Weed infestation of wheat fields by sheep grazing stubble in the Mediterranean semi-arid region.

Seasonal stubble grazing by sheep is a widespread practice in the Mediterranean basin. However, farmers frequently claim that turning the sheep from marginal lands to the wheat fields after grain harvest increases the risk of weed infestation because of potential weed seed dispersal. We tested this claim by: (1) examining the potential of seed dispersal by sheep from marginal land via faeces and fleece, and (2) evaluating effects of summer stubble grazing on the size and composition of the weed seedbank and weed vegetation. Analysis of seed density in faeces and fleece showed that the potential for infestation from marginal land by sheep was relatively high during late spring, but strongly decreased in mid-summer, when plants in the marginal land had already shed their seeds. The potential of weed transport by faeces was negligible, compared with the seedbank in the field, and little overlapping of species was found between them. Summer grazing by sheep in a no-tillage wheat field over 5 years enlarged the seedbank. This increase, however, was mainly due to the less competitive small-grass species that were not found in the faeces or in the fleece. Combined ploughing, crop rotation, and herbicide application strongly reduced the seedbank. These integrated management practices eliminated the effect of summer grazing on the seedbank.