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MECHANISED VINEYARD PRUNING: PREREQUISITES FOR THE VINEYARD AND REACTION OF THE VINE
E Archer, Department of Viticulture and Oenology, University of Stellenbosch
1. INTRODUCTION
Mechanised pruning of South African wine grape vineyards is a reality and already part of the decision making process when new vineyards are established. This kind of pruning is quite established in the Olifants River valley, while being increasingly incorporated in other areas. Due to increasing labour costs producers are compelled to consider mechanised pruning as an alternative despite the uncertainty surrounding the reaction of the vine as well as the prerequisites with which a vineyard has to comply for successful mechanised pruning.
2. THE REACTION OF THE VINE TO MECHANISED PRUNING
Any of the alternative pruning methods (mechanised, minimal, none) result in a significant increase in the number of buds per vine. The vine has built-in mechanisms which enable compensation for excessive bud loads. This ability of the vine to compensate depends on an inherent ability to make adjustments in the various yield components, which are the following:
2.1 Budding percentage
The number of buds per vine which bud successfully are determined by:
the satisfaction of the cold requirement during the end of May/beginning of June
the even distribution of mobilised reserve nutrients
apical dominance as influenced by an upright or creeping growth habit.
Compensation: the more buds left during pruning, the fewer actually bud and consequently the budding percentage is lower. This is ascribed to an uneven distribution of nutrients and hormones in particular, as well as the apical dominance effect of upright, longer bearers.
2.2 The number of flowers per flower cluster
This is determined during the induction phase of the creation of flower cluster primordia in the green buds during the spring of 18 months before. This process is influenced by light, temperature and cytokinin in particular. Light and temperature are usually not a problem, but cytokinin (produced in the root tips) is diluted when there are more demand points.
Compensation: the more buds left during pruning, the more green buds per vine. The accompanying dilatory effect of cytokinin in particular causes fewer flower bunch primordia to be formed, which results in fewer flowers per bunch and consequently smaller bunches. This reduction in size of the bunches is therefore ascribed mainly to fewer berries, but as there are a lot more buds per vine, the number of bunches per vine is significantly higher.
2.3 Berry mass
This is determined by the success of berry growth phases I (cell division) and III (cell expansion) and the success hereof is a function of the source: demand point ratio of photosynthetic products in the vine. With normally pruned vines the ratio is in favour of the developing berries, but with mechanically pruned vines the demand points (berries) are significantly more, thus causing a dilution of growth substances. Cell division and cell expansion are therefore impaired.
Compensation: the more buds left during pruning, the more berries per vine, with cell expansion in particular being impaired. The result is smaller berries.
2.4 Number of shoots per vine
This is determined by the number of budding buds and seeing that mechanically pruned vines have far more buds than normally pruned vines, there are far more shoots. Compared to the number of buds left during pruning, however, the mechanically pruned vines have fewer shoots.
Compensation: the more buds left during pruning, the lower the budding percentage, which means that even though there are still more shoots per vine, the foliage is not nearly as dense as would have been the case if all the buds had budded.
2.5 Mass per shoot
This is determined by the success of initial shoot growth in particular, which in turn depends on the success of the translocation and division of reserve nutrients. In this instance too, therefore, the source: demand point ratio plays an important role.
Compensation: the more demand points, the more growth substances have to be divided, the less being available per demand point. The subsequent weaker growth per demand point therefore causes shorter and lighter shoots.
In short, the total compensation of mechanically pruned vines may be summarised as follows:
More shoots per vine, but shorter and lighter.
More bunches per vine, but smaller, looser and lighter.
Smaller berries.
In due course the bunches are carried more and more towards the periphery of the foliage.
2.6 Influence of mechanised pruning on yield and quality
For the past five years, a Cabernet Sauvignon vineyard on Simonsig has been mechanically pruned in collaboration with Francois Malan. The results are shown in the table below.
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Results of mechanically pruned Cabernet Sauvignon on Simonsig, 1994/5 to 1997/98 |
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Yield (t/ha) |
Sugar concentration (°B) |
Acid concentration (g/l) |
pH |
| 1994/95 |
Manually |
12.1 |
25.6 |
7,25 |
3,64 |
| Mechanically |
22,4 |
23,5 |
5,85 |
3,63 |
| 1995/96 |
Manually |
12,5 |
24,0 |
6,8 |
3,83 |
| Mechanically |
23,0 |
22,8 |
6,9 |
3,55 |
| 1996/97 |
Manually |
9,01 |
23,0 |
6,7 |
3,55 |
| Mechanically |
18,43 |
22,4 |
6,9 |
3,46 |
| 1997/98 |
Manually |
7,9 |
22,4 |
6,7 |
3,57 |
| Mechanically |
17,4 |
22,6 |
6,4 |
3,58 |
| Average |
Manually |
10,4 |
23,8 |
6,9 |
3,65 |
| Mechanically |
20,3 |
22,8 |
6,5 |
3,58 |
This shows that mechanised pruning almost doubled the yield of Cabernet Sauvignon, slightly decreased the sugar and acid concentration and reduced the pH. The wines of each year are characterised by a pronounced fruitiness on the nose as well as softer tannins.
