The control of feed intake during the transition period of dairy cows is not completely understood, but research has shown that greater fermentability of starch and ruminal infusions of propionic acid (PA) can decrease intake of cows in early, mid- and late-lactation by decreasing size or frequency of meals. Evidence suggests that changes in feeding behavior (FB) are from the ability of PA to stimulate the oxidation of fuels likely increasing the energy status of the liver and stimulating satiety. Although the hypophagic effects of PA are well documented, the temporal effects of PA supply to the liver within the time frame of a meal is still unknown. To determine the temporal effects of PA infusion at the initiation of meals on the FB of cows in the postpartum (PP) period, we infused 1.25 mol of PA over 5 min (FST) or 15 min (SLW) into the rumen at each meal. Infusion of PA decreased DMI 45% compared with control with similar effects for FST and SLW treatments (10.4 vs. 10.0 kg DM). However, SLW decreased meal size 29% compared with FST (1.23 vs. 0.87 kg DM) and FST decreased meal frequency 24% compared with SLW (8.5 vs. 11.2 meals/day). Meal length was not affected by treatment (23.0 min), but PA increased intermeal interval compared with control (88.9 vs. 120.9 min) and FST increased intermeal interval 35% compared with SLW (139 vs. 103 min). To investigate if the temporal effects of PA infusion vary according to amount of PA infused, we conducted an experiment infusing 2.5 L of either 0.5M (HI) or 0.2M (LO) solutions of PA at initiation of meals over FST or SLW for 12 h following feeding. No interactions of treatments were detected for any FB parameter measured. FST did not affect DMI but tended to increase meal length (28.1 vs. 22.7) and decrease total eating time (108 vs. 122 min/12 h) compared with SLW. HI decreased DMI (7.4 vs. 11.5 kg/12 h) compared with LO by decreasing meal frequency (5.8 vs. 7.5 meals/12 h). The lack of effect of infusion rate on meal size, along with the reduction in DMI by HI compared with LO, by decreasing meal frequency rather than meal size, suggested that propionate flux to the liver might have exceeded the hepatic capacity for PA uptake from the blood, likely extending hepatic oxidation longer after meals. However, we could not sample liver and blood during meals without interfering with FB. Therefore, to determine the temporal effects of PA infusion on hepatic metabolism, we infused 1.25 mol of PA either FST or SLW, before feeding and after feeding. Blood and liver samples were collected before, and after 5, 15 and 30 minutes following the start of infusions. Our results indicated that FST did not reduce the efficiency of extraction of propionate by the liver compared with SLW. However, FST increased plasma glucose and insulin concentrations and decreased plasma NEFA concentration compared with SLW. Decreased NEFA concentration during infusion likely decreased the supply of acetyl CoA for oxidation in the liver. In addition, propionate from FST likely did not result in greater TCA cycling because it entered the gluconeogenesis pathway increasing glucose concentration faster than SLW, so less malate was likely available to be converted to oxaloacetate and continue the cycle. Reduced oxidation of acetyl CoA within a meal likely explains the greater meal size for FST compared with SLW, consistent with the hepatic oxidation theory and our FB results.Although rate of infusion can affect FB, these experiments show that the hypophagic effects of PA during the PP period are primarily from the amount of PA infused. Further, the reduction in DMI by increasing fermentability of starch is likely from greater, rather than faster PA production. We expect that this research will help nutritionists to better understand the complex mechanisms that control feed intake in the PP period.
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