True Metabolizable Energy of Submersed Aquatic Vegetation for Gadwall

Wetland vegetation communities provide critical foraging habitat for waterfowl, but many of the historical wetlands in the United States have been lost throughout the last two hundred years. The loss of wetlands has led to substantial declines in submersed aquatic vegetation species, which are important foods of waterfowl and other wildlife. Unfortunately, there is a lack of information about the implications of these losses on energetic carrying capacity for waterfowl, especially ducks. Waterfowl managers typically estimate the energetic carrying capacity for a wetland by using bioenergetics models. These models incorporate several parameters that predict energy demand, including population size, stopover duration, and the energetic value (i.e. true metabolizable energy) of foods available to ducks. Of these parameters, energetic carrying capacity models are especially sensitive to true metabolizable energy values, however, very few true metabolizable energy estimates are available for submersed aquatic vegetation. Most available true metabolizable energy values are from plant seeds and have only been estimated for a couple waterfowl species that do not primarily consume aquatic vegetation. I estimated TME$_N$ (true metabolizable energy corrected for non-dietary nitrogenous compounds) values of six common species of submersed aquatic vegetation for gadwall in order to parameterize energetic carrying capacity models and better understand the value of emergent marshes for ducks. Vegetation species was the most important predictor of true metabolizable values (mean ± SE; kcal/g[dry]) of submersed aquatic vegetation (Myriophyllum spicatum, 0.77 ± 0.32; Elodea Canadensis, 0.70 ± 0.31; Ceratophyllum demersum, 0.55 ± 0.28; Najas guadalupensis, –0.61 ± 0.34; Vallisneria americana, –0.98 ± 0.39; Stuckenia pectinata, –1.07 ± 0.33), but both sex and mass of birds was also influential.