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The influence of production levels and bud load on the quality of wine and wine grape cultivars in the Robertson area: Sauvignon Blanc
This article follows another that discussed the influence of production norms on grape and wine quality of Ruby Cabernet in the Robertson area (Van Schalkwyk et al., 1999). The conclusion of the prior investigation was that different production levels, induced by fixed bud load per running metre of cordon, did not have any significant influence on the quality of the wine. This concurs with the results of a previous study conducted on Sauvignon blanc in Stellenbosch, namely that bud load did not have a significant influence on the quality of grapes and wine (Van Schalkwyk & Fouché, 1994). In the latter case bud loads were linked to shoot mass and the number of bearers per vine differed from year to year. The vine’s vigour was therefore taken into account and to a certain extent served as a buffer to maintain a good yield : growth ratio and the vines could largely regulate the number of bearers allocated (Van Schalkwyk & De Villiers, 1991; De Villiers & Van Schalkwyk, 1995). In actual fact the vine itself regulated the bud load, e.g. where the bud load was high, not all the bearer eyes budded due to overshadowing. The shoots were also shorter and thinner, with the result that shoot mass was reduced and therefore fewer bearers were allocated the subsequent season. This trend caused production levels to differ considerably from one season to the next.
In South Africa, of the white grape cultivars, Sauvignon blanc is currently the fourth most planted (4.9% of all vine plantings) and occurs in all the climatic regions, except for the Below Orange River (Anon, 1999). As a result of the above findings, there was still a need to determine whether different production levels per se, viz. production per vine (kg) or unit surface (t/ha), influenced the wine quality of Sauvignon blanc and what the bud load and production should be for optimum wine quality.
The new approach, which is based on the number of buds allocated per running metre of cordon, monitors certain foliage characteristics which may influence the wine quality of Sauvignon blanc, since foliage density and exposure to sunlight usually have a noticeable influence on the wine aroma of Sauvignon blanc (Allen & Lacey, 1993; Marais et al., 1999).
Investigative procedure
The investigation was conducted on a seven-year-old Sauvignon blanc vineyard, grafted onto 99 Richter RY 13. This vineyard is established on calcareous Karoo soil, Sterkspruit/Swaerskloof soil type, on the Robertson experimental farm of the ARC. This soil type is representative of a large percentage of the soils in the Robertson area. The vines were planted 2.75 m x 1.5 m and trellised on a Six Strand Extended Perold. Micro-irrigation, using tensiometers, was scheduled to the extent that the vines were not exposed to moisture stress at any stage. To induce different production levels, five treatments were given in which the vines were pruned according to different norms (8, 16, 24, 32 and 40 buds per running metre of cordon) for three consecutive seasons. Two bud bearers, and where necessary "bokhorings" (double bearers on the same position), were used to allocate the desired number of bearers. A randomised block design, with five replicates of 15 vines each, was used per treatment. Since canopy has a significant influence on grape composition and therefore grape and wine quality (Hunter et al., 1991), canopy density was determined at the flowering and pea bud stages and again just before the harvest, by means of the point quadrant method (Smart, 1985). In sites where the total canopy density was equal to 4.0 points of contact, or higher in certain seasons, leaves were removed discerningly and at random so as to create uniform canopy density in all the treatments. Other standard foliage management practices such as tipping, topping and suckering were applied in all the treatments. Trial vines were suckered maintaining all the shoots budding out of the bearer buds. All the sites were harvested at 21.0 - 21.5°B to eliminate the effect of ripeness level. Wine was made according to standard Nietvoorbij experimental vinification methods and fermented dry. Hereafter wines were evaluated sensorially for aroma and wine quality six and eighteen months after bottling.
Findings
A significant increase in canopy density occurred when bud load increased, resulting in poor light penetration where the bud load was high (
Table 1
). Total canopy density in both the 8 and 16 bud treatments was significantly lower than in the higher bud loads. This is ascribed mainly to the fact that in these treatments the bearers were spaced more widely apart. No significant treatments occurred in the average total canopy densities of the 24, 32 and 40 bud treatments in November. It is clear that these norms result in excessively dense canopy, while the canopy density of the lower bud loads tends to be more ideal. Using "bokhorings" (double bearers) to allocate the high bud loads was conducive to canopy density. In those instances where leaves had to be broken out, the canopy density increased little up to the time of ripening in February. In actual fact, at that stage the canopy density of all the treatments was higher than that of the 8 bud treatment.
High bud loads clearly had a negative effect on the budding percentage and fertility of Sauvignon blanc (
Table 1
&
Fig. 1
). The biggest influence was noticeable in the 8 bud treatment, where fertility more than doubled after the start of the investigation. This may be ascribed to better sunlight exposure, since the bearers were spaced quite far apart (333 mm). In contrast with all the other treatments that showed a gradual increase in fertility as the seasons progressed, a decrease in the fertility of the 40 bud treatment was noted in the 1995/96 season. Most likely this is an indication of an imbalance in the vines’ yield : vigour ratio, thus creating a situation of excessive yield.
