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The effect of alternative pruning methods on viticultural and oenological performance (part 1): Cabernet Sauvignon in the Stellenbosch area
Danie van Schalkwyk1 & Eben Archer2
Key words: Alternative pruning, wine grapes, grape quality, wine quality.
Summary
Introduction
The enormous increase in production costs, inter alia expensive labour, in the national and international wine industries necessitated an investigation into alternative grape production methods to keep these costs as low as possible. In South Africa 60% of the annual total wine grape production cost consists of labour and the development of labour saving techniques is therefore of cardinal importance to the wine industry. Most vines in the Western Cape are pruned by hand and constitute thirty per cent of the annual labour costs. It is therefore important to investigate alternative pruning methods. Examples of such alternative pruning methods are mechanical pruning (May & Clingeleffer, 1977; Clingeleffer, 1988), minimal pruning (Clingeleffer, 1993) and no pruning (Bakonyi, 1987).
Mechanical pruning may increase yield (Morris et al., 1975; Archer, 1999). On the other hand it results in a reduction in bunch and berry mass (Reynolds, 1988; Archer & Van Schalkwyk, 2007), and sugar concentration (Shaulis et al., 1973; Anderson et al., 1996). Combined with a loss in colour, these factors are mainly responsible for the evident decrease in wine quality (Reynolds & Wardle, 1993). According to McCarthy & Cirami (1990) different cultivars reacted differently to mechanical pruning.
Research showed that minimal pruning, compared to normal manual pruning, caused crop mass, number of bunches and number of shoots per vine to increase (Cirami, McCarthy & Furkaliev, 1985; Clingeleffer, 1993). Most of these reports showed that minimal pruning caused sugar concentration and pH of the must to decrease, while total acid concentration increased. The results in respect of wine colour, aroma and wine quality differed depending on climate and vigour.
Little information is available about no pruning. Research on seven cultivars conducted by Bakonyi (1987) showed that no pruning caused crop mass to increase without a decrease in quality. Research by Martinez de Toda & Sancha (1988) on unirrigated Grenache showed that no pruning caused crop mass, dry material and sugar production per vine to increase. This was ascribed to the significant increase in the total leaf surface per vine.
Although general trends were observed in the effect of alternative pruning methods on the performance of the vine in the above-mentioned research, pertinent questions remain with regard to specific vine conditions in South Africa: 1) Can these alternative pruning methods be used to produce high quality red wines from noble cultivars? 2) Are these methods applicable to different climatic conditions? 3) Does the reaction to the various pruning methods differ among cultivars? A research project to address these questions was launched in 1996.
The project consisted of three field trials at three different venues (Elsenburg, Nietvoorbij and Robertson). This article discusses the results of the Nietvoorbij trial in the Stellenbosch area and will be followed by articles about the other two trials.
MATERIAL AND METHODS
The trial was conducted in an existing four-year-old Cabernet Sauvignon X 99 Richter vineyard established on an Avalon soil type at Nietvoorbij (Soil Classification Work Group, 1991). Before planting the soil was criss-crossed with a shift delve plough to a depth of 1.30 m after 12 t/ha calcitic lime was broadcasted on the soil surface (eventual pH = 5.9). The plant spacing was 2.75 m x 1.5 m and the vines were trellised on an existing five wire extended Perold. The trellis system (Photo 1) as well as the vines (except for the control sites) were converted to a 1.2 m high single wire trellis system (Photo 2 & 3) according to the procedures described by Archer & Van Schalkwyk (1998). The following treatments were applied:
Control - hand pruning to 12 two-bud spurs per vine, 24 buds per vine.
Mechanical pruning - vines were pruned using mechanical hedge cutters (Photo 4) to approximately 10 cm above the cordon, no thinning of spurs was done.
Minimal pruning - there was no winter pruning, but vines were mechanical skirted approximately 30 cm above ground level during December.
The sides of the mechanical and minimal pruning treatments were trimmed with mechanical hedge cutters to keep the rows open for spraying. The tops were untouched.
No pruning - no pruning or trimming was applied.
The pruning treatments were randomised in blocks and consisted of five replicates. Thirty vines with uniform vigour per trial site were used for data collection. Vines from the control treatments were split-cordon trained 70 cm above the soil surface on a five-wire extended Perold with moveable foliage wires (Zeeman, 1981). The alternative pruning treatments were split-cordon trained on a single wire hedge 1.20 m above the soil surface (Zeeman, 1981). Depending on soil moisture measurements, supplementary irrigation was given using microjets, up to a maximum of three irrigations depending on the season.
