The objective of this study was to determine the temporal effects of intraruminal infusion of propionic acid at the initiation of meals on feeding behavior of cows in the postpartum period. Propionic acid derived from ruminal fermentation can reduce energy intake of dairy cows. The suppression of appetite by propionic acid is likely caused by a signal related to the hepatic oxidation of fuels. Greater propionate flux to the liver is expected to result in faster oxidation of acetyl coenzyme A, which can stimulate satiety and reduce feed intake. Therefore, the rate of propionate supply to the liver, within the timeframe of meals, might be an important limitation to feed intake. Our hypothesis was that faster rate of propionate infusion during meals would decrease meal size and feed intake by decreasing the time required to stimulate satiety within a meal. Six ruminally cannulated, multiparous Holstein cows in the postpartum period were used in a duplicated 3 × 3 Latin square design experiment balanced for carryover effects. Treatments included control (no infusion) or 1.25 mol of propionic acid infused over 5 min (FST) or 15 min (SLW) at each meal. Infusions were initiated at the conditioned meal at feeding (1200 h) and were triggered at each spontaneous meal for 22 h. Contrary to our hypothesis, SLW decreased meal size 29% (0.87 vs. 1.23 kg of dry matter) compared with FST, and FST decreased meal frequency 27% (8.5 vs. 11.2 per d) compared with SLW. Dry matter intake was similar between FST and SLW, but propionic acid decreased dry matter intake 46% compared with control. A potential explanation is that FST resulted in greater liver bypass of propionate compared with SLW, extending anaplerosis of the tricarboxylic acid cycle, hepatic oxidation of acetyl coenzyme A, and satiety over a longer time after meals.

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A faster rate of infusion of propionic acid into the rumen of cows in the postpartum period increased meal size compared with a slower rate of infusion in a previous experiment. Because propionate is anaplerotic and stimulates oxidation of acetyl coenzyme A (CoA) in the liver, and hepatic oxidation has been linked to satiety, this result was opposite to our expected response. We then hypothesized that the faster rate of infusion might have saturated the pathway for propionate metabolism in hepatocytes resulting in lower first-pass extraction by the liver. Because we were measuring feeding behavior, we could not sample blood and liver tissue over time in that experiment. Therefore, to determine the temporal effects of propionic acid (PA) infusion on hepatic metabolism and plasma metabolites over the time course of a meal, we infused 1.25 mol of PA (2.5 L of 0.5M PA) over 5 min (FST) or 15 min (SLW) into the rumen. We evaluated response to PA infusions both before feeding, when ruminal PA production by rumen microbes is lower and hepatic acetyl CoA concentration is greater, and 4 h after feeding, when PA production is greater and hepatic acetyl CoA concentration is lower. Blood and liver samples were collected before, and after 5, 15, and 30 min of infusion. Contrary to our hypothesis, the rate of PA infusion into the rumen did not affect plasma propionate concentration, indicating the FST effects on feeding behavior were not because of a limitation on propionate uptake by the liver. However, FST increased plasma glucose and insulin concentrations faster than SLW, resulting in a reduction in plasma nonesterified fatty acid concentration during the time frame of meals. Decreased plasma nonesterified fatty acid concentration during infusion likely decreased the supply of acetyl CoA for oxidation in the liver. The FST treatment also increased fumarate concentration at 5 min after the initiation of infusion but did not affect oxaloacetate concentration compared with SLW, consistent with a limitation to propionate metabolism at that reaction. A metabolic bottleneck at the malate dehydrogenase reaction for FST compared with SLW would further contribute to a reduction in hepatic oxidation within the time frame of a meal, allowing greater meal size, consistent with the hepatic oxidation theory and our previous results.

The objective of this study was to determine the effects of 2 amounts of propionic acid (PA) infused intraruminally at 2 rates of infusion at the initiation of meals on the feeding behavior of Holstein cows in the postpartum period. We hypothesized that the amount and length of time of infusions would interact to affect feeding behavior: rapid infusion of a higher dose of PA would result in larger meal size with greater time between meals compared with a slower rate, whereas faster infusion of a lower dose of PA would reduce meal size and the time between meals compared with a slower infusion of the same dose. Eight ruminally cannulated, multiparous Holstein cows were used in a 4 × 4 Latin square design experiment. Cows were blocked by parturition date and randomly assigned to treatment sequence within square. Treatments were infusion of 2.5 L of 0.5 M (HI) or 0.2 M (LO) solutions of PA at initiation of meals over 5 min (FST) or 15 min (SLW) for 12 h following feed delivery. Contrary to our hypothesis, no interaction between amount and rate of infusion was detected for any feeding behavior parameter measured. The FST treatments did not affect dry matter intake or metabolizable energy intake compared with SLW. The FST treatments tended to increase meal length compared with SLW (28.1 vs. 22.7) but did not affect meal size. The FST treatments tended to decrease total eating time (108 vs. 122 min/12 h) but did not affect meal frequency compared with SLW. The HI treatments decreased dry matter intake (7.4 vs. 11.5 kg/12 h) and total metabolizable energy intake (22.5 vs. 29.1 Mcal/12 h) compared with LO by decreasing meal frequency (5.8 vs. 7.5 meals/12 h). The HI treatments decreased eating time (103 vs. 127 min/12 h) compared with LO but did not affect meal size. Further research is warranted on the effects of the temporal supply of propionate on propionate metabolism and feeding behavior.

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