COVER CROPS in South African Vineyards

J. FOURIE IN SOUTH AFRICAN VINEYARDS

Cover crops in South African vineyards

By Johan Fourie

COVER CROPS IN SOUTHAFRICANVINEYARDS COVER IMAGE: Emma Carkeek All rights reserved Wine Industry Network of Expertise &Technology NPC (Winetech) VinPro Building Cecilia Street Suider Paarl 7624 www.winetech.co.za AND South AfricanTable Grape Industry (SATI) 1st Floor 63 Main Street Suider Paarl Paarl 7464 www.satgi.co.za The author has made every effort to obtain permission for and acknowledge the use of copyrighted material. Please refer enquiries to the author. No part of this book may be reproduced or transmitted in any form or by any electronic, photographic or mechanical means, including photocopying and recording on record, tape or laser disk, on microfilm, via the Internet, by e-mail, or by any other information storage and retrieval system, without prior written permission by the author. First edition 2021 ISBN 978-0-620-97123-2 (print) ISBN 978-0-620-97124-9 (electronic) Set in Perpetua 13pt/15 PHOTOS: Johan Fourie, Lucinda Heyns, Emma Carkeek & Shutterstock LAYOUT AND DESIGN by Avant-Garde | 021 863 3165 PRINTING by ABC Press

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COVER CROPS IN SOUTH AFRICAN VINEYARDS

Johan Fourie Photo: Cor Langeberg

3

FOREWORD

FOREWORD By Kobus Louw

This book is the culmination of more than 50 years of research on cover crops at the Nietvoorbij research centre based in Stellenbosch. Research on cover crops and tillage were started in the early 1970’s by then youngsters Dawid Saayman, Leopoldt van Huyssteen and Johan van Zyl. The research, however, really got going in the mid-1980’s when Johan Fourie joined the team as a young researcher fresh out of University.

The work at Nietvoorbij became World renowned and was awarded many prizes and honours over the years. It must also be noted that tillage and cover crop research are long-term endeavours. Results sometimes take many years to manifest. The research was started when agricultural research was still fully funded by Government. With the privatisation of research in the early 1990’s, most of the trials initiated by Johan Fourie still had 10–12 years to go and were in danger of being terminated due to the costs thereof and budget restraints. It took the vision of one man, Jan Booysen then Manager ofWinetech one of the industry funders of research, to realise the importance of these long-term projects and to support the projects with the necessary funding.The other industries then followed suit. One should also understand that these were the early days of cover crop research. Monocultures were adopted to give very specific answers absent at

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COVER CROPS IN SOUTH AFRICAN VINEYARDS

LUCINDA HEYNS

that time. The effects, and importance, of rotations and mixed cover crops were researched only to some extent but are acknowledged throughout this book. Rotations and mixed cover crops became the subject of numerous other studies by different researchers, but one should never loose sight of the fact that all of that build on the information described in this book. Using monocultures are also sometimes necessary and beneficial. For instance, as described in this book, by planting a monoculture of a grass species, a problematic broadleaf species can be controlled with targeted chemicals that control broad leaf species only. Cover crops are also very important to control water run-off and erosion. In the book it is also clearly shown that using cover crops, and irrigating them for better growth, reduced the overall water usage of a vineyard. This was a very important finding as one of the serious concerns raised against cover crops was that it might increase the water usage of vineyards. In a water scarce country this would have been a serious drawback. Proving that cover crops, and using it as a mulch, decreased the overall water usage was thus a very important breakthrough. Research, especially long-term research, can sometimes be very monoto- nous.When you thus get lucky, and an unplanned event allows you to observe and measure something important it enlightens the day. One such an event was the heat wave experienced during one of the trials and the resultant measurements on the damage to bunches that differed markedly between treatments. See page 158.The researcher must, however, have the presence of mind to take advantage of these opportunities. Enjoy this book. It is the lifework of dedicated researchers.