MEASUREMENTS
Time studies
Time studies to determine man hours needed for each operation were done during pruning, canopy management (suckering, positioning of shoots, tipping, topping and leaf removal) and harvesting and from these the labour needed to manage the vines of each of the pruning systems was calculated (machine and machine operator costs not included).
Yield, bunch mass, peduncle mass, berry mass and berry volume
During harvest, the physical condition of the grapes was visually evaluated and the percentage rot, millerandage, etc. noted by the same person. Thirty vines per plot were harvested in crates, weighed, and the average yield per vine calculated. One crate per plot was randomly selected, weighed, and the number of bunches counted to calculate the average bunch mass. Five bunches from each of those crates were randomly selected and stored at -20°C until after harvest whereafter the peduncle mass, berry mass and volume were determined according to the method described by Van Schalkwyk (2004). Five vines from each of the alternative pruning treatments were also harvested by hand and used to determined the above-mentioned information. The alternative treatments were subsequently harvested using mechanical harvesters. Grapes from all treatments were harvested at between 23°B and 24°B.
Berry skin, wine colour and phenols
Berry skin (Hunter, et al., 1991) and wine colour (Ribéreau-Gayon, et al., 2000) were analysed using a spectrophotometer. Total phenolic compounds were quantified using a HPLC (Ribéreau-Gayon, et al., 2000).
Wines
A total of 80kg of grapes from all treatments and replicates were harvested and used to make experimental wines. Wines were made according to the standard Nietvoorbij procedure for small-scale winemaking at the ARC Infruitec-Nietvoorbij Research Institute. The wines were stored at 14°C until sensorial evaluation. The wine quality was evaluated for aroma intensity and quality based on a descriptive sensorial analysis (10-point line scale method), 6 and 18 months after bottling each season’s wine.
Cane mass and number of buds
During winter, the number of shoots per vine and internodes per shoot were counted. Cane length was measured using all the canes of five vines per replicate and the average internode length was calculated. The same vines were used each year. To calculate the average cane mass per vine, 30 vines of each replicate of the hand and mechanically pruned treatments were weighed. In one year the number of winter buds were counted on five randomly selected vines per plot. The same vines were used to count the shoots during spring and from this percentage bud burst was calculated.
Statistical analysis
The data were subjected to an analysis of variance. Student’s least significant difference (LSD) values were calculated to facilitate comparison between treatment means.
RESULTS AND DISCUSSION
All three alternative pruning methods significantly reduced labour costs compared to hand pruning (Table 1). Not only was labour input for pruning reduced, but also for canopy management and harvest. Minimal and no pruning also required significantly less labour than mechanical pruning. It is imperative that mechanical harvesting is used when any one of the alternative pruning methods are employed as it is extremely labour intensive and costly to harvest these systems by hand.
Hand pruning, together with the necessary canopy management as well as harvest, require 273.6 man hours/ha compared to 43.8 and 14.4 man hours/ha for mechanical and minimal pruning respectively (Table 6). This saving in man hours was true for all the varieties and there is a clear economic advantage to use anyone of the alternative pruning methods. During this trial, time studies showed that it is not viable to harvest any of the alternatively pruned plots by hand (data not shown). A mechanical harvester is a prerequisite when these methods are used.
Time studies showed that harvesting by hand of the mechanical and no pruning treatments took 189 and 575 man hours per hectare compared to 98 man hours for the hand pruning treatments. Grapes from the no pruning treatment hang from soil surface to approximately 1.8 m above the surface of the soil, thereby complicating harvesting and mechanical harvesters have to be adjusted accordingly (Photo 5).
Viticultural performance
Bunches from the minimal and no pruning treatments were more exposed to direct sunlight with the result that more sunburn occurred than with the hand pruning treatment (Table 2). The bunches were significantly smaller and a high percentage of runoff occurred. Windy conditions during flowering probably contributed to the higher percentage of runoff in the alternative pruning treatments due to greater bunch exposure. The canes of the alternative prune treatment were more open and resulted in greater bunch exposure and were more subject to sunburn in abnormally warm seasons (Photo 6). The bunches of the no pruning treatments were much looser compared to the other pruning treatments (Table 2). In the hand pruning treatment canes were positioned vertically in the canopy thereby protecting the bunches against direct sunlight (Photo 7).
The alternative pruning methods produced more shoots, but decreased shoot length significantly compared to the hand pruned control (Table 3), although more bearer buds occurred. This can be ascribed to the shorter internode length (Photo 8). Mechanically pruned canes had the longest shoots of all the alternative prune treatments. This can be related to the fact that the capacity of alternatively pruned vines was expressed in significantly more shoots per vine than in the case of the hand pruned control. The eventual result was that the highest cane mass was obtained from the hand pruned control vines.
