Photoperiodicity in cattle. I. Seasonal changes in coat character and their importance in heat regulation

Abstract

The seasonal changes in coat character of a control group of four young Poll Shorthorn cattle have been studied and compared with those of a similar group in which the daily photoperiod was altered to simulate the synchronous duration of daily lighting of the opposite (northern) hemisphere. Heat tolerance tests were conducted periodically as the animals assumed different coat types. The results show that the full range of coat changes which characterize well-nourished cattle of the type used in this experiment may, irrespective of seasonal temperature, be reversed by artificially reversing the seasonal trend of daylight duration. This is presumptive evidence that the natural light environment is a major controlling factor in the normal pattern of seasonal coat change which cattle of European origin exhibit. Cattle with sleek, summer-type coats have shown decisive heat tolerance superiority over those with deep, winter-type coats under the conditions of this experiment. Clipping tests indicate that it is the heavy coat itself which lowers heat tolerance, not a particular internal condition (of metabolism, for example) with which heavy-coatedness might be associated.