Using food quality and retention time to predict digestion efficiency in geese

Investigations of food digestibility are important in nutritional studies of herbivores, but accurate assessments in the field are usually difficult and time consuming. This study explored the possibility of predicting digestibility and metabolisability of organic matter from the chemical composition and retention time of food in barnacle geese Branta leucopsis. Captive birds were used in indoor trials to allow accurate assessments of digestion in terms of energy, organic matter and nutritional components. Retention times were estimated from dropping rates which were recorded by an electronic device. The regression models that best explained the variation in apparent digestibility and metabolisability of organic matter incorporated the proportion of cell walls in the food (acid detergent fibre had a negative effect) and the retention time (longer retention had a positive effect). To test the predictive power of the regression model, we performed seven field trials with captive barnacle geese on Festuca rubra salt marshes. The regression model proved reliable except when salt concentrations in the food were high following inundation by spring tides. The results obtained from this study further demonstrated its value in estimating the individual digestibility of each plant species in mixed diets. This is illustrated with reference to a study of food selection by brent geese Branta bernicla.