The decrease in fertility as a result of higher bud loads also contributed to a significant increase in crop mass up to and including the 24 bud treatment, whereafter yield decreased (
Fig. 2
). This decrease is also related to the significantly lower bunch mass of the 32 and 40 bud treatment of 157.0 g and 151.2 g respectively (
Table 1
). An increase in bud load from 8 to 16 buds even caused an increase in bunch mass and to a lesser extent berry mass. A significant decrease in shoot mass was observed only in the 24 bud treatment. The low shoot mass and high yield: shoot mass ratios indicate that an imbalance occurred in the 32 and 40 bud treatments (respectively 10.5 & 11.2). A decrease in vegetative growth was clearly noticeable in the vineyard. Shorter shoots occurred in the two highest bud loads (32 and 40 buds). The results of this investigation indicate that in Robertson, a bud load of 24 buds per running metre of cordon, with a crop : shoot mass ratio of 8 : 9, appears to be the maximum production norm for Sauvignon blanc in order to realise optimum production of 6.6 kg/running metre of cordon, given the specific conditions of the trial.
Since an attempt was made throughout to harvest all the treatments at optimum ripeness (21.0 - 21.5?B) and by so doing to eliminate the effect of degree of ripeness on wine quality, there were differences in the average harvest dates. There were differences of up to 12 days between the harvest dates of the 8 and 40 bud treatments, indicating delayed ripening in the higher bud loads (
Table 1
).
Despite the attempt to create uniform microclimatic conditions through the application of good canopy management practices, the grape and must composition of Sauvignon blanc was significantly influenced by bud load (
Table 2
). Despite stable sugar concentrations before skin contact and higher sugar contents at 8 and 16 bud treatments after skin contact, a decrease in total acid content occurred as and when bud load increased. The less vigorous shoot growth and fewer lateral shoots in the foliage with the higher bud loads, which reduced the photosynthetic capacity, may have played a role. The possibility that in these treatments some bunches may have been exposed to direct sunlight, could also have been a contributing factor. However, skin contact had a significant influence on the total acid concentration of the must and decreased by up to 1.4 g/l and 1.5 g/l acid respectively in wines of the 8 and 40 bud treatments, as a result of potassium bitartrate discharges. Taking into account the production yields of these two treatments, the trend in the 40 bud treatments differs slightly from the general trend, namely that in lower production more acid is discharged during fermentation. This is probably due to the increased density of the 40 bud treatment and the fact that a large percentage of this treatment’s bunches actually ripens under shade conditions within the foliage. The increase in the average tartaric acid and the decrease in the malic acid concentrations were also fairly even, in line with increased bud load. It is important, however, to note that the opposite trend occurred in the 1995/96 must compositions. The uniqueness of this season probably has to do with cooler weather conditions. As with the Ruby Cabernet investigation, it was found that, contrary to the general trend of free amino nitrogen levels in must decreasing when production increases, there were no significant differences in these levels in Sauvignon blanc grapes. The levels of the different treatments were more or less the same throughout (
Table 2
).
Grapes used for vinification were selected at the time of picking since a fairly high percentage of rot sometimes occurred at harvest time (
Table 3
). Although it is not apparent from the average physical condition of the grapes, in most seasons the general trend was less sour rot in the high bud loads.
The decrease in acid content in the must during skin contact, and other differences in must quality, are also reflected in the wine composition (
Table 4
). The average sensorial evaluation of young and matured wines indicated that bud loads had a significant influence on the wine quality of Sauvignon blanc (
Fig. 2
). This investigation clearly shows that, contrary to expectations that high yields decrease wine quality, high yields resulted in a significant improvement rather than a decline in wine quality of young wines. On average young wines (6 months after bottling) of the 32 and 40 bud treatments produced the highest wine quality over three seasons (
Fig. 3
). This was also the case in individual seasons, except in 1994 where the 8 bud treatment produced a significantly higher wine quality than the 32 bud treatment. The 24 bud treatment, on the other hand, produced the lowest wine quality on average. Except in the 1994 season, young Sauvignon blanc wines showed an increase in wine quality that corresponded with an increase in bud load and the same trend also occurred in the cultivar character (
Fig. 3
). However, in the course of maturation this trend was to a large extent reversed by a considerable decrease in wine quality and cultivar character. The only bud load that showed an average increase both in cultivar character and wine quality with maturation, was the 16 eye treatment. From a wine maturation point of view (18 months old) the latter bud load thus seems to be the ideal treatment for the best quality Sauvignon blanc wines.
