Effect of drastic undernutrition on digestion in Zebu cattle receiving a diet based on rice straw

The effect of drastic undernutrition was studied in Bos indicus cows. Four nonlactating cows (body weight = 208 kg) fitted with ruminal cannulas were used in a crossover design. They received, in two 1-mo periods, a diet of 8 0% rice straw and 20% cottonseed meal fed either at energy maintenance [3.9 1 kg of dry matter (DM)/d] or at one-third of this intake (1.30 kg of DM/d) . The variation in digestibility was studied in relation to particle retent ion time and microbial events. Organic matter digestibility decreased with underfeeding (64.5 and 53.7%, respectively, at high and low intakes) withou t modification of mean ruminal or total tract retention times. This lower d igestibility can be related to a decrease in protozoal population in the ru men; however, other characteristics of microbial activity did not vary, suc h as in situ DM degradability (41.9 and 43.3%, respectively, at high and lo w intakes) and ruminal soluble Ca concentration, a factor involved in the a ttachment of microorganisms to feed particles, Mean size of luminal particl es was not modified, although more time spent chewing was observed at the l ow intake (165 and 221 min/kg of DM intake, respectively, at high and low i ntakes). This experiment confirms that, at intakes below maintenance, diges tibility can decrease when feed intake is restricted, contrary to the gener al view described in the literature.