Neuroendocrine regulation of dry matter intake in grazing dairy cows

Abstract

Supplementary feeds are offered to grazing dairy cows to increase dry matter and metabolizable energy intakes; however, offering feed supplements reduces pasture dry matter intake, a phenomenon known as substitution. The objective of this research was to understand variations in grazing behaviour in pasture-fed cows and the effects of supplementation on grazing time and feeding intake rate throughout the day and to investigate humoral profiles of factors known to be associated with intake regulation in monogastric species and quantify their role in ruminant species. Grazing occurred predominately during daylight hours, with minimal grazing during the hours of darkness. Distinct grazing bouts were evident post sunrise and pre-sunset. Supplementation reduced time spent grazing; however, this was an accumulation of reduced grazing time throughout the day and was not restricted to the period following the consumption of supplement, as fundamentally, the profile of grazing behaviour in supplemented cows followed the same pattern as unsupplemented cows. The effects of supplementation on time spent grazing differed depending on the time of day. Time spent grazing linearly reduced with increasing supplement in the a.m., whereas, time spent grazing was unaffected by supplementation during the pre-sunset grazing bout, irrespective of supplement level or timing of sunset. The differences in grazing behaviour during the major post-sunrise and pre-sunset grazing events lead to the hypothesis that different factors regulate dry matter intake at these times. In the a.m., products of digestion and associated physiological factors regulate grazing behaviour. Whereas, in the p.m., environmental cues (i.e. sunset) override physiological signals that regulate grazing behaviour in the a.m. to ensure maximal grazing occurs prior to darkness, irrespective of supplementation or energy balance status. Humoral profiles of factors implicated in intake regulation in monogastric species were similar in the dairy cow. Humoral factors associated with a fasted or pre-prandial state were elevated and declined after meal initiation, whereas, factors indicating a change from a negative to a positive energy state increased after meal initiation. Despite the similar humoral profiles, the profile of plasma ghrelin during the major p.m. feeding event differed from its reported decrease in concentration after feeding, establishing a unique profile for ghrelin. Plasma ghrelin increased in the p.m. despite intensive grazing/feeding and cows being in a positive energy state prior to the p.m. feeding event, which had not been previously reported in ruminant species. The increase in ghrelin was coincident with an increase in the intensity of grazing/feeding that lead to the hypothesis that ghrelin increases in diurnal species ensuring animals maximise dry matter intake prior to darkness, which is a major environmental cue to cease grazing/feeding.