Digestive function in Australian magpie geese (Anseranas semipalmata)

Abstract

The Australian magpie goose (Anseranas semipalmata) is not really a ‘goose’ but an aberrant species representing the monotypic family Anseranatidae. It is herbivorous but its ability to utilise dietary fibre is uncertain. We examined digestive processes in tame birds fed natural forages in metabolism cages and in wild birds. An examination of the gross anatomy of the gut showed features similar to those in waterfowl of the family Anatidae, the true ducks and geese.

In a total-collection feeding trial geese were fed either unhusked rice grain or fresh aquatic grass. The aquatic grass was high in fibre (neutral detergent fibre (NDF) was 74% of dry matter) and magpie geese could not maintain energy or nitrogen balance on this feed. The maintenance energy requirement of the caged magpie geese, as estimated on the rice diet, was 573 kJ kg^–1 day^–1, which was similar to that found for other species of geese. The maintenance nitrogen requirement was 0.44 g N kg^–1 day^–1 or 0.52 g N kg^–0.75 day^–1, which also was similar to the average value for birds. Fibre digestion on both diets was small, 19% and 27% of NDF for rice and grass respectively. Rates of passage of fibrous digesta through the gut of magpie geese varied with diet. The mean retention time for fibre was longer when feeding on the aquatic grass than on unhusked grain, 7.7 3.0 h v. 3.7 0.6 h

Data from wild magpie geese clarified the process of digestion. The patterns of pH and short-chain fatty acids along the gut pointed to acid and enzymic digestion occurring in most of the tract, down to the ileocaecal junction. Fermentation appeared restricted to the caeca, rectum and cloaca, though, of note, the caeca contained little fibre, 5% NDF. Higher levels of fibre digestion were indicated in wild geese but fibre still was not a major contributor to the energy intake of these birds.

The digestive capabilities of the magpie geese were considered in relation to their impact on the feeding and reproductive biology of these ‘geese’ in monsoonal, northern Australia