5

partners and industry grower bodies with the work that our researchers and postgraduate students do. The educa- tion leg of this endeavour strives not only to augment formal qualifications, but also to ensure continuous training options for agriculturalists and others already working in the local industry. The envisaged accredited short courses to be presented through the Chair in Crop Health will build on short courses, The crop chemicals business is rapidly evolving, and it has become highly technical. Villa is passionate about educating the people we work with in our industry to ensure that we have well equipped, well trained people in the agricultural business. We also actively participate and endorse publications of books such as this one to grow the knowledge of our industry even further. We’ve been through a decade of experiential and self-training, which just isn’t good enough for the environment we operate in. Knowledge is key! The local crop protection industry is worth around R9 billion at wholesale level and Villa is one of the leading suppliers of crop protection solutions to distributors in South Africa. The foundations of Villa were already laid in 1989. It has since grown into a formid- able force in crop protection solutions in South Africa. In its first commercial venture in Africa, the Fortune 250 global agribusiness and food company Land O’Lakes, through it’s crop input business Winfield United, acquired majority ownership of Villa Crop Protection in 2015. Since then, Villa has grown from strength to strength and will continue to do so in the future. certificate and diploma courses previously developed by the Villa Academy.

Crop protection, like any other industry, needs smart people. Companies tuned in to their workforce understand that people want to succeed and be good at what they do. Education is just as valued as practical experience to achieve the professional success and status which workers strive to achieve. Villa is committed to provide a new generation of skilled people in agri- culture and for our industry to be equipped with the latest knowledge, scientific facts and experience in the field of crop protection. The company therefore contributed funding for the establishment of a Chair in Plant Health at Stellenbosch University. This venture endeavours to focus on research driven by the needs of the agricultural industry, the education and financial support of students and the provision of further skills training for employees in the agronomic sector. The Chair in Plant Health is a first for South African agriculture. This is a major collaborative effort, and will see the integration of industry needs, industry

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ACKNOWLEDGEMENTS

SHUTTERSTOCK

ACKNOWLEDGEMENTS

The author expresses his sincere gratitude to:

• The South AfricanWine Industry throughWinetech and the South African Table Grape Industry (SATI) for co-funding the writing of this book. • Villa for their financial contribution towards this book and enabling the printing of hard copies. • The Agricultural Research Council (ARC),Winetech, SATI and the dried fruit industry through the Dried Fruit Technical Services for co-funding cover-crop research in South Africa. • The ARC for the opportunity and infrastructure to carry out the research.

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COVER CROPS IN SOUTH AFRICAN VINEYARDS

• The Soil &Water Science research team at the ARC Infruitec-Nietvoorbij for their collaboration and assistance over the years. • Kobus Louw for his technical review and for writing the foreword. • Lucinda Heyns for her organization and liaison with the different parties and for reviewing the book. • Jan Booysen for his technical assistance and guidance. • Anel Andrag fromWinetech and TarrynWettergreen from SATI for their help in securing funding. • Karen Freitag for assisting with the figures and photos in this book. • Anna Mouton for editing, proofreading and data visualisation. • Leandri van Heerden fromAvant-Garde for the design and layout.

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LUCINDA HEYNS

Acknowledgements .....................................................................................................10 CHAPTER 1 INTRODUCTION . ................................................................18 An exploration of the potential of cover crops . ................................................20 Weed suppression ................................................................................................20 Water infiltration, retention and drainage ..........................................................21 Grapevine nutrition . ..............................................................................................23 Pests .......................................................................................................................24 The importance of the physical condition of the soil ......................................25 How to use this book ................................................................................................27 Reference list ..............................................................................................................28 TABLE OF CONTENTS

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COVER CROPS IN SOUTH AFRICAN VINEYARDS

