COVER CROPS in South African Vineyards

CHAPTER 3 COVER-CROP SELECTION

Cover crops received 18 millimetres of water per week for the first ten weeks after sowing, and 18 millimetres every two weeks thereafter. Rainfall was supplemented by full-surface sprinkler irrigation, except in three years (1995, 1998 and 2000) when canal maintenance led to periodic water shortages. Dry-matter production, especially for legumes, was reduced in those three years. The inconsistent water supply is probably the main reason why dry-matter production in the vineyard trials was lower than in the open-land trials. In addition, shading of the work row by the grapevine canopy and covering of the seedlings at the two- to four-leaf stage by fallen leaves may have impacted cover-crop performance. Dry-matter production for the grasses is illustrated in Figure 3.21 and for the legumes in Figure 3.22. In both charts, the dry-matter production of the natural weed stand is presented for comparison.

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Rye Black oats Overberg oats Triticale Weeds

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1,00

0,00 1993 1994 1995* 1996 1997 1998* 1999 2000* 2001 2002 Year Dry matter in tonnes per hectare FIGURE 3.21. Annual dry-matter production of grass cover crops in vineyard trials in the Olifants River valley. *Years in which canal maintenance interrupted irrigation. Adapted from Fourie et al (2005). Dry-matter production of the grasses and legumes did not decline over time. The trials showed that the selected cover crops could be sown as monocultures over the long term on sandy soils. However, although cover crops should be sown as a monoculture in any given year, it is recommended that grasses and legumes should be rotated between years to add nitrogen to these sandy soils. Rotation will also help prevent the build-up of soil-borne diseases that may target a specific cover-crop.

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