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A Technical Guide for Wine Producers
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RECENT ARTICLES | WYNBOER HOME
Increased Yield in Wine Grapes for Specific Production Goals
INTRODUCTION A permanent increase in yield is only possible if the number of effective buds per vine (per ha) can be increased sustainably. The only way to retain the quality of such an increased yield is by having sufficient effective leaves to achieve optimal ripening (Archer, 1984). Maximum performance per fertile bud is only possible if the microclimate and source: point of demand ratios in the foliage during critical periods throughout the entire growing season can be maintained at optimal levels (Hunter, 2000). Whether effective buds will be obtained therefore depends on sufficient and effective leaf surface. A vine bud is effective when sufficient fertility has been induced so that it can bring forth two normal bunches (for the particular cultivar). It goes without saying that for the bud to make a contribution to yield, it should burgeon. The shoot that develops out of such a bud must have sufficient trellis space so that it does not overlap with adjacent shoots on the one hand and may reach an effective length of 1,2 to 1,6 m (depending on cultivar and growing conditions) on the other hand. One of the numerous important functions of a vine shoot is to carry the leaves in such a way as to expose the leaves on it to sufficient sunlight energy over the longest possible period. The accommodation of shoot growth on the trellis system is therefore of critical importance in order to fulfil this function. A balanced and well-functioning shoot that supports efficient bunch nutrition, is one in which the spring growth between veraison and ripeness comes to a spontaneous arrest (Jamain, 1901), while it has sufficient young leaves (primarily on lateral shoots) to support the final stages of ripening (Hunter, 2000). Any effort to achieve a sustainable increase in yield must comply with the above-mentioned principles in order to be successful. Furthermore it is very important to remember that a bigger crop in any existing vineyard causes additional stress to the vine. In such instances the vine frame and the root system in particular must be capable of supporting the additional stress. Soil preparation should not only ensure good root distribution and composition, but practices such as fertilisation and irrigation must be adjusted to supply the increased demand for water and nutrients. INFLUENCE OF GENETICS ON YIELDCultivars differ from each other genetically with regard to bud fertility (Fig. 1), bunch and berry size (Table 1). Consequently they differ as to the amount of grapes produced per ha (Table 1).
Table 1: Bunch and berry size and yield per ha of a number of wine grape cultivars (Carstens, Burger and Kriel, 1981; De Villiers and Theron, 1987)
* Lowest and highest yield obtained with vertical trellising in various cultivar trials in Stellenbosch, Robertson and Lutzville over a 10 year period. The scion cultivar clearly has a big influence on the eventual yield per ha. The rootstock also makes an important contribution to yield (Table 2 and Table 3). Table 2: The influence of rootstock cultivars on the yield (t/ha) of wine grape cultivars in Stellenbosch over a 9 year period (Carstens, Burger and Kriel, 1981)
Table 3: The influence of rootstock cultivars on the yield of Chenin blanc in Oudtshoorn and Montagu over a 5 year period (Carstens, Burger and Kriel, 1981)
Each scion/rootstock combination therefore has a genetic limitation above which yield cannot be increased successfully. It is also clear that the environment plays a significant role in the yield level at which this genetic character is expressed, e.g. it would be futile to try and produce 20 t/ha Cabernet Sauvignon/Richter 99 in Stellenbosch with normal row widths and a vertical trellis. The yield levels of scion cultivars differ from each other due to inherent differences in bud fertility and bunch mass. The yield levels of rootstock cultivars differ from each other due to differences in inter alia manner of growth, the accumulation of reserve nutrients and the amount of natural growth hormones produced by each (Champagnol, 2003). In each instance the environment and soil conditions will influence the eventual performance. It is clear that when the crop is to be increased in existing vineyards, genetics deserve proper consideration. An additional factor that cannot be disregarded, is the degree of virus infection (leafroll in particular) to which the plant material is subject. Leafroll infection often means that the second principle of yield increase, namely sufficient effective leaf surface, cannot be satisfied. Yield increase in leafroll infected vineyards easily exceed the level at which the leaves are able to sustain optimum ripening. In most cases it is therefore not possible to achieve a successful increase in yield in such vineyards. INFLUENCE OF VINEYARD LAYOUT AND GROWTH ON YIELDThe number of rows per hectare, in other words the row width, have a huge impact on the amount of grapes produced per ha (Table 4). Table 4: The influence of row width on the yield of wine grapes