CHAPTER 2 REGIONAL TRIAL SITES . .............................................30 Breede River valley ...................................................................................................32 Climate ....................................................................................................................32 Soil types ................................................................................................................33 Irrigation practices . ...............................................................................................33 Cultivar and management . ..................................................................................34 Where else do these results apply? ...................................................................34 Where to find details of trials in this region .......................................................34 Reference list .........................................................................................................35 Coastal region ............................................................................................................35 Climate ....................................................................................................................35 Soil types ................................................................................................................36 Irrigation practices . ...............................................................................................36 Cultivar and management . ..................................................................................37 Where else do these results apply? ...................................................................37 Where to find details of trials in this region .......................................................38 Reference list .........................................................................................................38 Little Karoo ..................................................................................................................39 Climate ....................................................................................................................39 Soil types ................................................................................................................40 Irrigation practices . ...............................................................................................40 Cultivar and management . ..................................................................................40 Where to find details of trials in this region .......................................................40 Reference list .........................................................................................................40 Lower Orange River region .....................................................................................40 Climate ....................................................................................................................40 Soil types ................................................................................................................41 Irrigation practices . ...............................................................................................41 Cultivar and management . ..................................................................................41 Where else do these results apply? ...................................................................41 Where to find details of trials in this region .......................................................41 Reference list .........................................................................................................42 Olifants River valley ..................................................................................................42 Climate ....................................................................................................................42 Soil types ................................................................................................................43 Irrigation practices . ...............................................................................................43 Cultivar and management . ..................................................................................43 Where else do these results apply? ...................................................................43 Where to find details of trials in this region .......................................................43 Reference list .........................................................................................................44

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TABLE OF CONTENTS

CHAPTER 3 COVER-CROP SELECTION ........................................46 Reference list .........................................................................................................49 Cover-crop plants ......................................................................................................50 Grains . ....................................................................................................................50 Legumes .................................................................................................................51 Brassicas ................................................................................................................52 Breede River valley ...................................................................................................53 Open-land trials .....................................................................................................53 Vineyard trials ........................................................................................................56 Triticale under drip irrigation ................................................................................57 Which cover crops were successful? . ...............................................................57 Reference list .........................................................................................................58 Coastal region ............................................................................................................59 Open-land trials .....................................................................................................59 Vineyard trials ........................................................................................................61 Evaluation of cover crops selected to suppress nematodes . ........................62 Which cover crops were successful? . ...............................................................63 Reference list .........................................................................................................65 Little Karoo ..................................................................................................................65 Vineyard trials ........................................................................................................65 Reference list .........................................................................................................66 Lower Orange River region .....................................................................................66 Vineyard trials ........................................................................................................66 Which cover crops were successful? . ...............................................................67 Reference list .........................................................................................................67 Olifants River valley ..................................................................................................68 Open-land trials .....................................................................................................68 Vineyard trials ........................................................................................................68 Which cover crops were successful? . ...............................................................71 Reference list .........................................................................................................72 Winter-growing weeds that can be used as cover crops . ..............................72 Oat-seed grass ( Ehrharta longiflora ) . ................................................................73 Ripgut brome ( Bromus diandrus ) .......................................................................74 Common wild oats ( Avena fatua ) .......................................................................74

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COVER CROPS IN SOUTH AFRICAN VINEYARDS

