Effects of rumen fill on short-term ingestive behavior and circulating concentrations of ghrelin, insulin, and glucose of dairy cows foraging vegetative micro-swards

The objective of this study was to investigate changes in foraging behavior, hunger-related hormones, and metabolites of dairy cows in response to short-term variations in rumen fill (RF). The effect of RF on intake rate, jaw movements, bite rate and dimensions, and concentrations of plasma ghrelin, and serum insulin and glucose were measured in 4 rumen-cannulated lactating dairy cows (612 +/- 68 kg, empty live weight; 237 +/- 29d in milk) foraging micro-swards of vegetative orchardgrass (Dactylis glomerata L.). The treatments compared were the removal of different proportions of total rumen contents: 1.00 (RF0), 0.66 (RF33), 0.33 (RF66), or 0 (RF100). Treatments were randomly applied 2 h before foraging sessions in a 4 x 4 Latin square design. Micro-swards were weighed before and after foraging sessions. Cows were allowed to take a maximum of 15 bites with no time restriction. Eating time, intake rate, total jaw movements, and bite mass, depth, area, and rate were determined. Plasma was analyzed for ghrelin and serum for insulin and glucose immediately before and 2 h after the treatments were applied. Intake rate, bite mass, and bite area increased, whereas bite depth decreased as RF decreased. The RF did not affect bite rate or total jaw movements. Decreasing RF resulted in increased plasma concentrations of ghrelin and tended to increase serum insulin, with reduced concentrations of serum glucose. Incremental variation in plasma ghrelin and serum insulin correlated with bite depth and mass, whereas changes in serum glucose correlated with intake rate, bite area, depth and mass, as well as with herbage intake per jaw movement. The present study elucidates some of the underlying endocrine physiology of cattle with short-term temporal variations of RF and their effects on some components of foraging behavior.