Handbook for Irrigation of Wine Grapes in South Africa

2012 2013

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the grapevines experienced almost no water constraints. Mean cane mass and yield were 2.5 and 15 t/ha, respectively. These levels of vegetative growth and yield were comparable to those previously reported for Cabernet Sauvignon vineyards without a summer cover crop. Therefore, the results suggested that the grapevines’ growth vigour and yield were not affected by the pearl millet in the work rows during summer. The primary reason is probably because the pearl millet was sown after most of the bunch initiation and differentiation, as well as vegetative growth, had occurred. Furthermore, visual observations revealed that the root system of this cover crop was shallow, compared to that of the grapevines. Therefore, competition for water and nutrients was probably not strong enough to have induced negative effects on grapevine growth and yield. The fact that organic carbon did not accumulate in the soil during the study period indicated that the soil was sufficiently aerated between irrigations to allow organic matter breakdown via oxidation (Howell, 2016). This probably explains why grapevine growth and yield did not respond to level of COD per se . Furthermore, it must be noted that the salinity and sodicity levels in the diluted winery wastewater were below the thresholds for grapevines. Therefore, accumulation of elements in the leaves and shoots was not expected, which was confirmed by the lack of response to the element levels in the diluted winery wastewater. This is a further, and maybe more realistic reason, why the grapevines did not respond to the wastewater irrigation. Since vegetative growth did not differ, ET was not affected by the wastewater irrigation under the prevailing conditions (Myburgh & Howell, 2014b). However, due to the vigorous pearl millet growth (Fig. 5.29), ET showed an unexpectedly high peak during berry ripening (Fig. 5.30). 2000 3000 ab a ab b ab

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FIGURE 5.29. Pearl millet planted between grapevine rows as an interception crop.

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142 CHAPTER 5 – IRRIGATION WATER QUALITY