Cover Crop Management in a Chardonnay/99 Richter Vineyard in the Coastal Region, South Africa. 2. Effect of Different Cover Crops and Cover Crop Management Practices on Grapevine Performance

Abstract

The trial was conducted over a period of 10 years (1993/94 to 2002/03) on a medium textured soil in a Chardonnay/99 Richter vineyard near Stellenbosch (33°55'S, 18°52'E), situated in the Coastal Wine Grape Region of the Western Cape. Sixteen treatments, consisting of three grain species and five N-fixing broadleaf species managed according to two cover crop management practices, were included. These treatments were compared to a control treatment, in which no cover crop was sown and the weeds were controlled mechanically in the work row and chemically in the vine row from the first week of September to the end of March (grapevine growing season). A treatment in which no cover crop was sown and full surface post-emergence chemical control was applied during the grapevine growing season was also included. The different weed control actions were carried out during the first week of September and/or at the end of November, as well as mid October (1999/00 to 2002/03). Secale cereale L. v. Henog (rye), Avena sativa L. v. Overberg ('Overberg' oats), Avena strigosa L. v. Saia ('Saia' oats) and Viciafaba L. v. Fiord [only if sown annually and controlled chemically before bud break (BB)], showed the ability to produce, on average, significantly more dry matter during winter than the weeds in the region. The dry matter production of all the cover crops increased from the end of August to the end of November if left to complete their life cycles, with the exception of rye and 'Overberg' oats sown in early April. None of the cover crop species were able to re-establish successfully. Continuous effective suppression of winter growing weeds (less than 20% of the weed stand in the control) was achieved with 'Overberg' oats (BB) and 'Saia' oats (BB), while total suppression was achieved for six and five of the 10 years, respectively. Effective, long-term control of the summer growing weeds was obtained with rye (BB), 'Overberg' oats (BB) and 'Saia' oats (BB). An increasing number of weed species are developing resistance to herbicides and even to groups of herbicides with different modes of action (Anonymous, 1997; Henkes, 1997; LeBaron, 1991). Certain weed species can be controlled with biological agents (Cullen et al, 1973; Daniel et al, 1973; Phatak et al, 1983; Woodhead, 1981). The vacated niche will, however, be filled by other species not susceptible to the agent (Putnam, 1990). Cover crop management is a non-specific biological method of pre-emergence weed control, which has many advan-tages (Buckerfield & Webster, 1996; Khan et al, 1986, Louw & Bennie, 1992; Radcliffe et al, 1988; Roth et al, 1988; Van Huyssteen et al, 1984). The difference in climate between regions, as well as the spec-trum of winter growing weeds in a specific region, has an effect on the performance of a cover crop species (Fourie et al, 2001). A selection of species suitable for cover crop management in the different grapevine regions is required in order to apply this envi-ronment friendly practice in a sustainable manner, as part of an integrated production strategy (Fourie et al, 2001). In this regard it was indicated that Secale cereale L. (rye), two Avena (oat) species and Triticale v. Usgen 18 (triticale) could be considered for cover crop management on the medium textured soils of the Coastal wine grape region, situated in the winter rainfall region of South Africa (Fourie et al, 2001). They also indicated that three Trifolium subterraneum (subterranean clover) species, four Medicago (medic) species, three Vicia (vetch) species and Ornithopus sativus L. v. Emena (pink Seradella) should be included in further studies to determine whether the norm of eight t/ha of dry matter, considered by Van Huyssteen et al. (1984) to be necessary for the effective control of summer growing weeds, was applicable to these species under the edaphic conditions of the Coastal region.