The physiological consequences of ingesting a toxic plant (Diplotaxis tenuifolia) and medicinal supplements influence subsequent foraging decisions by sheep

Abstract

There is a large group of highly nutritious plants that are commonly avoided by grazing livestock due to the presence of toxic plant secondary compounds. Our hypothesis was that aversion toward toxic plants is learned and that their negative postingestive effects could be attenuated by specifc nutrients. Therefore, we determined the impact of supplements on physiological parameters and feeding behavior in Merino sheep consuming Diplotaxis tenuifolia (“Wild rocket”), a plant with high concentration of glucosinolates (37.2 ± 3.6 ȝmolUg)U Thirty-six sheep were randomly assigned to four feeding treatments in a split-plot design with lambs (n = 9) nested within treatment: Wild rocket (DT), Wild rocket and a protein (160 g/d) supplement (DT+P), Wild rocket and a protein supplement containing iodine (10 mg/d) and copper (40 mg/d) (DT+P+M), or alfalfa pellets (CTRL) in amounts that paired the ingestion of Wild rocket in DT. Toward the end of a 35-d exposure period, sheep in DT showed the lowest intake of Wild rocket (P = 0.04) as well as reduced concentrations of plasma thyroid hormones (T3 and T4; P < 0.001 and P = 0.05, respectively) and the enzyme alanine aminotransferase (P = 0.02) and a trend toward reduced hemoglobin concentration (P = 0.07) relative to sheep in DT+P and DT+P+M, which, in turn, showed concentrations of hormones and hepatic enzymes similar to those recorded in CTRL. Total serum protein and albumin levels were greater (P = 0.03 and P = 0.04, respectively) in supplemented than in unsupplemented sheep fed Wild rocket, which could have elicited a protective effect on glucosinolate ingestion. Foraging behavior was evaluated in an experimental arena where animals could select among randomly distributed buckets containing a fxed amount of Wild rocket or variable amounts of barley grain. Regardless of barley grain availability, sheep in DT showed lower intake and lower time spent eating Wild rocket than sheep in DT+P or in DT+P+M (19.0 vs. 48.5 and 43.2 ± 6.0 g [P = 0.007] and 26.8 vs. 54.4 and 48.9 ± 4.78 s [P = 0.005], respectively). Sheep in CTRL showed intake levels of Wild rocket and behavioral responses similar to those observed in supplemented sheep. In conclusion, nutritional supplements have the potential to attenuate the negative postingestive effects of glucosinolates in Wild rocket and enhance the utilization of the plant at pasture. A negative feeding experience with Wild rocket is needed for animals to display the typical pattern of aversion commonly observed in grazing conditions.