The smaller berries of the mechanically pruned section were smaller than the hand pruned control, which probably caused a better skin: juice ratio. Furthermore, the grapes on the mechanically pruned vines are quite obviously carried closer to the periphery of the foliage, causing the more pronounced flavour of the wine (grapes more exposed to environmental factors).
3. PREREQUISITES FOR THE VINEYARD
3.1 Prerequisites for the soil
Alternative pruning methods cause greater vine volumes. With the vine striving towards a 1:1 ratio between growth above and below the soil, this means that relatively large root volume is required to balance the above-ground volume. This requires deeper and more fertile soil for more root tips (sufficient production of cytokinins) as well as more deep roots to absorb water from the subsoil in order to provide enough transpiration water for the bigger leaf surface. The soil must therefore have sufficient buffer capacity to protect the vineyard against inclement climatic conditions, especially during ripening. Vineyards that are transformed for the purposes of mechanised pruning require more water and nutrients than before the transformation.
3.2 Prerequisites for the vineyard
3.2.1 Existing vineyards
A prerequisite for the transformation of existing vineyards to vineyards which qualify for mechanised pruning, is vigour. The more vigorous the vineyard, the better the chances of success. The main reason for this is that the cordon height has to be lifted to 1,2 m to 1,8 m above the surface of the soil. This means furthermore that the poles of the existing vineyard should be high enough to accommodate the higher cordon wire. Provision should also be made for the installation of additional inside poles to carry the increased weight of mechanically pruned vines. The distance between adjacent inside poles in the row should preferably not exceed 6,0 m. When vineyards are to be transformed for mechanised pruning, the vines should be spaced at least 3,0 m x 1,4 m. It is not necessary to put up a new cordon wire at the new height; two foliage wires strung together fulfil the same function.
3.2.2 New vineyards
Soil preparation should be very thorough for new vineyards planted for mechanised pruning. There is a choice of two trellis systems, i.e. a high single strand Hedge system or a high double strand Hedge system. For these systems the cordon wire should preferably be strung through a hole in the poles, or alternatively be attached to the pole with heavy duty staples. The weight of the above-ground growth plus the increased crop of mechanically pruned vineyards is much bigger than hand pruned vines and consequently the trellis system should be sturdier than usual. The recommended trellis system is shown in Figure 1.
3.3 Prerequisites for the climate
With vigour being such an important prerequisite for the successful application of mechanised pruning, it seems better suited to the warmer climatic regions of South Africa. Under these circumstances it also seems that the quality may be improved due to smaller berries and more balanced growth.
Vineyards which grow strongly in the cooler climatic areas react equally well to mechanised pruning, especially since the bunches are better exposed and therefore develop stronger fruit flavours. At present it looks as though the prevailing climate in various viticultural areas in South Africa is not too limiting and mechanised pruning is applicable should vigour allow.
3.4 Prerequisites for the cultivar
With mechanised pruning far more short and weak shoots with grapes are encountered in the foliage than with manually pruned vines. It is a well-known fact that the quality of any bunch of grapes is equal to the physiological quality of the shoot on which it is situated. As a result it seems possible that mechanised pruning, in certain cases, can cause a reduction in quality, with the extraction quality of the wine in particular suffering. For this reason it remains to be seen what the ageability is of wine deriving from mechanically pruned vineyards. As a result mechanised pruning cannot be recommended yet for vineyard blocks destined for flagship wines.
4. CONCLUSIONS AND RECOMMENDATIONS
Mechanised pruning in the South African wine grape industry has come to stay and it can be applied with great success to suitable vineyards. The majority of vigorous vineyards can be pruned in this way and great labour savings are possible. Furthermore, in most cases this kind of pruning imparts another, fruity dimension to the wine, which holds great possibilities for blending. Therefore it is recommended that only certain vineyards on the farm be pruned using this method. In this way labour may be freed to pay more attention to the top blocks with regard to both winter pruning and foliage management.
The only foliage management applied to mechanically pruned vineyards is the topping of shoots to keep the working row open and keep the shoots away from the soil surface for proper aeration at the bottom. This process can also be mechanised. With sensible planning of the blocks that can be pruned mechanically, the existing labour force on any wine farm can be applied a lot more efficiently in the top blocks.
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