Herbivores prefer foods that supply required nutrients and avoid those with excess nutrients and plant secondary compounds (PSC). Nevertheless, PSC such as condensed tannins can provide beneficial medicinal effects to herbivores. The objectives of this study were to determine: (1) if parasitized lambs increased preference for a tannin-rich food after they experienced the beneficial antiparasitic effects of tannins relative to parasitized lambs that did not experience such benefits, (2) if preference for the tannin-containing food in the former group decreased when parasite burdens subsided, and (3) if the tannin-enriched food decreased parasitic burdens.

Twenty-two lambs were familiarized with beet pulp and beet pulp + 8% quebracho tannins and choices were given between the two foods (initial preference tests). Subsequently, all animals were dosed with 10,000 L ₃ stage larvae of Haemonchous contortus. Twenty-two days later, animals were exposed to beet pulp (Control group; n=11) or beet pulp+tannins (Treatment group; n=11) during a span of 24 d. After exposure (during a parasitic infection) animals in both groups were given choices between the two foods. Lastly, animals in both groups received an antiparasitic drench and were again given a choice between both foods (after a parasitic infection). Lambs preferred beet pulp to beet pulp + tannins throughout the study (P< 0.001) and no difference in preference for the tannin-rich food was detected between groups during initial preference tests (P > 0.05). However, during a parasitic infection, intake of and preference for the tannin-containing food was higher for Treatment lambs than for Control lambs (P < 0.05). When parasitic infections were terminated by chemotherapy, differences between groups disappeared (P > 0.05). Preference by the Treatment group for the tannincontaining food was lower after than during a parasite infection (P < 0.05). In contrast, preference by the Control group did not change during these periods (P > 0.05). Lambs in the Treatment group displayed lower FEC than lambs in the Control group (Group x Sampling Date; P < 0.05). These results show lambs needed to learn about the beneficial antiparasitic effects of tannins (Treatment) to increase their preference for the tannin-containing food.