Relationship of Periploca laevigata (Asclepidaceae) tannins to livestock herbivory

Periploca laevigata is an abundant and palatable tannin-containing shrub present in the south-east coastal area of Spain. Sheep and goats usually consume this species, which represents 7•2% of total dry matter (DM) intake per year. Periploca laevigata is an important grazing species in the dry season, despite its high tannin content. The objective of the present work was to examine the tannin level of P. laevigata in relation to the extent of utilization by small ruminants, as well as in response to simulated browsing. Periploca sprouts were collected in three areas from plants exhibiting either medium (0•51 animals/HA), low (0•34 animals/HA) or no (0 animals/HA) browsing by small ruminants. These sprouts were also collected in plants with simulated herbivory twice (in two successive months), once, and with no simulated herbivory. A greater concentration of tannins (p<0•001) was detected in the sprouts of plants experiencing greater damage (3•3%, 2•0% and 1•2% w/w in DM basis for high-, medium- and low-stocking rates, respectively). Similarly, a greater tannin concentration (p=0•037) was documented in the sprouts subjected to simulated browsing (4•8%, 4•2% and 3•8% w/w in DM basis for two, one or no browsing, respectively). In summary, P. laevigata increases the level of tannins under higher grazing pressure. Nevertheless, this response does not seem to be effective in defending these plants against herbivores, as the livestock consumes them, perhaps because animals browse P. laevigata moderately throughout the day, mixing it with other plants, or because they possess some kind of physiological mechanism to overcome the effects of tannins.