The concept that feed intake is controlled by hepatic oxidation of fuels has developed over the last several decades, mostly with laboratory species. Propionate has been implicated in the control of feed intake in ruminants, and the mechanism is consistent with simulation of hepatic oxidation. Previous research in our laboratory has demonstrated that intraruminal propionate infusion, compared with iso-osmotic acetate infusion, is more hypophagic in early lactation than in mid-lactation dairy cows. Three experiments were conducted to investigate the mechanisms related to the control of feed intake in early lactation dairy cows. The first experiment was a randomized block design with 30 lactating Holstein cows from 3 - 40 days in milk (DIM) to evaluate the extent of hypophagia from intraruminal propionate infusion for cows in early lactation. Intraruminal propionate infusion, compared with iso-osmotic acetate infusion, depressed dry matter intake (DMI) 20%. There was an interaction between treatment and DMI such that propionate was increasingly hypophagic for cows with greater hepatic acetyl CoA concentration. The second experiment evaluated longer-term infusions over 3 d to identify adaptive mechanisms using 12 lactating Holstein cows from 2 - 13 DIM in a cross-over design experiment. Propionate infusion decreased DMI over the 3-d infusion period, however a treatment by period interaction was detected. During period 1, propionate infusion, compared to acetate infusion, decreased DMI by 18% by decreasing meal size by 21%. Similar to the results of the first experiment, propionate was more hypophagic than acetate for cows with greater hepatic acetyl CoA concentration. However, there was no treatment by day interaction for DMI suggesting that cows did not adapt to intraruminal propionate over the 3 d infusion period. The third experiment was designed to evaluate the effects of intravenous lipid infusion on feed intake response to intraruminal propionic acid infusion by lactating cows. Eight lactating Holstein cows past-peak lactation were used in a replicated 4x4 Latin square experiment with a 2x2 factorial arrangement of treatments. Lipid infusion, compared to a saline control, did not affect plasma non-esterified fatty acid (NEFA) and β-hydroxybutyrate (BHBA) concentrations, but did increase hepatic acetyl CoA concentration. Intraruminal propionate infusion, compared to a sham control, decreased DMI 15%, but lipid did not affect DMI. There was no interaction between lipid and propionate for DMI, likely because lipid infusion failed to mimic the metabolite concentrations of cows in a lipolytic state. Collectively, these experiments provide strong evidence that propionate is more hypophagic for cows in a lipolytic state. Additional research is needed to evaluate the longer-term effects of propionate and its application to diet formulation for cows in the postpartum period.
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