Handbook for Irrigation of Wine Grapes in South Africa

no effect on midday Ψ S in grapevines where the maximum crop load was one bunch per shoot, (Chapter 6, Fig. 6.23), less bunches will probably not reduce ET significantly. Furthermore, one bunch per shoot did not increase midday Ψ S , compared to total crop removal of high yielding grapevines where the crop load was ca . two bunches per shoot (Chapter 6, Fig. 6.24). Another possibility is that high crop loads could delay sugar accumulation when grapevines experience severe water constraints. Therefore, it is advisable to remove unnecessary bunches before véraison. If the drought conditions caused a substantial reduction in vegetative growth, but not in fertility, bunches should be removed to improve the balance between vegetative growth and yield to prevent possible physiological stresses. Based on the foregoing, it seems that one bunch per shoot is unlikely to induce excessive water constraints or increase ET. Consequently, one bunch per shoot can be considered as a norm for bunch removal where irrigation water is limited or during critical droughts. Cover crops may use a substantial amount of winter water stored in the soil for transpiration, particularly if they grow vigorously (Fig. 7.35). If the cover crop continues to grow after budbreak, it will deprive the grapevines of crucial soil water resources, compared to bare or mulched soil (Fig. 7.36). The water lost during spring can be crucial in regions where irrigation resources are limited, e.g. in the Swartland region. The extent of soil water use by the growing cover crop will also depend on its growth vigour (Fig. 7.37). The effect of the growing cover crop will be less severe on the grapevine row, compared to the work row where it usually grows (Fig. 7.38). Soil water content in the topsoil layer was substantially higher 28 days after a Triticale cover crop was killed and rolled flat at budbreak, compared to the actively growing Triticale (Fig. 7.38B). During this period, the growing cover crop used ca. 44 mm more water, compared to the killed cover crop (unpublished data). In this particular case, the dried matter of the Triticale amounted to 5.1 t/ha based on the total vineyard area. Since the soil water content was higher, compared to clean cultivated soil, it suggested that the flattened cover crop effectively served as a mulch. If the cover crop is slashed instead of controlling it chemically, there will probably be no mulch effect with respect to soil water losses. Channels formed where cover crop roots have died back will also improve water infiltration into the soil and reduce the risk of runoff if rainfall occurs during summer. Based on the foregoing, growers are advised to kill cover crops earlier than usual if winter rainfall was abnormally low. Likewise, timely chemical control of winter weeds is crucial for reducing transpiration losses (Fig. 7.39).

222 CHAPTER 7 – IRRIGATION STRATEGIES