Trans fatty acids: Rumen in vitro production and their subsequent metabolic effects on energy metabolism and endocrine responses in the lactating dairy cow

Milk fat depression can be induced by dietary trans-18:1 fatty acids (FA) or conditions that lead to the abnormal accumulation of trans-18:1 in the rumen of lactating dairy cows. Several substrates and two pH (5.8 vs. 6.8) were tested using an in vitro rumen system. Low PH increased the accumulation of trans-18:1 in high grain diets (80% corn, 20% alfalfa). Diets supplemented with fish meal decreased the hydrogenation of trans-18:1 to 18:0. Results indicate that the last step of biohydrogenation is pH and substrate dependent.;The metabolic effects of trans-18:1 in vivo were studied in six multiparous lactating Holstein cows in midlactation. Cows were abomasally infused with fat mixtures high in cis-18:1 (iCIS), high in trans-18:1 isomers (iTRS) or no infusion in a 3 x 3 latin square design. On week 4 of each experimental period, energy balance measurements were made by indirect calorimetry. Fat infusion increased apparent digestibility of dry matter, energy and acid detergent fiber by 1 to 4 percentage units, and metabolizable energy and net energy of lactation of the diet by 11 and 15%. Milk fat percentage and yield were higher when cows were infused with iCIS than with iTRS (4.12%, 1.41 kg/d vs. 3.15%, 1.06 kg/d). The finding of lower yield of milk fat, higher milk content of long chain FA, particularly trans-18:1, and higher citrate when cows received iTRS indicated depressed milk fat synthesis. Infusion of iCIS and iTRS increased milk production but only iCIS increased milk energy output.;Hormone responses to arginine and thyrotropin releasing hormone (TRH) were studied on days 10 and 12 after initiation of fat infusion in each period. Hormone responses to TRH challenge were not differentially affected by predominance of cis- or trans-18:1. Because fat infusion accentuated end organ response in thyroid axis to TRH challenge, the influence of FA (0 to 1 mM) on the in vitro hepatic 5...