Variation in True Metabolizable Energy Among Aquatic Vegetation and Ducks

Avian diet quality is typically measured using true metabolizable energy (TME_N), which is a measure of assimilable energy of food items accounting for innate endogenous losses. Originally developed for use in the poultry industry, TME_N methods have been adapted to determine the value of natural foods consumed by waterfowl to parameterize bioenergetics models for conservation planning. Because there is little knowledge of the variation in TME_N estimates among food items and waterfowl species, we investigated TME_N of 6 common species of submersed aquatic vegetation for mallards (Anas platyrhynchos; i.e., a diet generalist) and gadwall (Mareca strepera; i.e., a diet specialist) in the midwestern United States during autumn 2015–2017. We precision fed and collected excreta from ducks using standard bioassays to estimate TME_N. Mallards had slightly greater TME_N than gadwall, but there was considerable variation in TME_N among vegetation species, duck species, and individuals within each species. True metabolizable energy (±SE; kcal/g[dry]) for mallards was greatest for Canadian waterweed (Elodea canadensis; 1.66 ± 0.26), followed by coontail (Ceratophyllum demersum; 1.51 ± 0.28), southern naiad (Najas guadalupensis; 1.37 ± 0.39), sago pondweed (Stuckenia pectinata; 0.50 ± 0.22), wild celery (Vallisneria americana; 0.05 ± 0.42), and Eurasian watermilfoil (Myriophyllum spicatum; –0.13 ± 0.42). Mean TME_N for gadwall was greatest for Eurasian watermilfoil (0.77 ± 0.32), followed by Canadian waterweed (0.70 ± 0.31), coontail (0.55 ± 0.28), southern naiad (–0.61 ± 0.34), wild celery (–0.98 ± 0.39), and sago pondweed (–1.07 ± 0.33). Generally, TME_N for most vegetation species was less than agricultural grains, but it was similar to ranges reported for seeds of naturally occurring hydrophytic vegetation and aquatic macroinvertebrates. We recommend that conservation planners incorporate species-specific TME_N estimates in bioenergetics models and that future researchers improve TME_N assays for wild waterfowl following our recommendations.