COVER CROPS in South African Vineyards

CHAPTER 5 WEED CONTROL

After five years, the dominant weed in the conventional treatment, which had no cover crop, was bur clover (an annual), which did not increase in either of the cover-crop treatments. The dominant weeds in the integrated treatment were yellow sorrel and wild radish (an annual). Broadleaf purple vetch produced the largest amount of dry matter in the organic treatment, followed by approximately equal quantities of yellow sorrel, narrow-leafed ribwort, and wild radish. The increase in yellow sorrel in the integrated and organic production systems is of little consequence, as this weed is readily controlled by chemicals. Narrow-leafed ribwort, on the other hand, is a perennial weed that is not easily controlled chemically, and that can have a negative impact on production in the long term. Summer-growing weeds were collected from the work row each year at the end of January.The contributions to total dry-matter production of each winter-growing weed species at the start and end of the trial are shown in Figure 5.30. During the first summer, the dominant weed in the conventional treatment was bristly oxtongue, whereas hairy wild lettuce dominated in both the integrated and organic production treatments. The conventional treatment had mostly annual weeds, whereas perennial weeds, namely hairy wild lettuce, narrow-leaf ribwort, and wild lettuce, contributed most of the dry-matter production of weeds in the cover-crop treatments. The dominant species changed in all three treatments over the five-year period. At the end of the trial, the annual common blackjack dominated the conventional treatment, while the annual flax-leaf fleabane dominated the integrated production treatment.The organic treatment had equal amounts of the annual bristly oxtongue and the perennial narrow-leaved ribwort. Common blackjack, flax-leaf fleabane, and narrow-leaved ribwort are all problem weeds. These results show that, irrespective of the weed control applied, weed- population shifts occur during both winter and summer.The dominant weed species varied depending on whether a cover crop was grown, and on how a cover crop was managed. Each practice favoured a certain problem weed. Not applying herbicides will lead to dominance of narrow-leaf ribwort, a problem perennial weed.

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