NUTRITIONAL QUALITY OF WILLOWS FOR MOOSE: EFFECTS OF TWIG AGE AND DIAMETER

Alaskan moose (Alces alces gigas) consume willow (Salix spp.) as a fundamental component of their winter diet. We collected Barclay willow (S. barclayi) from 5 nearby sites (15-80 m apart) on the Kenai Peninsula, Alaska, USA, during winter 1999-2000. We tested effects of diameter and age of twigs on nutritional quality of willows for moose. Smaller-diameter twigs had higher in vitro dry matter digestibility (IVDMD), and protein content, but lower fiber content (P < 0.001) than larger twigs. An inverse relationship occurred between the age of twigs and protein content (P < 0.001), with older-aged twigs containing less protein. Accordingly, age of twigs was negatively related to fiber content (P = 0.002). Conversely, no relation existed between age of twigs and IVDMD (P = 0.34). Tannin content (P < 0.001) and age of twigs (P = 0.04) varied among sites, with older twigs possessing more tannins than younger ones. No difference in tannins, however, occurred between diameter categories of twigs (P = 0.48). Digestible energy differed between diameter categories (P = 0.02) and among ages of twigs (P = 0.02), as well as among collection sites (P < 0.001). Thus, structural components of the twig to support growth were more important in affecting digestibility, whereas age of the twig was more influential in determining nitrogen and tannin content. The relation between twig age and tannin content, however, was the inverse of that expected. More research is needed to understand how quality of winter browse interacts with additional factors, such as predation risk, population density, and allometric differences between sexes, to affect diet selection and foraging behavior of moose and other large herbivores