COVER CROPS in South African Vineyards

CHAPTER 8 EFFECT OF COVER CROPS ON GRAPEVINE NUTRITION AND PERFORMANCE

Nitrogen status of vines and grape juice Cover crops had a significant effect on the nitrogen status of grapevines over a period of nine years. Figure 8.21 shows the nitrogen levels measured as nitrates in the leaf petioles of grapevines in the different grass cover-crop treatments. Figure 8.22 shows the nitrogen levels in the different legume cover-crop treatments. During the first two seasons, grapevine nitrogen levels were generally higher in the treatments where cover-crop treatments were sown annually and chemical weed control started at grapevine bud-break, than in the treatments where cover crops were sown every two years and chemical weed control started in November, or where grasses were controlled mechanically. These results suggest that earlier weed control is especially beneficial in young vines. In the case of Henog rye, later control appeared to lead to competition with the young vines. trol rol

1200

1000

800

600

400

1994/95 1995/96 1997/98 1998/99 2002/03

200

0

Nitrogen in milligram per kilogram

Sown annually

Sown annually

Sown annually

Sown two-yearly

Sown two-yearly

Sown two-yearly

Chemical control

Mechanical control

Henog rye

Overberg oats

Saia black oats

No cover crop

FIGURE 8.21. Nitrogen measured as nitrates in the leaf petioles of grapevines in grass cover-crop treatments compared to a control in the Olifants River valley. Box indicates acceptable range. Adapted from Fourie et al (2007). According to research by Conradie (1994), nitrogen fertilisation at grapevine berry set is not necessary when nitrogen measured as nitrates in the leaf petioles of grapevines exceeds 1 000 milligram per kilogram. It can be seen from Figure 8.21 that these levels were not achieved in the grass cover crops,

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