Effects of tannin on selection by sheep of forages containing alkaloids, tannins and saponins

BACKGROUND: Mixtures of plant species provide biochemical diversity to pastures which may enhance productivity while decreasing reliance on herbicides and insecticides. All plants contain secondary metabolites (PSMs) that interact in plant communities in a variety of ways. For instance, tannins are a group of highly reactive chemical compounds with the potential to interact with other PSM such as alkaloids and saponins, neutralizing their negative effects. Our objective was to determine whether tannins modify the foraging behavior of sheep grazing on varieties of alfalfa, birdsfoot trefoil and tall fescue with high and low concentrations of saponins, tannins and alkaloids, respectively. RESULTS: Lambs that received intraruminal infusions of tannins increased their consumption of the high-saponin variety of alfalfa and the high-alkaloid variety of tall fescue relative to lambs not infused with tannins (controls). Lambs infused with tannins and then offered choices among the three high-PSM varieties of the forages also manifested higher consumption of the high-alkaloid variety of tall fescue than control lambs. In contrast, lambs infused with tannins reduced their consumption of the high-tannin variety of birdsfoot trefoil. Thus lambs modified their foraging behavior as a function of the presence/absence of tannins in their rumens. CONCLUSIONS: These results indicate that ruminants are able to discriminate the specific post-ingestive effects of forage varieties with high concentrations of PSM, and that PSM complementarities are likely to increase the efficiency of use of diverse forages with different biochemistries. (C) 2009 Society of Chemical Industry