Wild barley ( Hordeum murinum ) . .......................................................................75 Reference lis t .........................................................................................................75 Alternative cover crops . ..........................................................................................75 Creeping saltbush ( Atriplex semibaccata ) ........................................................75 Fynbos and renosterveld species . .....................................................................76 Reference list .........................................................................................................77 CHAPTER 4 COVER-CROP ESTABLISHMENT ...........................78 Re-establishment of cover crops ..........................................................................79 Coastal region . ......................................................................................................79 Lower Orange River region .................................................................................81 Olifants River vall ey ..............................................................................................83 Fertilisation of cover crops .....................................................................................85 Reference list .........................................................................................................87 Use of different implements for cover-crop establishment ...........................87 Reference list .........................................................................................................90 CHAPTER 5 WEED CONTROL ...............................................................92 Reference list .........................................................................................................94 Weed species ..............................................................................................................95 Reference list .........................................................................................................98 Breede River valley ...................................................................................................99 Winter-growing weeds ..........................................................................................99 Summer-growing weeds ....................................................................................100 Reference list .......................................................................................................102 Coastal region ..........................................................................................................102 Winter-growing weeds ........................................................................................102 Summer-growing weeds ....................................................................................104 Weed succession and dominance . ..................................................................106 Reference list .......................................................................................................114 Little Karoo ................................................................................................................114 Reference list .......................................................................................................115 Lower Orange River region ................................................................................... 115 Winter-growing weeds ........................................................................................ 115 Reference list .......................................................................................................116

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TABLE OF CONTENTS

CHAPTER 6 EFFECT OF COVER CROPS ON WATER USE . .............................................................................................. 118 Coastal region ..........................................................................................................119 Reference list .......................................................................................................120 Breede River valley .................................................................................................120 Reference list .......................................................................................................121 Olifants River valley ................................................................................................122 Reference list .......................................................................................................123 CHAPTER 7 EFFECT OF COVER CROPS ON SOIL CARBON AND MINERALS . ...........................................................124 Breede River valley .................................................................................................125 Organic carbon ....................................................................................................126 Nitrogen in the soil ..............................................................................................127 Other minerals .....................................................................................................129 Reference list .......................................................................................................130 Coastal region ..........................................................................................................130 Organic carbon ....................................................................................................131 Nitrogen in the soil ..............................................................................................132 Other minerals .....................................................................................................134 Reference list .......................................................................................................134 Olifants River valley ................................................................................................135 Organic carbon ....................................................................................................135 Nitrogen in the soil ..............................................................................................136 Other minerals .....................................................................................................139 Reference list .......................................................................................................139 CHAPTER 8 EFFECT OF COVER CROPS ON GRAPEVINE NUTRITION AND PERFORMANCE .......................140 Breede River valley .................................................................................................141 Nitrogen status of vines . ....................................................................................142 Vegetative growth and grape yield ...................................................................145 Grape and wine quality ......................................................................................147 Root distribution ..................................................................................................147 Reference list .......................................................................................................149 Coastal region ..........................................................................................................149 Nitrogen status of vines . ....................................................................................149

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COVER CROPS IN SOUTH AFRICAN VINEYARDS

Vegetative growth and grape yield ...................................................................152 Grape and wine quality ......................................................................................155 Reference list .......................................................................................................156 Lower Orange River region ...................................................................................156 Nitrogen status of vines . ....................................................................................156 Bunch damage ....................................................................................................158 Grape yield and quality ......................................................................................158 Reference list .......................................................................................................158 Olifants River valley ................................................................................................159 Nitrogen status of vines and grape juice .........................................................160 Vegetative growth and grape yield ...................................................................163 Grape and wine quality ......................................................................................165 Reference list .......................................................................................................165 CHAPTER 9 PESTS ......................................................................................166 Snails ..........................................................................................................................167 Reference list .......................................................................................................169 Ants and mealy bugs ..............................................................................................169 Reference list .......................................................................................................170 Nematodes . ...............................................................................................................170 Coastal region . ....................................................................................................170 Lower Orange River region ...............................................................................173 Reference list .......................................................................................................175 CHAPTER 10 PRACTICAL GUIDELINES .......................................176 Order seed .................................................................................................................176 Cover-crop management before grapevines are planted .............................178 Cover-crop management in established vineyards .......................................179 Sowing dates and irrigation . ................................................................................180 Breede River valley . ...........................................................................................180 Coastal region . ....................................................................................................181 Lower Orange River region ...............................................................................181 Olifants River valley ............................................................................................182 Fertilisation ...............................................................................................................182 Control of cover crops ...........................................................................................182

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1

LUCINDA HEYNS

INTRODUCTION

Modern agriculture relies on chemical inputs to increase yields and production efficiency, but synthetic fertilisers and crop- protectants may damage the environment and threaten human health if not used judiciously. Therefore, producers must apply soil cultivation practices that are both sustainable and effective. The extent to which producers can satisfy consumer concerns and demands while also accommodating economic realities will determine the measure of their success.

