"Feeding behavior (FB) and dry matter intake (DMI) of ruminants has been linked to hepatic oxidation of fuels. The type and temporal pattern of fuels absorbed from the gastrointestinal tract of ruminants and available for oxidation are affected by diet starch concentration and fermentability. However, research considering the fuels produced from starch digestion is lacking for cows in the postpartum (PP) period. Our overall hypothesis is that fuels produced during digestion affect hepatic metabolism and FB differently. To investigate the effect of anaplerosis of the tricarboxylic acid (TCA) cycle on FB, experiments were conducted using cows in PP period. Propionic acid (PA, 99.5%) and glycerol (GL, 99.7%) were infused into the abomasum (AB) in isoenergetic (193 kcal/h) and isomolar (0.03035 mol/h) solutions using a double crossover design. Infusion of PA decreased DMI compared with GL 17 and 23%, by decreasing meal size 13 and 15% with isoenergetic and isomolar infusions respectively, and PA decreased total metabolizable energy intake (MEI, diet plus treatment infusions) compared with GL. Compared with PA, GL increased plasma glucose and insulin concentrations for isomolar infusions. Pulse-dose (2 moles) of PA into the AB in a crossover design decreased hepatic acetyl CoA (AcCoA) content whereas GL increased 32%, resulting in lower content for PA compared with GL at 30 min (18.0 vs. 36.9 nmol/g), which persisted at 60 min after dosing (21.9 vs. 32.8 nmol/g). These results are consistent with anaplerosis from PA stimulating oxidation of AcCoA in the liver and satiety compared with GL. To investigate the effect of fuels from starch digestion on FB, continuous isoenergetic AB infusions (150 kcal/h) of PA (99.5%, 0.41 mol/h), lactic acid (LA, 88.0%, 0.46 mol/h) or glucose (GLU, 99.9%, 0.22 mol/h) were studied with cows in PP period in a duplicated 4 x 4 Latin square design. A control (CON, no infusion) treatment was also included. Treatment solutions were infused at 500 mL/h for 22 h/d and provided 3.03033 Mcal/d. Compared with CON, PA decreased DMI by 24% (14.3 vs. 18.9 kg/d) and MEI by 13% (34.8 vs. 40.2 Mcal/d) by a tendency to decrease meal frequency; and LA decreased DMI by 14% (16.3 vs. 18.9 kg/d) by decreasing meal size 20%, but did not affect MEI. Glucose infusion did not affect DMI or MEI. While ruminal infusions of sodium acetate have little effect on DMI, infusion of acetic acid (AA) into the AB of cows in the PP period decreased DMI 64% compared with a sham control (Gualdrón-Duarte and Allen, unpublished). To understand the mechanism, we investigated the effects of continuous isomolar infusions (0.030375 mol/h; 0.5 L/h; 8 h) of AA, sodium acetate (NA) or CON (NaCl) into the rumen (RU) or into the AB on FB, DMI, and metabolic responses of cows in the PP period in a 6 x 6 Latin square design. Treatments were infused for the first 8 h following feeding with a rest day between infusion days. During infusions treatments decreased DMI 30% (4.3 kg vs. 6.1 kg) compared with CON by decreasing meal frequency but their effects differed; NA-RU did not reduce DMI (7.0 kg), whereas AA-RU (2.6 kg), AA-AB (3.7 kg) and NA-AB (4.0 kg) decreased DMI compared with CON. Contrary to the other treatments, NA-RU did not decrease DMI compared with CON, but the reason cannot be determined from the results available from this study. Collectively, these experiments provide strong evidence that anaplerotic metabolites for the TCA cycle decreased DMI and MEI on cows in the PP period consistent with their expected effects on hepatic oxidation. Depression of feed intake by diets including highly fermentable starch is likely because of the ability of propionate to stimulate hepatic oxidation."--Pages ii-iii.
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