Aroma profiles of the different seasons’ wines show that in aroma distribution, maturation causes a move away from a vegetative cultivar character (
Fig. 4
). This change in aroma occurred mainly towards increased caramel and tropical fruit characters. The general trend in young wines was that the aroma distribution tended to be more vegetative as and when bud load increased. Increased density in the foliage of the high bud loads was in all likelihood partially responsible for this occurrence. This trend corresponds with findings from an investigation into the use of micro-climatic factors to determine grape and wine quality. In this instance it was found, inter alia, that the increase in methoxypyrazines (group components describing the aroma as vegetative, grassy, herbaceous, potatoey, soil-like, reminiscent of gooseberry, asparagus and greenpepper) corresponded with a decrease in light exposure of the bunch. During maturation this situation was reversed to a certain extent, however, in that generally the lower bud load displayed a more vegetative character. What emerges without any doubt, however, is that in the warmer areas Sauvignon blanc prefers slightly denser, well-aerated foliage and requires less bunch exposure to produce a wine with a typical vegetative (greenpeppery/asparagus-like) character (Marais, 1994; Marais et al., 1995; Marais et al., 1999).
Summary
From the investigation it is clear that in Robertson, Sauvignon blanc is extremely sensitive to variation in foliage density, considerably more so than Ruby Cabernet, with regard to grape and wine quality. Although the vines compensate for an increase in bud load by a decrease in berry and bunch mass, this is not as dramatic as in Ruby Cabernet. The slightly looser bunches in the high bud loads are beneficial and promote physical grape quality since sour rot is reduced. With an increase in bud load, acid concentration is probably reduced as a result of decreased foliage capacity, but this did not have a detrimental effect on the Sauvignon blanc wine quality.
In Robertson Sauvignon blanc reached optimum production where the vines were pruned at 24 bearer eyes per running metre of cordon. Yield was then reduced as a result of decreased budding percentage and fertility. Although high bud loads such as 32 and 40 buds produced higher quality young wines, it is clear that the low bud loads, such as for example the 16 buds, produce wines with a longer shelf life due to the higher sugar concentration. Taking into account these factors it is recommended that Sauvignon blanc established on calcareous soils in the Robertson area, be pruned to between 16 and 24 buds per running metre of cordon to create optimum production and an ideal canopy with a yield: shoot mass ratio of approximately 5 to 8. This norm should create an ideal canopy and microclimate without requiring leaves to be removed (creating sufficient space for indirect light exposure and aeration through the foliage). By spacing the bearers 91 to 143 mm apart, there is sufficient space to apply foliage management to the extent that grape composition and quality in the vineyard may be optimised.
ACKNOWLEDGEMENT
The technical assistance of the Wine Grapes plant physiology group, and the staff of Robertson Experimental Farm, as well as the partial funding of the investigation by Winetech, are appreciated. Thanks also to Distillers Corporation for the determination of free amino nitrogen in the must.
LITERATURE
ALLEN, M.S. & LACEY, M.J., 1993. Methoxypyrazine grapeflavour: Influence of climate, cultivar and viticulture. Wein-Wiss. 48, 211-213.
ANONYMOUS, 1999. Suid-Afrikaanse wynbedryfstatistiek no.18. KWV, Skakeldienste Afdeling, Suider-Paarl, Republiek van Suid-Afrika.
DE VILLIERS, F.S. & VAN SCHALKWYK, D., 1995. The effect of crop levels induced by bud load on grape and wine quality. Poster at 1995 Vth International Symposium of Oenology AActualités Oenologiques 95", France.
HUNTER, J.J., DE VILLIERS, O.T. & WATTS, J.E., 1991. The effect of partial defoliation on quality characterstics of Vitis vinifera L. cv Cabernet Sauvignon grapes 1. Sugar, acids and pH. S. Afr. J. Enol. Vitic. 12, 42-50.
MARAIS, J., 1994. Sauvignon blanc cultivar aroma B A review. S. Afr. J. Enol. Vitic. 15, 41-45.
MARAIS, J., HUNTER, J.J., HAASBROEK, P.D. & AUGUSTYN, O.P.H., 1995. Cluster exposure on Sauvignon blanc grape aroma composition. In: Goussard, P.G., Archer, E., Saayman, D., Tromp, A. & Van Wyk, J., (Eds). Proceedings of the first SASEV International Congress, Cape Town, South Africa, 8-10 November 1995, 32-34.`
MARAIS, J., HUNTER, J.J. & HAASBROEK, P.D., 1999. Effect of microclimate, season and region on Sauvignon blanc grape composition and wine quality. S. Afr. J. Enol. Vitic. 20,19-30.
SMART, E., 1985. Principles of grapevine canopy microclimate manipulation with implications for yield and quality. A review. Am. J. Enol. Vitic. 33, 230-239.
VAN SCHALKWYK, D. & DE VILLIERS, F.S., 1991. Die invloed van ooglading op die wingerdkundige prestasie van Emerald Riesling te Lutzville. Wynboer Tegnies 51, 14-15.
VAN SCHALKWYK, D. & FOUCHÉ, G.W., 1994. Die invloed van ooglading op die wingerd- en wynkundige prestasie van Sauvignon blanc te Stellenbosch. Wynboer Tegnies 64, 12-14.
VAN SCHALKWYK, D., DE VILLIERS, F.E. & DE VILLIERS, F.S., 1999. Die invloed van produksienorme op die wyngehalte van wyndruifkultivars in die Robertson omgewing : Ruby Cabernet. Wynboer Tegnies 123 , t8-t12.
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