1) With bearer spacing of 10 cm between bearers. 2) For an average of 3 bunches per short bearer. 3) e.g. Cabernet Sauvignon, Cabernet franc, Pinot noir 4) e.g. Merlot, Gewürztraminer, Pinotage, Malbec 5) e.g. Cape Riesling, Morio Muscat, Sauvignon blanc 6) e.g. Bukettraube, Carignan, Colombar, Hárslevelü, Mourvèdre When planning the vineyard layout it is therefore obvious that the rows should be as narrow as practically possible. For this reason it is often necessary to plant downslope since contour planting results in rows that are too wide. For the same reason the viticultural industry should insist on the design of narrower equipment and stop planning vineyards around mechanics. It is extremely important, however, that the bigger total cordon length per hectare obtained with narrower rows, should be effective with regard to crop mass and quality and consequently careful attention should be paid to the in-row vine spacing and trellis system. Due to the big variation in soil types in the Western Cape, there are few vineyard blocks in which the same in-row vine spacing can be used. By varying the in-row vine spacing according to the variation in soil potential for vigour, the total cordon length per hectare can be effectively retained. Wider in-row spacing on higher potential soils (or sections in the same block) and narrower in-row spacing on lower potential soils (or section in the same block) satisfy the principle of balanced growth below and above the surface. By so doing more vigorous vines obtain a bigger and less vigorous vines a smaller cordon space on which a balanced bud load may be practiced when pruning. This not only prevents internal foliage shading and shoot crowding, but also ensures homogeneous shoot length and strength per vineyard block. An additional benefit is that the occurrence of weak shoots and the accompanying thinning out of grapes can be largely prevented. At the insistence of winemakers in a mostly futile effort to increase wine quality, producers are often forced to thin out bunches. In many instances the fact that grapes were removed had no positive effect on the quality. It has also been proved by research that ill-judged bunch thinning (without proof of noticeable stress conditions) has no quality benefit (Van Schalkwyk & De Villiers, 1992; Van Schalkwyk, et al., 1996). As long as a vine shoot reaches a length of 120 cm and lateral shoots are present (Hunter, 2000), two bunches per shoot can be ripened optimally and bunch thinning will only bring about a mass loss per hectare. On the whole too many bunches are thinned out in South Africa. INFLUENCE OF VINE SHAPE ON YIELDIn some vineyards poor vine shape is responsible for up to 20% crop loss (Zeeman, 1984). Vine shaping practices come into play as well as years of maintenance by means of correct pruning practices. Unbalanced cordon arms as well as poorly shaped cordons play a significant role in the crop load that is realised per bearer. Weak and thin bearers must be pruned back to one bud. This results in one shoot from which one bunch must be removed in most instances. Overly vigorous bearers, on the other hand, usually result in overly vigorous shoots, the grape composition of which is hampered by strong growth. A build-up of bearers results in sub-optimal use of the trellis system and reduces the efficient leaf surface. This in turn reduces the yield and quality potential of the vineyard.
Incorrect vine shape is a major source of uneven shoot length. Short shoots with insufficient leaves necessitate the practice of thinning out grapes and the more they occur in a vineyard, the more grapes must be removed and the higher the labour costs. A good, balanced vine shape that allows for the maximum number of uniform shoots ensures ripening of a much bigger crop than when the shoot length is inconstant.
Practices that contribute to homogeneous shoot length include: differentiated pruning based on shoot circumference; purposeful and regular tipping of vigorous shoots and efficient shoot positioning. In this regard it goes without saying that long term practices should be suited to the soil potential and that balanced cordons be obtained during vine shaping. INFLUENCE OF PRUNING METHOD ON YIELDThroughout the world one finds numerous hand pruning methods, but all of them can be categorised in one of four pruning systems, viz: (i) short cut (2 buds), (ii) half long cut (±6 buds), (iii) long cut (±12 buds) and (iv) combinations of the aforementioned (Zeeman & Archer, 1981). The influence exerted by the pruning method on the yield is directly dependent on the number of effective buds allowed by the method. An effective bud is fertile and budding results in a normal, good quality shoot. Table 5: The influence of various pruning methods on the performance of Cabernet Sauvignon over a 5 year period (Archer, 1982)
Although significantly fewer buds per vine were allocated by the short cut system, it did not result in fewer grapes than the other pruning treatments (except for the bruised long bearers). This may be ascribed to a significantly higher budding percentage. The higher yield obtained with the bruised, horizontal long bearers is ascribed to a better fertility coefficient and a higher bunch mass (data not indicated). If the budding percentage of buds on the long bearer pruning system can be increased, this pruning system will result in the highest yield. On the other hand, if the number of effective buds in the case of the short cut system can be increased, this system will produce the highest yield. The occurrence of dead arm disease increases the risk involved with the long cut method and consequently the short cut is the pruning method of choice in South Africa. To increase the crop using the short cut method involves more than simply allowing for more short bearers per vine. When short bearers become crowded over time the result is a decrease in yield as a result of the negative effect of canopy shade on bud fertility. Lower bud fertility means fewer bunches and fewer grapes on the vine mean the vigour:yield balance swings in favour of growth. Case studies in the industry often show that in due course of time such vines produce more and more infertile shoots and in each instance this catch 22 situation can only be remedied by applying correct bearer spacing and backing it up with suckering. An efficient bud load increase can be obtained by the addition of Cazenave bearer units.