Bud counts made during one data year showed that 24, 72, 191 and 227 buds per vine were left with hand, mechanical, minimal and no pruning respectively. This resulted in percentage budburst of 108%, 60%, 49% and 47% respectively.
A significantly lower yield was obtained from the hand pruned control vines (Table 3). Climatic differences such as temperatures during bunch initiation, bud, windy conditions during flowering (2004) as well as the occurrence of downy mildew (2002) had a negative impact on production (Fig. 1A). In most years the hand pruned plots yielded less but at least it was relatively consistent from year to year, while the alternative pruning treatments varied much more. The 2003 season was extremely favourable for high yields for alternatively pruned treatments (Fig. 1B). The hand pruned treatments accumulatively produced fewer grapes than the other treatments, while the accumulated yields of mechanical and minimal pruning over eight years were practically the same.
Despite the reduction in bunch mass, the increase in the number of bunches per vine caused the yields of the alternative pruning treatments to increase significantly (Table 3). Hand pruning also produced significantly larger berries than the other pruning treatments.
Must and wine
Vines that had minimal and no pruning treatments produced grapes that did not reach the required ripeness of 23°B to 24°B in some seasons due to their high yields (2003 & 2004). In those years the average sugar concentration of the grapes from the hand and mechanical pruning treatments was significantly higher than minimally and non-pruned vines (Table 3). The grapes from the alternative pruning treatments had a lower pH and this tendency was carried forward into the must (Table 4). The higher pH of the hand pruning treatment can largely be ascribed to the higher shade levels in the bunch zone of these trellis treatments.
In general, the alternative pruning methods induced better colour in the skins and wines (Table 4) and this is ascribed to better sunlight penetration into the bunch zone of these vines (data not shown). These results are in accordance with those reported by Clingeleffer (1983 & 1988). During the mechanical harvest of the alternative pruning treatments it was noted that the free run juice was pink to light red even at that stage.
Mechanical pruning produced the best wine quality in 6-month-old Cabernet Sauvignon during three of the six seasons (2000, 2002 & 2005) (Fig. 2). In three of the six seasons (2001, 2003 & 2004), however, hand pruned vines nevertheless yielded similar wine quality. Similar trends occurred in the 18-month-old vines in two seasons (2001 & 2002).
The aroma profiles and overall quality of 6- and 18-month old Cabernet Sauvignon wines are indicated in Figs. 2 & 3. Vintage played an important role and over years succeeded in blanketing the treatment effects in the case of 6-month-old wines (Fig. 2). In spite of this, the wines from the hand pruned vines consistently showed the most pronounced vegetative character. This is expected due to a higher canopy density (more shade) in the bunch zone. This tendency also held true in most vintages for the 18-month-old wines (Fig. 3). These results are in accordance with those of numerous other researchers who found a better expression of fruitiness in wines from alternatively prunes vines. In spite of this, an evaluation of overall quality showed that the trained panel of judges had a preference for wines from the hand pruned vines (Table 5). Both the minimal and no pruning treatments displayed less intense cultivar character over the period of the trial and also produced wines of significantly less quality.
CONCLUSION AND RECOMMENDATION
Compared to hand pruning, alternative pruning methods are hugely labour saving and contribute greatly to reducing production costs of wine grapes. A prerequisite for these methods is the availability of mechanical harvest machines because hand harvesting the resultant numerous small bunches by hand, is not viable (up to 60% more labour).
Alternative pruning methods induced shorter canes with shorter internodes and consequently lower cane mass. Although berry mass and volume as well as physical bunch size were reduced by alternative pruning methods, the increased number of bunches not only prevented a yield decrease, it was responsible for considerable yield increases compared to hand pruning. In general hand and mechanically pruned Cabernet Sauvignon produced wines with a more intense cultivar character and higher quality than minimal and no pruning. Hand pruning yields dense canopies in the bunch zone and therefore less bunch exposure, resulting in a more vegetative character in the wine, while alternative pruning methods produce wines with a more fruity (berry) character due to a larger percentage of bunch exposure. Minimal and no pruning are therefore not recommended for the vinification of high quality Cabernet Sauvignon in the Stellenbosch area. The difference in the canopies of the mechanical and hand pruning treatments induced different aromas, consequently grapes from these two treatments have to be used for different wine styles, although there were no quality differences.