Soil underpins the success or failure of a vineyard. Therefore, attention to soil health is of the utmost importance to producers and to their heirs. Maintenance and improvement of soils also concern everybody else, as we all depend on soils to sustain agricultural productivity and environmental quality.

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COVER CROPS IN SOUTH AFRICAN VINEYARDS

Tillage has always been central to agriculture. In viticulture, soil tillage can be defined as the management of the soil to maintain the favourable conditions created for grapevine roots by expensive soil preparation. Soil preparation improves the physical properties of the soil to promote root penetration and water infiltration. It may also include incorporation of amendments such as lime and phosphate. Soil management aims to maintain the improved soil physical conditions created by effective soil preparation. Goals include eliminating weeds, increas- ing organic matter, limiting compaction, preventing erosion, and optimising water use.

Mechanical tillage is one method for managing soils. However, mechanical tillage disturbs the soil, which can have negative effects on soil health and the physical structure of the soil (Figure 1.1). These impacts include loss of soil organic matter, reduced soil biodiversity, increased erosion, loss of water-infiltration capacity and, to a lesser extent, reduction of water-holding capacity.

MECHANICAL TILLAGE CAN HAVE NEGATIVE EFFECTS ON SOIL HEALTH AND THE PHYSICAL STRUCTURE OF THE SOIL.

FIGURE 1.1. Irresponsible cultivation has compacted the soil in this full-bearing vineyard, leading to restricted grapevine root growth. (Photo: ARC Infruitec-Nietvoorbij).

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CHAPTER 1 INTRODUCTION

Growing evidence of the drawbacks of mechanical tillage has led to greater recognition of the value of cover crops as an alternative to traditional soil- management practices.This book is a resource for growers who are interested in adopting cover crops, in that it summarises the results of long-term research trials conducted in vineyards in the main grape-growing regions of SouthAfrica.The characteristics of the trial sites are described under Regional Trial Sites in Chapter 2 (p30). An exploration of the potential of cover crops Weed suppression In recent years, chemical weed control has come under pressure as consumer opposition to agrichemical use has increased, and as more weeds have developed herbicide resistance. Cover crops offer a biological means to control weeds by preventing their germination and suppressing their growth. Besides being more environmentally friendly, biological weed control has the advantage of slowing the development of herbicide-resistant weeds. The efficacy of cover crops as weed-control agents is strongly dependent on how much biomass the cover crop produces. Firstly, the cover crop needs to grow strongly to outcompete winter-growing weeds. Secondly, the dry matter produced by the cover crop must translate into enough mulch to prevent germination of weed seeds. Biomass production by any given cover crop will depend on the climate and soil of the site where it is grown. For this reason, the long-term trials described in this book began by testing a wide range of cover-crop candidates in open-land conditions in different regions.Trials in vineyards then evaluated the most promising candidates. This work is described in Cover-Crop Selection in Chapter 3 (p46) andWeed Control in Chapter 5 (p92). Most of the cover crops discussed in this book are annuals.Throughout the book, the reader will find reference to two main management strategies for these annual cover crops in vineyard trials. In the first, the cover crops were sown every year, and full-surface chemical control was applied to the vineyard just before bud-break.As the cover crops had usually not yet flowered by this time, they did not reseed.

