow—A comparison of two dynamic models

Abstract

In this paper, we examine the behavior of two dairy cow models, with particular reference to glucose metabolism, when challenged with different substrate mixtures. The body sub-models of the two dairy cow models Molly [Baldwin, R.L., 1995. Modeling Ruminant Digestion and Metabolism. Chapman & Hall, London] and Karoline [Danfær, A., Huhtanen, P., Udén, P., Sveinbjörnsson J., Volden, H., 2006. Karoline—a Nordic cow model for feed evaluation-model description. In: Kebreab, E., Dijkstra, J., Bannink, A., Gerrits, W.J.J., France, J. (Eds.), Nutrient Digestion and Utilisation in Farm Animals: Modelling Approaches. CAB Int., Wallingford, UK, pp. 407–415] were compared. The Karoline gut sub-model was used to generate substrate uptakes for both body sub-models. For these simulations, we used lactation week 25 and a body weight of 614 kg. The volatile fatty acid (VFA) patterns in Karoline were based on equations derived from a Nordic database [Sveinbjörnsson, J., Huhtanen, P. Udén, P., 2006. The Nordic dairy cow model Karoline—development of VFA sub-model. In: Kebreab, E., Dijkstra, J., Bannink, A., Gerrits, W.J.J., France, J. (Eds.), Nutrient Digestion and Utilisation in Farm Animals: Modelling Approaches. CAB Int., Wallingford, UK, pp. 1–14] and differed from Molly by giving considerably less propionate from sugars, starch and hemicellulose. In Test 1, we simulated two basal diets (∼17 kg dry matter/d), consisting of grass silage with either 500 (C500) or 700 g (C700) barley/wheat-based concentrate/kg diet dry matter. The VFA molar proportions generated by Karoline differed only slightly between the two diets and were approximately 0.660 acetate, 0.200 propionate and 0.130 butyrate. Milk fat and protein synthesis in Molly were regulated to achieve similar milk fat and protein levels as to the corresponding treatments in Karoline and energy balances were regulated to approximately zero in both models by changing lactose synthesis. Results showed a 5–7% higher lactose production in Karoline and slightly positive protein and lipid balances. Molly, however, showed a negative protein balance and positive adipose triglyceride, acetate and long-chain fatty acid (LCFA) balances. Approximately 14% more propionate, glucose and lactate were oxidized in Molly, which resulted in more gluconeogenesis from amino acids, compared to Karoline. In Test 2, we tried to rectify the negative protein and positive acetate and fatty acids balances in Molly by increasing propionate and glucose absorption. The C700 propionate inputs for Molly were increased by 40% (C700P) at the expense of acetate and butyrate, giving a VFA proportion of 0.310 for propionate. In addition, C700P was also altered to give three times more glucose at the expense of VFA (C700PG), resulting in a change in glucose uptake from 12.4 to 37.1 mol C/d (equivalent to 355 and 1064 g of starch). Total carbon absorption was identical for all the treatments and milk fat and protein levels were regulated to be similar to C700. These changes resulted in approximately 18% more lactose produced from the manipulated substrates, compared with C700. Amino acid catabolism was simultaneously reduced by 19 and 29% in C700P and C700PG, respectively, and protein balances became slightly positive. Acetate, free LCFA and adipose LCFA balances were reduced from highly positive to nearly zero. It is concluded that Molly requires a higher uptake of glucose or glucose precursors than Karoline to behave sensibly.