An increase in bud load normally results in increased yield, but the effect is usually influenced by the compensatory ability of the vine. Table 6: Influence of bud load on the performance of Chenin blanc (Jooste, 1983)
Figures in the same line followed by the same letter do not differ significantly at p < 0,05. An increase of 100% in the bud load caused an average increase of 55% in yield as a result of the compensatory ability of the vine. This compensation occurred mostly with regard to reduced bunch mass (data not indicated) as well as a decrease in bud fertility. The data in Table 6 emphasises the important principle that increased bud load should be accompanied by an increase in the effective leaf surface. The impact of increased bud load on yield also depends on the cultivar (Table 7). Table 7: The influence of bud load on the performance of a number of wine grape cultivars at NIWW, Stellenbosch (Archer, 1984)
An upward adjustment in the bud load resulted in increased yield in all cultivars except Morio Muscat, which received the same bud load as a control. However, the increased yield percentage did not coincide with the increased bud load percentage and differed from cultivar to cultivar. In most cases increased bud load went hand in hand with decreased wine quality, but it is clear that there are major differences from one cultivar to the next. Cultivars with outspoken colour and flavour components show a bigger decrease in quality than the more neutral cultivars.
Table 8: The influence of bud load on the performance of Red Muscadel, Robertson (Archer & Fouché, 1987)
The compensatory ability of the vine means that the percentual increase in bud load is not reflected by the percentual increase in yield. The decrease in budding percentage and fertility that accompanies an increase in bud load, prevents a rectilinear increase in yield. The data in Table 8 indicate once again that the grape and wine quality of cultivars with pronounced colour and flavour characteristics are more readily impaired by an increase in yield. These results support the trends shown in Table 7 and Fig. 7. Table 9: The influence of bud load on the performance of Merlot in Bulgaria (Nikov, 1987)
Yield increases with an increase in bud load to the level where a bigger percentage of the allocated buds becomes ineffective due to overshadowing and the bunch mass begins to decrease. A further increase in bud load simply causes a decrease in grape quality. Unpredictable yield and grape quality are therefore some of the bigger dangers of increased bud load. INFLUENCE OF THE TIME OF PRUNING ON YIELDAs early as 1867 Cazenave declared: “Prune early for leaves, prune late for grapes”. Seeing that the time of pruning (the time of clean pruning) drastically influences the budding date, it also has a significant influence on the time of flowering. During flowering the induction of flower bunch primordia occurs in the young, green buds and this process is influenced by environmental conditions (light and temperature in particular) as well as cytoquinine production in the root tips. The later the pruning – within limits – the later the budding and the later floweing and induction. Light intensity and temperature are then more favorable and soil temperature (better root activity) is higher. Better induction occurs and consequently the bud fertility is higher (more and bigger flower bunch primordia). These effects are clearly illustrated by Archer & Champagnol (1979). Table 10: The influence of the time of pruning on the performance of Mourvèdre (Archer & Champagnol, 1979)
* Pruning times: 1 = just after the harvest 2 = just after leaf drop 3 = mid-winter (total dormancy) 4 = late winter (“huilsap”) 5 = beginning of budding 6 = shoots + 10 cm long at the tips. Table 10 indicates that with the same bud load yield can be increased by pruning later. Although it is a globally accepted principle that later pruning increases the crop mass, it must be accepted that there may be cultivar differences and the delay of pruning should obviously not be taken too far. INFLUENCE OF ALTERNATIVE PRUNING METHODS ON YIELDAlternative pruning methods entail mechanical pruning, minimum pruning and no pruning. The effect of these pruning methods on the performance of the vineyard is largely controlled by the inherent compensatory ability of the vine and consequently there are lots of differences among cultivars. In general the yield is considerably increased by the alternative pruning methods. This is a global trend.
