Pesticide residues
The grapevine requires many pesticide treatments. For instance, no less than 37 pesticides were approved for grapevine treatment in the UK in 2014 (Agrii, 2014). Two of them, Boscalid (a fungicide) and Fipronil (an insecticide) have been reported to be of concern for animal feeding. Leaves and shoots spread with these products should not be grazed by or fed to animals (MacLachlan, 2010).
Copper toxicity
The use of copper-based fungicides, such as the Bordeaux mixture, may cause copper toxicosis in sheep, which are sensitive to copper. There have been reported deaths of sheep kept in vineyards treated with copper sulphate (Reis et al., 2015). Feeding grape-based fodder to sheep is, therefore, not recommended unless it has been washed (Magnier, 1991). In Italy, subclinical symptoms of copper toxicity were also reported in heifers fed vine shoots. However, the symptoms disappeared after the addition of ammonium molybdate and sodium sulphate to the ration (Tartari et al., 1979b).
Condensed tannins
Grapes leaves and shoots contain high and variable amounts of condensed tannins that may impair digestibility. Condensed tannins bind to plant proteins and cell wall carbohydrates thereby decreasing protein and fibre digestibility (Waghorn, 2008). However, tannins may have beneficial effects such as increasing by-pass protein in ruminants and decreasing gas production, thus reducing gas emissions (Makkar, 2003). Tannins that have been reported to reduce palatability in some forages do not appear to have a negative effect on the intake of grape leaves by sheep (Romero et al., 2000). Grape leaves have been reported to contain 2-10% of DM as condensed tannins (Gurbuz, 2007; Kamalak, 2005; Romero et al., 2000).