Effect of underfeeding on digestion in cows. Interaction with rumen degradable N supply

Abstract

The effect of underfeeding on digestion was studied in cows fitted with ruminal cannulas. Four cows in a Latin square design received four diets made up of 60% hay, 30% straw and 10% maize. They were given either 80% or 27% of maintenance energy requirements, with or without urea to compensate for the shortage of rumen degradable N in the rumen. Contrary to the trend observed at higher levels of intake, the decreased intake resulted in a decrease in digestibility of OM (59.1% vs. 53.6%) and fibre. Neither N supply nor the interaction between intake and N supply significantly modified digestibility. The fall in digestibility could not be explained by retention time of particles in the rumen or by a disturbance of feeding behaviour, and in situ measurements did not reveal any impairment of theoretical DM degradability (56.2% vs. 55.0% for high and low intakes, respectively). Ruminal ammonia and non-protein non-ammonia N concentrations, and plasma urea concentration were decreased at low intake and/or low N supply, but there was no evidence that shortage of N compounds for ruminal microbes is responsible for decreased ruminal degradation at low intake. Further research has to be done in order to explain such disturbances in digestibility at very low intake.