White-tailed deer (Odocoileus virginanus) damage to soybean crops is a concern for soybean producers. Although researchers have documented decreases in the intensity of deer-browse on soybean plants as the growing season progresses, an understanding of the mechanisms driving the decrease in deer-browse is necessary for reduction and mitigation of deer damage to soybean crops. We tested 4 hypotheses to determine why deer-browse rates decrease 3 weeks after plant emergence: (1) plant phenology affects plant palatability; (2) diet change occurs; (3) deer damage induces a plant response making soybean leaves less palatable: and (4) deer consume fewer leaves but the same amount of leaf biomass as the season progresses. We recorded deer-browse in double- and single-crop soybean fields in Little Creek, Delaware, during the 2005 to 2006 growing seasons. To test if plant phenology affected deer-browse, we conducted a forage analysis of soybean leaves at different growth stages. Although forage quality components were variable across the growing season, white-tailed deer dietary requirements were met or exceeded in all cases. We compared deer diet composition using microhistological analyses across the early soybean growing season. The proportion of soybeans in the diet increased from 13 to 37% from late May to early July. We tested for an induced plant response by comparing the browse rates of plots that were protected from deerbrowsing until 4 weeks after plant emergence to plots that received no protection. Although we documented greater browse rates in the protected plots once protection was removed, we also documented that protected plots had taller plants, suggesting that deer may have been attracted to the taller plants. The amount of soybean leaf biomass that deer were consuming across the growing season increased from the early to late growth stages of soybeans. Based on our results, we believe that the increasing biomass of soybean leaves is the most plausible explanation for the decrease in browsing rate that we observed as soybeans matured.

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