HAZARD, L.C.*; SHEMANSKI, D.R.; NAGY, K.A.: Digestibility of native and exotic food plants eaten by juvenile desert tortoises

Exotic plants can comprise a major component of the diet for some desert tortoises (Gopherus agassizii) in the Mojave Desert. Introduced plants may not be as nutritious as native plants. Nutrient availability in a native grass (Achnatherum hymenoides), an introduced grass (Schismus barbatus), a native forb (Malacothrix glabrata) and an introduced forb (Erodium cicutarium) were measured in one to two year old juvenile desert tortoises. We fed tortoises measured amounts of chopped foods daily for ~130 days (dry grass) or ~90 days (fresh or thawed forb). Orts and feces were collected daily and dried to constant mass, and energy, water, calcium, phosphorus, and nitrogen content of food and feces were measured. Apparent dry matter and energy digestibilities (percent of ingested material retained) were low for A. hymenoides and S. barbatus (dry matter 42% and 45%; energy 39% and 45%, respectively), and did not differ significantly between grass species. Apparent dry matter digestibility was higher for M. glabrata (56%) and E. cicutarium (71%). Tortoises did not grow while feeding on either grass; mass changes were primarily due to water imbalances. Tortoises grew on both forb diets. Voluntary intake rates were higher for the forbs than for the grasses. Therefore, the higher growth rates for forbs were the result of both higher intake rate and higher digestibility of these foods. Adult tortoises fed the same diets had similar digestibilities as did the juvenile tortoises. Dietary digestibility is better explained by the type of food (forb vs. grass) than by its geographic origin (native vs. exotic).