Handbook for Irrigation of Wine Grapes in South Africa

Chapter 7

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25

15

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20

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Cane mass measured at pruning of low frequency drip irrigated Sauvignon blanc in shallow, shale derived soil, as well as that of Cabernet Sauvignon in deep red and yellow soil near Philadelphia in the Coastal region, increased linearly as the irrigation volume increased (Fig. 7.8A). In contrast to vegetative growth, yield of both cultivars showed no significant yield increase as the low frequency irrigation increased from 300 to 600 mm (Fig. 7.8B). In this example, the higher irrigation volume will probably produce higher yields if the bearing capacity of the grapevines could be extended. In practice, it means that the excessive growth must be converted to yield. By doing so, more grapes can be produced with the same volume of irrigation water, thereby improving the irrigation water use efficiency as discussed in Section 7.3 below. 5 10 0 1 2 3 4 5 Yield (t/ha) Number of irrigations 10 15 0 1 2 3 Yield (t/ha) Number of irrigations Figure 7.8 5 10 0 1 2 3 4 5 Yield (t/ha) Nu ber of ir igations 10 15 0 1 2 3 Yield (t/ha) Nu ber of ir igations Figure 7.8

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10 12 14 16

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Sauvignon blanc Cabernet Sauvignon Sauvignon blanc Cabernet Sauvignon

Sauvignon blanc Cabernet Sauvignon Sauvignon blanc Cabernet Sauvignon

A

B

3

3

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2

0 2 4 6 8

0 2 4 6 8

Yield (t/ha) Yield (t/ha)

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Cane mass (t/ha) Cane ass (t/ha)

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Dryland

Dryland

300 mm 600 mm 30 m 60 m

Dryland

Dryland

300 mm 600 mm 30 m 60 m

FIGURE 7.8. Effect of low frequency irrigation on the (A) cane mass at pruning and (B) yield of two cultivars near Philadelphia in the Coastal region (unpublished data). Data are means for three seasons.

Although three irrigations applied at pea size, véraison and post-harvest, respectively, resulted in wetter soil conditions, compared to no irrigation (Fig. 7.9), it did not affect the water status of Merlot/99 R near Wellington in the Coastal region (Table 7.3). However, the three irrigations tended to increase berry mass and yield, compared to non-irrigated grapevines. These trends indicate that low frequency irrigation is likely to reduce the risk of yield losses during heat waves. In fact, when a severe heat spike occurred in the Western Cape on 6 February 2005, the dryland Merlot grapevines yielded only 9.3 t/ha, compared to 12.2 t/ha where two irrigations had been applied at that stage. More frequent irrigations, i.e. at pea size, midway between pea size and véraison, at véraison, midway between véraison and harvest and post-harvest increased berry mass and yield (Table 7.3).

IRRIGATION OF WINE GRAPES 185