EFFECT OF PREVIOUS EXPERIENCE ON SHEEP GRAZING LEAFY SPURGE

Herbivores develop preferences for certain plant species in several wa ys. One way is to learn from role models, another is through postinges tive feedback. In this study we determined (1) whether yearling sheep exposed to leafy spurge (Euphorbia esula L.) as lambs graze it more re adily than yearlings that were not exposed to it as lambs, and (2) whe ther this difference, if present, persists through the grazing season. On a leafy spurge-infested, Idaho fescue (Festuca idahoensis Elmer) r ange site, we compared the grazing behavior of naive groups of Targhee yearlings with grazing behavior of experienced groups for 3 years. Us ing focal animals, we assessed diet selectivity by counting bites and by determining time spent grazing different forage types. During the g razing season, nutritive value (crude protein, neutral detergent fiber , in vitro dry matter digestibility) of leafy spurge was always higher than nutritive value of Idaho fescue. Experienced yearlings spent mor e than four times as much time grazing leafy spurge in early summer co mpared with naive yearlings (averaged across 1992 and 1993, experience d 13.6% vs. naive 2.9%), whereas in mid- (39% vs. 31%) and late summer (37% vs. 38%) both groups spent similar amounts of time grazing leafy spurge. Neither group readily grazed the plant in early summer. Exper ienced yearlings had higher bite rates of leafy spurge than naive year lings during the early summer sampling periods in 1992 and 1993 (avera ged across 1992 and 1993, 15.9 vs. 5.0 bites min(-1)) but their bite r ates were similar by mid-summer (22.7 vs. 20.6 bites min(-1)). In 1994 , we observed their grazing behavior every 5 days for 35 days. Initial ly, naive yearlings spent less time grazing leafy spurge than experien ced yearlings, but were spending similar amounts of time grazing leafy spurge by day 25. Previous experience with leafy spurge, compared wit h no experience, resulted in a slight but short-lived advantage in ear ly summer use of the species. Within 3-4 weeks, sheep that had never p reviously been exposed to leafy spurge readily grazed the plant.