COVER CROPS in South African Vineyards

COVER CROPS IN SOUTH AFRICAN VINEYARDS

ryegrass when chemical control was not applied at bud-break. Mechanical control appeared to either allow some ryegrass to survive and set seed, or to promote the germination of ryegrasses by removing fibrous material from the soil surface. Ryegrasses and musk herons bill were also present during early summer in the first season of the trial. These weeds were eventually completely suppressed using full-surface chemical control at grapevine bud-break. However, full-surface chemical control appeared to facilitate an increase in crab fingergrass. The summer-growing weeds that had significant stands were common dubbeltjie, crab fingergrasses, flax-leaf fleabane, prostrate knotweed, Natal red-top, and common couch. In this trial, Natal red-top was the dominant weed overall before the treatments began, after which it lost its dominance. The reduction in common couch and prostrate knotweed in 2011 may have been due to low rainfall that summer. However, these weeds recovered when rainfall increased. In general, crab fingergrass, common couch, and prostrate knotweed tended to establish better in late summer when mechanical cultivation was applied at grapevine bud-break. In these trials, common couch was reduced by Pallinup oats, white mustard, canola, and Caliente. To summarise, the suppression of certain weeds by cover crops will favour other weeds. In this study, mechanical control at bud-break was associated with an increase of problem weeds, partly due to persistence of weed seeds in the soil. Mechanical control also incorporated cover-crop residues in the soil, which led to the loss of potentially weed-suppressing surface mulches. Full-surface herbicide application at bud-break was shown to be necessary for weed control with all cover crops. Some weeds, such as musk herons bill, may set seed early, and therefore increase despite herbicide application at bud-break.A further application of a post-emergence herbicide is needed to control crab fingergrass, common dubbeltjie, and flax-leaf fleabane, all weeds that germinate later in summer. It is suggested that a high dosage of a directed spray of glyphosate should be applied to flax-leaf fleabane on a windless day before it reaches ankle-height. Common dubbeltjie can be controlled with a contact herbicide before it produces flowers or seeds, again as a directed spray on a windless day.

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