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COVER CROPS IN SOUTH AFRICAN VINEYARDS

In the second strategy, full-surface chemical control was applied at the end of November, to allow the cover crops to set seed, thereby reducing the need to sow every year.As part of a third strategy, the cover crops were controlled chemically during the first week of October, to extend the growing season and thereby increase dry-matter production. Mechanical weed control was applied from just before grapevine bud- break.Where mechanical control is mentioned, it was only used in the work

row.The vine row was still chemically treated. It should be obvious from the above that cover crops are not seen as a stand-alone weed-control solution, but that they form part of an integrated production system. Nonetheless, cover crops ren- der the use of pre-emergence herbicides unneces- sary, and reduce the requirement for post-emer- gence herbicides. Research on the best practices for cover-crop establishment, with the aim of maximising dry- matter production, are discussed in Cover-Crop Establishment in Chapter 4 (p78).

COVER CROPS RENDER THE USE OF PRE- EMERGENCE HERBICIDES UNNECESSARY, AND REDUCE THE REQUIREMENT FOR POST-

EMERGENCE HERBICIDES.

The effects of cover crops on weeds are described in Weed Control in Chapter 5 (p92). Over time, certain weed species adapt to the weed-control method applied, whether these are chemical or biological. Weed Control therefore includes results of studies on the effect of different cover crops, and their management, on weed populations. Knowing the dynamics of a weed population under cover-crop management allows the producer to adapt his weed-control strategies, thereby preventing the problem weeds from becoming dominant over time. Water infiltration, retention and drainage Bare soil disrupts optimal water use in several ways. Firstly, bare soil tends to form an impermeable crust when impacted by water droplets. Although the crust can be broken by mechanical cultivation, it may recur after a single rain or irrigation event.Water is also more likely to run off bare soil.The net result is reduced water infiltration.

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CHAPTER 1 INTRODUCTION

40,00

4,00

35,00

3,50

30,00

3,00

25,00

2,50

Water run-off Irrigation Water run-off Rainfall Erosion Rainfall

20,00

2,00

15,00

1,50

10,00

1,00

Soil erosion in tonnes per hectare

5,00 Water run-off as percentage of total

0,50

0,00

0,00

Bare soil

Straw mulch

Cover crop

Research conducted in vineyards by Louw and Bennie (1992) showed that rainwater runoff was approximately 27% on bare soils compared to about 1% on soils covered by either mulch or a cover crop. Irrigation runoff was approximately 38% on bare soils compared to about 0.6% on soils covered by either a mulch or a cover crop (Figure 1.2). Bare soil is vulnerable to erosion by wind and water, with the most fertile topsoil the first to be lost (Figure 1.3). Soil loosened by preparation is especially vulnerable to erosion, so it is wise to lay out the vineyard and establish a cover crop immediately after preparation. The mulch created by this cover crop can also help to suppress weed emergence in newly established vineyards. In the work done by Louw and Bennie (1992), approximately 3.8 tonnes of soil was lost from bare soil per hectare after seven rain events, compared to losses of 0.5 tonnes per hectare from soils that had cover crops, and 0.03 tonnes per hectare from mulched soils. Louw and Bennie (1992) found that a mulch of 6–8 tonnes per hectare should eliminate runoff from and erosion of most soils. Other studies have found that mulches of 5 tonnes per hectare prevented crust formation, water runoff, and soil erosion. Loch and Donnollan (1988) indicated that as little as 0.1 tonnes per hectare significantly reduced erosion. FIGURE 1.2. Water run-off during a 46 millimetre irrigation event, and measured over 34 rainfall events, and soil erosion during seven rainfall events, on a Glenrosa soil. Adapted from Louw (1991).

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COVER CROPS IN SOUTH AFRICAN VINEYARDS

FIGURE 1.3. Soil erosion in a vineyard with no protective mulch or winter-growing cover crop. (Photo: ARC Infruitec-Nietvoorbij).

