Seasonality of food intake in ruminants: recent developments in understanding.

Domestic ruminants are used to exploit many vegetation resources that would otherwise be unproductive. For maximal effectiveness, there is a need to understand underlying mechanisms controlling animal performance, including seasonal variations in appetite and food intake. Potentially useful experimental approaches, recent findings and aspects for future study are discussed. Seasonal variation in intake is expressed through changes in the pattern of meals (duration, frequency, inter-meal interval and ingestion rate). These changes are signalled through alterations in both structure and function of the gastrointestinal tract and physiological signals. Studies suggest that multiple, interactive signals are involved, including hormones such as cholecystokinin, insulin, leptin and triiodothyronine. However, baseline concentrations in the peripheral circulation are not appropriate measurements of some of these hormones since there can be seasonal differences in postprandial profiles or changes in rate of dilution in the bloodstream or in the rate of degradation in the liver. Interactions between these circulating signals, liver function and neural signals to the brain need clarification. Systemic nutritional signals also act directly in the brain where they are integrated with seasonal photoperiod (melatonin) signalling within the hypothalamus. Melatonin target sites critical to appetite regulation have still to be identified, but leptin receptors and downstream neuropeptides have been localised within the ovine hypothalamus. These orexigenic and anorexigenic 'compensatory' pathways are sensitive to imposed changes in nutritional status but, with the exception perhaps of cocaine- and amphetamine-regulated transcript, do not appear to drive seasonal 'anticipatory' changes in intake. Mechanisms underlying seasonal changes in hypothalamic sensitivity to nutritional feedback clearly deserve further study.