EFFECT OF THE THERMAL ENVIRONMENT ON DIGESTION IN RUMINANTS

Abstract

Literature relating to the influence of temperature on digestibility in ruminants has been reviewed. Decreases in digestibility of certain diets induced by exposing animals to low environmental temperatures are related to changes in the rate of passage of digesta through the gastrointestinal tract. The increased rates of passage are due to enhanced gastrointestinal motility and likely, for some diets, to enhanced rumination activity. Opposite changes in these digestive parameters appear to occur in response to heat stress. The digestibility of 100% concentrate diets appears not to be temperature dependent. Digestion of ground and pelleted forage diets is more consistently affected by temperature than chopped forages and the response appears to differ between forage plant species. The effects on digestion influence the availability of dietary energy to a greater extent than protein. For some diets cold exposure results in increased urea nitrogen recycling to the rumen, increased efficiency of rumen microbial synthesis and reduced rumen degradation of dietary protein; responses which help to favor the nitrogen economy of the animal even though digestible energy is reduced. The increased rate of passage of digesta residues in the cold may help to support increases in the voluntary intake of diets offered ad libitum. Key words: Digestion, ruminants, temperature, environment