THE EVALUATION AND MANAGEMENT OF DIFFERENT GRASSES AND LEGUMES AS POTENTIAL COVER CROPS IN THE VINEYARDS OF SOUTH AFRICA

A selection of species suitable for cover crop management in the different wine grape regions is required to enable more producers to apply this environment friendly practice in a sustainable manner as part of an integrated production strategy. The correct management practice(s) to be applied to these species over both the short and long term in a cooler and warmer wine grape region needed clarification. The effect of seeding date on the dry matter production (DMP) and weed control efficacy of seven grasses and sixteen legumes, as well as varieties of three of these species, was determined during 1991 and 1992. The decomposition rate of the mulches was determined. In the cooler climate of Stellenbosch (3355'S, 1852'E), the Medicago species, subterranean clovers, pink Seradella and three Vicia species did not compete effectively with the winter weeds if the weekly precipitation from mid-March to mid-May (autumn) exceeded 18 mm. The two oat species, as well as rye and triticale produced more than five t/ha of dry matter if the precipitation exceeded 18 mm per week. The DMP of the above-mentioned species indicated that these species could be considered for cover crop management in Lutzville (3135'S, 1852'E), if full surface irrigation of 18 mm per week could be applied for 10 weeks directly after sowing, followed by 18 mm fortnightly. Seeding date had a significant effect on DMP in both regions. A highly significant correlation (r = 0.85, p ≤ 0.0001) existed between the decomposition rate of the mulches and the initial amount of dry matter present on the soil surface. A second trial was conducted on a sandy soil in a Sauvignon blanc/Ramsey vineyard near Lutzville from 1993/94 to 2002/03. Twenty four treatments, consisting of three management practices and three fertilizer rates applied selectively to four cereals and four legumes, were evaluated. Rye and pink Seradella produced, on average, the highest amount of dry matter at the end of August receiving on average 278 mm of full surface irrigation and rain. A P and K concentration of 10 mg/kg and 78 mg/kg, respectively, in the top 300 mm soil layer, seemed to supply in the needs of grazing vetch. Saia oats performed poorly unless 30 kg of P, 30 kg of K and 42 kg of N were applied during establishment and early growing phase. All the species, except ‘Parabinga’ medic, produced additional fibre from September to November following a dry winter (201 mm of both rain and irrigation), while none produced additional fibre if the water supply was luxurious. The cover crops did not produce enough seeds to reestablish successfully. The shoot mass of the young grapevines in the grazing vetch and ‘Paraggio’ medic treatments in which post-emergence chemical control was applied from just before bud break (BB), was significantly higher than that of the treatments in which post-emergence chemical control was applied from berry set (AB). It was also significantly higher than that of the treatments in which mechanical control was applied in the working row and chemical control in the vine row from just before bud break (MC), with the exception of rye (AB) and grazing vetch (MC). The first harvest from the young grapevines in the BB treatments was significantly higher than that of the treatment in which no cover crop was sown and BB was applied (weedchem) and the MC treatments. When the grapevines reached full production, the grape yield in the BB treatments, grazing vetch (AB) and pink Seradella (AB) was significantly more than that of weedchem and the control. The NO3-N concentration in the leaf petioles of all the cover crop treatments was, with the exception of the AB treatments of rye, ‘Parabinga’ medic and grazing vetch, significantly more than that of weedchem and the treatment in which no cover crop was sown and MC was applied (control). The NO3-N concentration in the leaf petioles of the BB and AB treatment of a species differed significantly. The N concentration in the grape juice of the cover crop treatments was, with the exception of ‘Saia’ oats (MC) and ‘Parabinga’ medic (AB), significantly higher than that of weedchem and the control. The N concentration of the juice in the BB and AB treatments of grazing vetch and pink Seradella was significantly higher than that of the MC treatments, the two rye treatments, weedchem and the AB treatments of the other cover crop species. The concentration of Ca in the juice of the cover crop treatments was, with the exception of the pink Seradella treatments, significantly higher than that of weedchem and the control. Wine quality did not differ between treatments. After 10 years of applying the treatments, the soil organic matter (SOM) content in the 0-600 mm soil layer of grazing vetch (AB), as well as the 0-150 mm soil layer of pink Seradella (AB) and rye (BB), was significantly higher than that of the control and weedchem. The total inorganic N concentration (TIN) of pink Seradella (BB) was the highest in the 0-150 mm soil layer during the full bloom stage of the grapevines and significantly higher than that of the other treatments in the 150-300 mm soil layer. The TIN in the 0-150 mm soil layer of the legumes was, with the exception of pink Seradella (BB), significantly more than that of the control, weedchem and the BB treatments of the grain species as measured after the grapevines were harvested. The TIN in the 150-300 mm soil layers of the AB treatments of pink Seradella, ‘Parabinga’ medic and ‘Paraggio’ medic, was significantly higher than that of the control, weedchem and the grain treatments. Potassium concentrations in the 0-150 mm soil layer of the two pink Seradella treatments, the AB treatment of rye, ‘Paraggio’ medic and grazing vetch, as well as the 150-300 mm soil layer of grazing vetch (BB) and pink Seradella (BB), were significantly higher than that of the control, weedchem and ‘Saia’ oats (MC) after harvest. A third trial was conducted) on a medium textured soil in a Chardonnay/99 Richter vineyard near Stellenbosch from 1993/94 to 2002/03. Sixteen treatments, consisting of three cereals and five legumes managed according to two cover crop management practices (BB and AB), were included. These treatments were compared to a control (as above) and weedchem treatment managed the same as described for the trial in Lutzville. Rye, ‘Overberg’ oats, ‘Saia’ oats and faba bean (only if BB was applied) showed the ability to produce, on average, significantly more dry matter during winter than the weeds in the region. The DMP 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. Total suppression of the winter growing weeds was achieved for six and five of the 10 years with ‘Overberg’ oats (BB) and ‘Saia’ oats (BB), respectively. Effective, long-term control of the summer growing weeds was obtained with rye (BB), ‘Overberg’ oats (BB) and ‘Saia’ oats (BB). The shoot mass of the two year old grapevines in the BB treatments was significantly higher than that of the control and the AB treatments. In the following season, the shoot mass and grape yield of the BB treatments was, with the exception of faba bean and ‘Overberg’ oats, significantly higher than that of the control and weedchem. The grape yield of the control and AB treatments was significantly less than that of weedchem. The petiole NO3-N and juice N concentrations in ‘Kelson’ medic (BB) were significantly higher than that of the control and weedchem. The juice N concentration of the control and weedchem was significantly less than that of the legume treatments, with the exception of ‘Paraggio’ medic (AB) and ‘Woogenellup’ subterranean clover (AB). Wine quality did not differ between treatments. After five seasons the SOM content in the 0-300 mm soil layer increased in all the cover crop treatments during this period, while that of weedchem and the control remained unchanged and decreased by 16%, respectively. The SOM in the 0-150 mm soil layer of the cover crop treatments was, with the exception of grazing vetch, significantly higher than that of the mechanically cultivated control after a period of 10 years. The SOM in the 0-300 mm soil layer of rye and the treatments in which the legumes were sown (except grazing vetch) was significantly higher than that of weedchem. The TIN of the 0-150 mm soil layer in the BB treatments of the two Medicago species and ‘Woogenellup’ subterranean clover, was significantly higher than that of the control, weedchem, and the AB treatments during the full bloom phase of the grapevines. The TIN of the 0-600 mm soil layer in the AB treatment of a species, as measured after the grapes were harvested, tended to be higher than that of the BB treatment of that species.