PRODUCTION RESEARCH PAPERS Net Metabolism of Volatile Fatty Acids, D-(/-Hydroxybutyrate, Nonesterified Fatty Acids, and Blood Gasses by Portal-Drained Viscera and Liver of Lactating Holstein Cows 1'2 CHRISTOPHER K. REYNOLDS 3 Department of Animal Science University of Maryland College Park 20742 GERALD B. HUNTINGTON, HENRY F. TYRRELL, and PAUL J. REYNOLDS 4 USDA, ARS Livestock and Poultry Sciences Institute Ruminant Nutrition Laboratory Beltsville, MD 20705 ABSTRACT Net flux of VFA, D-~-hydroxybutyr- ate, nonesterified fatty acids, and blood gasses across portal-drained viscera and liver was measured in four lactating Holstein cows fed a 60:40 corn silage: concentrate diet ad libitum and milk- ed at 12-h intervals. Twelve consecutive hourly measurements of net flux (venous- arterial concentration difference times blood flow) were obtained during wk 4 and 8 postpartum for each cow. Milk yield and DM intake averaged 32.2 and 15.6 kg/d. On a net basis, hepatic tissues produced acetate and removed 63 to 101% of other VFA absorbed by portal- drained viscera. Hepatic and portal- drained visceral tissues produced 60 and 40%, respectively, of D-~-hydroxybu- tyrate produced by splanchnic tissues. Hepatic tissues removed 9.3% of non- esterified fatty acids in portal vein and hepatic arterial blood. Oxygen use was greater by liver than for portal-drained viscera (3062 vs. 2394 mmol/h). Net Received January 1, 1988. Accepted April 19, 1988. Mention of a trade name, proprietary product, or specific equipment does not constitute a guarantee or warranty by the US Department of Agriculture and does not imply its approval to the exclusion of other products that may be suitable. 2Scientific Article Number A-4731, Contribution Number 7727 of the Maryland Agricultural Experi- ment Station, Department of Animal Sciences. 3Present address: Ruminant Nutrition Laboratory, USDA, ARS, BARC-East, Beltsville, MD 20705. 4 Deceased. portal-drained visceral flux of VFA, D- ~-hydroxybutyrate, a-amino nitrogen, L- lactate, and oxygen together accounted for 84.9% of calculated metabolizable energy intake. Net hepatic removal of propionate, L-lactate, and 0l-amino nitro- gen maximally accounted for 55.1, 17.4, and 16.5% of carbon in glucose produced by hepatic tissues. INTRODUCTION The absorption of fermentative end products from the gut is of central importance to the energetic nutrition of the lactating dairy cow. Volatile fatty acids represented 69% of net energy absorption by portal-drained viscera (PDV) of a lactating dairy cow (2). Acetate and propionate are the predominant VFA absorbed; each represents 30% of net PDV energy ab- sorption in lactating Holstein cows (17). Postabsorptive metabolism of VFA by PDV is extensive in sheep (4); therefore, measurements of net PDV absorption describe amounts of VFA available for metabolism by hepatic and other body tissues, not amounts truly absorbed. The proportionally high metabolic activity of PDV tissues is reflected by amounts of total body oxygen (O2) consumption attributable to PDV of lactating Holstein cows (9). In the liver, VFA metabolism is integrated with glucose, ketone body, and nonesterified fatty acid (NEFA) metabolism. In lactating dairy cows, 93% of propionate absorbed by PDV is removed by hepatic tissues and it is a predominant glucose precursor (12). Like PDV, energetic requirements and 02 consumption of hepatic tissues are high relative to other body tissues of lactating dairy cows (21). 1988 J Dairy Sci 71:2395-2405 2395