Restricting time at pasture: Effects on dairy cow herbage intake, foraging behavior, hunger-related hormones, and metabolite concentration during the first grazing session

Abstract

This study investigated the effect of restricting grazing time on circulating concentrations of ghrelin, nonesterified fatty acids (NEFA), and glucose before, and foraging behavior of dairy cows during, the first grazing session of the day (GS, 0800–1200h). Forty-eight Holstein-Friesian cows (470±47kg of BW; 35±9 d in milk) were strip-grazed on a perennial ryegrass pasture for either 4h after each milking (2×4), 8h between milkings (1×8), or the 24-h period excluding milking times (CTL). Cows were bled before the GS; plasma was analyzed for ghrelin and serum for glucose and NEFA. Herbage mass was measured pregrazing (0730h), during and at the end of the GS (1200h), and postgrazing (24h after the first measurement). Herbage mass data were fitted to a model to estimate herbage disappearance rates. Herbage intake and bite mass were calculated using herbage mass disappearance and behavioral measurements. Bite rate, eating, searching, ruminating, and idling time were determined during the GS for each cow. No difference in glucose concentration was found between treatments. Concentrations of NEFA and ghrelin were the greatest for cows in the 1×8 treatment. Daily herbage intake did not differ between treatments; however, during the GS 1×8 had a greater herbage intake than 2×4 and CTL. Bite mass differed between treatments and throughout the GS. Bite mass was smallest for CTL during the first 60min and greatest during the last 90min, when cows in the 2×4 treatment had the smallest bite mass. Cows in 1×8 spent the longest time eating and the least time searching and ruminating. Eating time was greatest for 1×8 during the first 60 and last 90min of the GS. Searching time only differed in the second 60min, when it was the lowest for 1×8. Cows from all treatments did not ruminate during the first 120min. Cows in CTL had the greatest rumination time during the last 90min. The model fitted to represent dynamics of herbage mass disappearance presented differences in the fractional herbage disappearance rate. There was an interaction between treatment and time in herbage depletion rate. The results of this study present a fuller picture of foraging dynamics during the first 4h of grazing and its potential relationship with physiological markers of hunger as affected by grazing management.