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

During two growing seasons, we evaluated forage quality tin terms of digest ibility and nitrogen content) of eight (five woody and three herbaceous) na tive species of plants commonly consumed by white-tailed deer (Odocoileus v irginianus) at four sites along a climatic gradient. At one site, we also c ompared the composition of native forages of deer with that of three cultiv ated species consumed occasionally by deer. We predicted that forage qualit y would improve with increasing latitude because the cool climate and short growing season should allow forage to remain more digestible during the pe riod of availability. We also predicted that the quality of cultivated plan ts should be higher than that of forest plants throughout the growing seaso n, and particularly towards its end, because of successive harvesting of cu ltivated plants. Depending on the plant species, estimated dry-matter diges tibility varied between 55.2% and 79.2%, whereas nitrogen content averaged between 0.7% and 5.7%. Both variables decreased progressively throughout th e summer. Our results did not support the first prediction despite a reduct ion of 50% in the number of degree-days between the southern and the northe rn extremes of the climatic gradient. Inter-annual climate variations may h ave masked subtle differences in plant quality between sites. As predicted, cultivated forages tended to be of higher quality than native species grow ing in neighboring woodlots, and particularly at the end of the summer beca use of mowing and regrowth. Our results indicated that white-tailed deer li ving at the northern fringe of their range cannot count on highly digestibl e forage to compensate for the short growing season.