Fertilisation and irrigation of alternative pruning methods have to be adjusted as a result of the dense canopy and higher yields. To accommodate the dense canopies of the alternative pruning methods, plant rows should not be narrower than 3 m. Sturdy poles should be used and planted closer to each other. A strong cordon wire that is properly attached to the poles should be used to prevent the trellis system from collapsing under the increased weight.
Acknowledgements
The Viticulture personnel of ARC Infruitec-Nietvoorbij for technical assistance. Thanks also to Winetech and ARC Infruitec-Nietvoorbij for financial support. Thanks to New Holland South Africa and Unique Agri Trade respectively for making available their mechanical harvesters and the mechanical harvesting of the alternative pruning treatments in the Nietvoorbij trial.
For more information contact Danie van Schalkwyk at ARC Infruitec-Nietvoorbij, tel (021) 809-3156, fax (021) 809-3261 or email vschalkwykd@arc.agric.za.
Literature cited
Archer, E., 1999. Meganisering van snoei by wingerd: Vereistes gestel aan die wingerd en reaksie van wingerdstok. Wynboer Tegnies 117, 46 - 48.
Archer, E. & Van Schalkwyk, D., 1998. Die omskakeling van die prieelstelsel sowel as wingerdstokke vir meganiese snoei van wyndruiwe. Wynboer Tegnies 102, 10 - 11.
Archer, E. & Van Schalkwyk, D., 2007. The effect of alternative pruning methods on the viticultural and oenological performance of some wine grape varieties. S. Afr. J. Enol. Vitic. 28(2), 107 - 139.
Bakonyi, I., 1987. Non-pruning: A new technique in vineyard management. The Australian Grapegrower & Winemaker 280, 112 - 113.
Cirami, R.M., McCarthy, M.G. & Furkaliev, D.G., 1985. Minimum pruning of Shiraz vines - effects on yield and wine colour. The Australian Grapegrower & Winemaker 263, 24 - 26.
Clingeleffer, P.R., 1988. Response of Riesling clones to mechanical hedging and minimal pruning of cordon trained vines - implications for clonal selection. Vitis 27, 87 - 83.
Clingeleffer, P.R., 1993. Vine response to modified pruning practices. In: R.M. Pool (ed). Pruning mechanization and crop control. Proc. 2nd Nelson J. Shaulis Gr. Symp., July 13 - 14, Fredonia New York, 20 - 30.
Hunter, J.J., De Villiers, O.T. & Watts, J.E., 1991. The effect of partial defoliation on quality characteristics of Vitis vinifera L. cv. Cabernet Sauvignon grapes. II. Skin color, skin sugar, and wine quality. Am. J. Enol. Vitic. 42, 13 - 18.
May, P. & Clingeleffer, P.R., 1977. Mechanical pruning of grapevines. Austr. Wine Brew. Spirit Rev. 96(11), 36 - 38.
Martinez de Toda, F. & Sancha, J.C., 1998. Long-term effects of zero pruning on Grenache vines under drought conditions. Vitis 37, 155 - 157.
McCarthy, M.G. & Cirami, R.M., 1990. Minimal pruning effects on the performance of selections of four Vitis vinifera cultivars. Vitis 29, 85 - 96.
Morris, J.R., Cawthon, D.L. & Fleming, J.W., 1975. Effect of mechanical pruning on yield and quality of Concord grapes. Ark. Farm. Res. 24(3), 12.
Reynolds, A.G., 1988. Response of Okanagan Riesling vines to training system and simulated mechanical pruning. Am. J. Enol. Vitic. 39, 205 - 212.
Reynolds, A.G. & Wardle, D.A., 1993. Yield component path analysis of Okanagan Riesling vines conventionally pruned or subjected to simulated mechanical pruning. Am. J. Enol. Vitic. 44, 173 - 179.
Ribénreau-Gayon, P., Glories, Y., Maujean, A. & Dubourdieu, D., 2000. The chemistry of wine and stabilization and treatments. In: John Wiley & Sons Ltd., Handbook for Enology, Vol. 2, 129 - 185.
Shaulis, N.J., Pollock, J., Crowe, D.E. & Shepardson, E.S., 1973. Mechanical pruning of grapevines: Progress 1968 - 1972. Proc. New York State Hort. Sci. 118, 61 - 69.
Soil Classification Work Group, 1991. Soil classification - a taxonomic system for South Africa. Memoirs on natural resources of South Africa no. 15. Dept. Agric. Developm., Pretoria.
Van Schalkwyk, D., 2004. Metodes om korrelmassa, korrelvolume en trosmassa te bepaal. Wynboer 182, 115 - 116.
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