Diurnal patterns of grazing behavior and humoral factors in supplemented dairy cows

Offering feed supplements to grazing dairy cows results in a reduction in grazing time. However, the effect differs depending on the time of day that feeds are offered. To understand the physiological basis for this, associations among circulating factors known to be associated with intake regulation in monogastric species and grazing behavior in the dairy cow were investigated. Seventeen multiparous cows at 28 +/- 5 d in milk grazed together and consumed 4.4 kg of dry matter/d of a pelleted concentrate feed supplement, equally split, at the a.m. and p.m. milking. Grazing behavior was recorded over 4 consecutive days in all 17 cows. Blood was sampled from 10 of the 17 cows every 4 h over a 48-h period following the grazing behavior measurements; sampling times were staggered by 2 h to provide a diurnal profile of humoral factors. Grazing profiles illustrated major grazing bouts after the a.m. and p.m. milking; however, the p.m. grazing bout was characterized as the most intensive and time spent grazing was unaffected by supplementation. Associations among proportion of cows grazing and circulating hormones and metabolites differed throughout the day. During the a.m., relationships were consistent with those reported in monogastric species, with ghrelin and nonesterified fatty acids decreasing and insulin increasing with feeding. In comparison, during the major grazing bout predusk, ghrelin concentrations increased until sunset, despite the large proportion of cows grazing, before declining; this is consistent with ghrelin stimulating the predusk grazing bout. Results indicate that humoral factors known to affect hunger and satiety in monogastric animals may also have a potential role in the physiological regulation of diurnal and feeding behavior in ruminants.