Besides improving infiltration, cover crops and mulches also reduce water loss by creating a barrier to evaporation and by maintaining cooler soil temperatures.Water savings associated with cover crops are described in Effect of Cover Crops onWater Use in Chapter 6 (p118). Researchers have also found that microbial biomass and activity increase under surface residues, facilitating the release of available forms of nutrients from inorganic and organic sources. Grapevine nutrition Nitrogen contribution by cover crops depends on the species and cultivar, length of growing season, climate, and soil conditions. Legumes are well- known for their ability to fix atmospheric nitrogen. Researchers have shown that the amount of nitrogen fixed by annual medics is related to their total dry-matter production, and nitrogen fixation by legumes is greatest during flowering and pod fill.Temperate legumes retain 10%–29% of the fixed nitrogen in their roots. Higher levels of organic matter promote nutrient cycling by microbes in the soil and thereby the availability of nutrients to vine roots.The slow release of nitrogen by microbial activity supplies vines with additional nitrogen throughout the growing season. For wine-grape vines, Conradie (1994) states that soil with a clay content of 6% or more may provide wine-grape vines with sufficient nitrogen when

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CHAPTER 1 INTRODUCTION

organic carbon in the soil is 0.9% or more. For soils with a clay content of less than 6%, the application of nitrogen may be unnecessary if the carbon content of the soil exceeds 0.6%. Besides increasing water loss, high soil temperatures also accelerate microbial breakdown of organic matter. Organic matter is likewise broken down faster when the soil is aerated during mechanical cultivation.Therefore, soil-cultivation practices that keep the soil cool and do not aerate the soil excessively should be used, to facilitate the slow release of nutrients and to promote the build-up of organic matter. The results discussed in Effect of Cover Crops on Soil Carbon and Minerals in Chapter 7 (p124) show that soil carbon can be increased by planting cover crops, irrespective of climate or soil type. Soil carbon was also increased by winter-growing grasses in the work row. Effect of Cover Crops on Soil Carbon and Minerals in Chapter 7 (p124) also describes this impact of cover crops on nutrients such as nitrogen in the soil and the vine. It is important to note that the impacts of cover crops on soil are more strongly influenced by soil type than by climate, so readers should consult the section of Effect of Cover Crops on Soil Carbon and Minerals in Chapter 7 (p124) which deals with the soil type that most closely matches their own, and not only focus on the section pertaining to their region. Whereas cover crops can enhance soil fertility and improve growing conditions for grapevines, cover crops can also potentially compete with vines for resources. Effect of Cover Crops on Grapevine Nutrition and Performance in Chapter 8 (p140) examines the interaction between cover crops and the growth of grapevines, specifically root distribution, vegetative growth, and grape yield and quality. Pests When it comes to pests, cover crops can be a two-edged sword. On the one hand, cover crops can reduce the impact of pests directly, for example through releasing biofumigants, or indirectly, through providing habitat for predators. On the other hand, cover crops may be a breeding ground for pests that invade grapevines. Whether a cover crop has a positive or negative effect on pests in a vineyard will depend on the pest profile of that specific site, as well as on

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COVER CROPS IN SOUTH AFRICAN VINEYARDS

which cover crops are planted, and how they are managed. Pests in Chapter 9 (p166) describes trials that have examined the interactions between cover crops and snails, ants, mealy bugs, and nematodes. It is worth mentioning that the studies discussed in Pests in Chapter 9 (p166) did not find any evidence of cover crops encouraging pests. The importance of the physical condition of the soil Grapevine roots find it easier to penetrate loose than compacted soil. The correct physical soil characteristics will promote the development of a large, healthy root system, whereas soil compaction will limit root growth (Figure 1.4). This is why the soil is loosened to a depth of 800–1 000 millimetres when preparing to plant vines.

FIGURE 1.4. Restricted root growth in a non-bearing vineyard due to recompaction caused by mechanical soil cultivation after soil preparation. (Photo: ARC Infruitec-Nietvoorbij).