Figures followed by the same letter in the row do no differ significantly (p < 0,05) An annual evaluation of the wines indicates that the mechanically pruned vineyard gives rise to a more pronounced fruit character, while a lower extract results in wines with less maturation potential. Table 12: The influence of alternative pruning methods on the performance of Cabernet Sauvignon/R99 at Nietvoorbij, Stellenbosch, 1998 - 2002
Figures followed by the same letter in the row do not differ significantly (p < 0,05) Except for the sugar concentration, alternative pruning methods do not have a noticeable influence on the grape composition of Cabernet Sauvignon. On the other hand, it did cause a significant increase in yield. In this trial too the lower extract in wines deriving from alternative pruning methods possibly indicates less maturation potential. INFLUENCE OF EFFECTIVE LEAF SURFACE ON YIELDA fair amount of national and international research has shown that an increase in effective leaf surface may bring about a noticeable increase in yield. Table 13: Influence of effective leaf surface on the performance of Chenin blanc/R99 in Robertson, 1973 - 1980 (Zeeman, 1981)
Except for the bush vines the same bud load was allocated per vine annually, but the increase in the effective leaf surface caused a noticeable increase in yield. In a neutral cultivar such as Chenin blanc no influence on the quality of the wine was measured. The same trends were also measured in Colombar (data not shown). Table 14: Influence of effective leaf surface on the yield of Cabernet Sauvignon and Cape Riesling on R99 in Stellenbosch, 1972 – 1980 (Zeeman, 1981)
At the same bud load (except in the case of bush vines) not only the yield, but also the wine quality of both Cabernet Sauvignon and Cape Riesling improved noticeably with an increase in the effective leaf surface. This shows that an increase in the effective leaf surface by means of bigger trellises often goes hand in hand with improved foliage microclimate, thus improving the grape composition. In a study in which a vertical trellis is compared to the Lyre trellis, Volschenk & Hunter (2001) prove that despite a yield increase of 65%, quality can be retained provided the foliage is properly managed, the foliage climate is not disadvantaged and sufficient and effective leaf surface is thus obtained. Table 15: Influence of an increase in the effective leaf surface on the foliage microclimate and yield of Chenin blanc/R99 over a 5 year period (Volschenk & Hunter, 2001)
Figures followed by the same letter in the row do not differ significantly (p < 0,05). By increasing the effective leaf surface, an increase of 15 t/ha in yield (at the same bud load/m cordon) was achieved without compromising grape and qine quality. The improved foliage microclimate (light penetration, air flow, bunch exposure) that was achieved prevented a quality compromise in the case of a neutral cultivar such as Chenin blanc. In the case of the more noble cultivars the improved foliage microclimate will result in increased grape and wine quality as found with Shiraz in Australia by Smart (1990).
The effectiveness of any
vineyard foliage is influenced by the efficiency of the canopy
management programme. In this regard it is not only the short term
practices that play a role, but definitely also the long term
practices. The choice of trellis system in particular places
limitations on the amount of grapes that can be ripened optimally.
This problem was recently solved on 7 selected farms in the industry
(altogether 450 ha) by upgrading the trellis system through more
efficient wire spacing and in some instances pole extensions. The
improved canopy that was thus obtained not only increased the average
production with 3,2 t/ha, but was also responsible for a noticeable
improvement in grape quality.
SUMMARY AND RECOMMENDATIONS
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E Archer (pictured)1) and JJ Hunter2)
Fig. 1: Bud fertility of various cultivars of Vitis vinifera
Fig. 2: Incorrectly shaped cordon arms result in low production and poor quality
Fig. 3: Incorrect vine shape caused by a build-up of bearers
Fig. 4: A build-up of bearers reduces trellis space, effective leaf surface and
therefore yield and quality
Fig. 5: Good bunch distribution prevents crowding and consequently reduces the
necessity of bunch thinning
Fig. 6: Temporary increase of bud load by Cazenave bearer unit
Fig. 7: The influence of yield on wine quality (Archer, 1984)
Fig. 8: Influence of soil preparation of root growth and density: left: traditional deep trenching; right: effective soil preparation 15 months after planting
Fig. 10: Narrow row width ensures maximum yield per ha
Fig. 11: Five wire hedge trellis with moveable foliage wires
Fig. 12: Poor cordon shape causes short and long shoots and heterogeneous yield and quality
Fig. 13: Shoot positioning ensures diffused sunlight
Fig. 14 (above and below): Well-balanced vine development with good bearer spacing results in even shoot strength and evenly distributed shoots
Fig. 15: Before and after suckering
Fig. 16: Leaf removal brings about good sunlight penetration in the foliage
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