Quality of plant species utilized by northern white-tailed deer in summer along a climatic gradient

Abstract

During two growing seasons, we evaluated forage quality (in terms of digestibility and nitrogen content) of eight (five woody and three herbaceous) native species of plants commonly consumed by white-tailed deer (Odocoileus virginianus) at four sites along a climatic gradient. At one site, we also compared the composition of native forages of deer with that of three cultivated species consumed occasionally by deer. We predicted that forage quality would improve with increasing latitude because the cool climate and short growing season should allow forage to remain more digestible during the period of availability. We also predicted that the quality of cultivated plants should be higher than that of forest plants throughout the growing season, and particularly towards its end, because of successive harvesting of cultivated plants. Depending on the plant species, estimated dry-matter digestibility varied between 55.2% and 79.2%, whereas nitrogen content averaged between 0.7% and 5.7%. Both variables decreased progressively throughout the summer. Our results did not support the first prediction despite a reduction of 50% in the number of degree-days between the southern and the northern extremes of the climatic gradient. Inter-annual climate variations may have masked subtle differences in plant quality between sites. As predicted, cultivated forages tended to be of higher quality than native species growing in neighboring woodlots, and particularly at the end of the summer because of mowing and regrowth. Our results indicated that white-tailed deer living at the northern fringe of their range cannot count on highly digestible forage to compensate for the short growing season.