Results and discussion
Cover crop performance
All the cover crops produced more dry matter in 2011 than in 2010 (Table 4), due to the additional P and N applied one week before sowing, as well as the K applied at the two to six leaf stages. Although it did not rain during the second and third week after the cover crops were sown, the 71 mm rain that fell between 24 April and 7 May 2011 (Table 5), seemed sufficient to ensure the successful germination of the cover crop seeds and sustain growth during the first three weeks.
The four broadleaf cover crops selected for their biofumigation properties did not perform as well as Pallinup oats during 2012 and produced less fibre in 2012 than in 2010 and 2011 (Table 4). This was attributed to the average temperature from May 2012 to August 2012 being approximately 1.4°C lower than that of the previous two seasons (Table 6). This indicated that the four broadleaf species were more sensitive to lower temperatures than Pallinup oats. Although the cover crops could only be sown as late as 23 May 2013, the dry matter production of Pallinup oats still improved slightly, compared to 2012 (Table 4). This was attributed to the relatively high rainfall during the first five weeks following the sowing of the cover crops (Table 5) and the slightly cooler temperatures during June, July and August compared to 2010 and 2011 which seems to favour the oats (Table 6). The four broadleaf cover crops, however, still performed poorer than during the 2011 season, indicating that these species need to be sown earlier.
Suppression of winter growing weeds
The average weed stand was significantly reduced by all the cover crops (Figure 1). Effective suppression (less than 10% of the weed stand measured in the Weeds treatment) was achieved with Pallinup oats and Caliente 199.
Suppression of summer growing weeds
The average weed stand in the CC treatments was significantly lower than that of Weeds (MC), with the exception of Caliente 199 (CC) (Table 7). The average weed stand in the CC treatment of a cover crop species also tended to be lower than the corresponding MC treatment. The observed trends were attributed to the presence of a surface mulch in the CC treatments, as well as to the mechanical cultivation applied during September in the MC treatments which may have promoted the germination of weeds during early summer.
On sandy to sandy clay loam soils, cover crops should receive N, K and P from just before planting up until the six leaf stage to maximise their performance. However, the application of P should be done judiciously, as it may accumulate and reach very high concentrations in the top 75 mm of the soil within a very short period of time. Relatively low winter temperatures and the late establishment of the cover crops due to a dependency on the winter rain (drip irrigation), had a greater negative effect on the performance of the small seeded white mustard, canola, Caliente 199 and Nemat, than on the larger seeded Pallinup oats. All the cover crop species can compete successfully with the winter growing weeds of the region, especially Pallinup oats and Caliente 199. Chemical control of the cover crops or weeds during grapevine bud break seemed to improve the control of summer growing weeds.
Caliente 199, canola, white mustard and Nemat can therefore be included in a cover crop rotation system for grapevines without compromising weed control efficacy.
The authors thank the ARC, Winetech and Dried Fruit Technical Services for financial support, the staff of the Soil and Water Science Department at ARC Infruitec-Nietvoorbij for technical support and Blaauwklippen Wine Estate for supplying the trial site and farm support.
This publication is adapted from a manuscript titled: Effect of management practices applied to cover crops with biofumigation properties on cover crop performance and weed control in a vineyard. J.C. Fourie, D.H.M. Kruger, A.P. Malan, 2015. South African Journal of Enology and Viticulture 36, 146 – 153.
– For further information, contact Johan Fourie at email@example.com.