The problem is that the prepared soil is readily recompacted by the actions involved in establishing the vineyard.Tractor and implement traffic commonly cause recompaction when tractors are allowed on prepared soils, so this should be avoided at all costs (Figure 1.5).

25

CHAPTER 1 INTRODUCTION

Soil resistance in kPa x 10 2

0 2 4 6 8 10 12 14 16 *

10

*

*

20

* = soil surface

30

40

Soil after preparation After first pass After second pass

Soil depth in centimetres

50

60

FIGURE 1.5. Recompaction after two passes with a wheeled tractor on a prepared soil. Adapted from Van Huyssteen (1981).

Cultivation itself, also contributes to compaction.The ploughshares or discs from soil-cultivation implements that repeatedly pass at the same working depth will gradually seal the soil pores at that depth. Mechanical cultivation executed directly after soil preparation, to level the soil surface or establish cash crops before a vineyard is established, will cause the soil to compact at the working depth. Furthermore, rain and irrigation wash silt and clay particles from the loosened soil to the deeper soil layers, clogging the soil pores. This results in a compacted soil layer through which the grapevine roots penetrate with difficulty.The highest degree of compaction takes place when the soil water content is near field water capacity, which is unfortunately when soils are mostly cultivated (Figure 1.6).

26

Figure 1.6

COVER CROPS IN SOUTH AFRICAN VINEYARDS

2000

1950

1900 Soil bulk density in kilogram per metre 3 1850 1800 1750 1700 1650 1600

Field water capacity

1550

1500

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Soil water content as percentage mass

FIGURE 1.6. Compaction of a Clovelly soil shown as bulk density resulting from cultivation at different soil-moisture levels. Adapted from Van Huyssteen and Weber (1980).

The result of compaction is that deeper soil layers become nearly inaccessible to the grapevine roots.The root system is restricted, and so the potential for the plant to absorb water and nutrients is also restricted, which ultimately reduces its performance. One of the benefits of cover crops is their ability to counteract compaction. Cover crops protect the surface of the soil from impacts such as traffic and water. How to use this book Most of the chapters have a general introduction, followed by sections that describe research conducted in the different grapevine-growing regions. In most cases, there is information on the Breede River valley, Coastal region, and Olifants River valley, because these were the regions in which the main cover-crop trials were conducted.Where information is available for other regions, such as the Lower Orange River region, this is presented in separate sections.

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CHAPTER 1 INTRODUCTION

A list of references is provided at the end of each section, for those readers who wish to consult the original sources. Photos of the main cover crops discussed in this book are provided in Cover-crop Plants in Chapter 3 (p50). Photos of weeds can be found inWeed Control in Chapter 5 (p92). The final chapter of the book provides guidelines for establishing and managing cover crops in different regions. By adhering to these guidelines, the producer can maximise cover-crop growth and the resulting benefits. Reference list ConradieWJ. 1994, September. Vineyard fertilisation. In Proceedings of work- shop on vineyard fertilization. Nietvoorbij (Vol. 30). Loch RJ and DonnollanTE. 1988. Effects of the amount of stubble mulch and overland flow on erosion of a cracking clay soil under simulated rain. Soil Research , 26(4), 661–672. Louw PJE. 1991. Die beskerming van onstabiele bogrondstruktuur vir ’n volgehoue hoë infiltrasietempo in wingerdgronde. Masters dissertation. Bloemfontein: University of the Free State. Louw PJE and BennieATP. 1992.Water runoff and soil erosion in vineyard soils. Australian Grapegrower & Winemaker, Annual Technical Issue , 100–113. Van Huyssteen L. 1981. Soil cultivation. In: Burger J and Deist J. Win­ gerdbou in Suid-Afrika .ARC Infruitec-Nietvoorbij, Stellenbosch, South Africa, 283–307. Van Huyssteen L and Weber HW. 1980. The effect of conventional and minimum tillage practices on some soil properties in a dryland vineyard. South African Journal of Enology and Viticulture , 1(1), 35–45.

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