Abstract
The impact of mechanical thinning using a machine harvester on yield components and fruit composition of Tempranillo and Grenache (Vitis vinifera L.) vines was investigated. Experiments were conducted in two Spanish vineyards, both trained to a vertical shoot-positioned canopy over two consecutive seasons. The height of the harvester and the position of the bow rods were adjusted so that fruit was removed by the vibration of the canopy caused by the bow rods striking the vine trunks. The Tempranillo was thinned ~10 days before veraison and the Grenache was thinned when 10 to 15% of berries were colored. Mechanical thinning significantly reduced berry number per cluster, cluster weight, and cluster compactness; a similar amount of fruit (~65%) was removed from both varieties. Botrytis incidence was reduced by mechanical thinning of the Grenache in 2007 and was otherwise not affected by thinning treatments. Soluble solid concentration was higher under mechanical thinning treatments over the two seasons in Tempranillo and in 2006 in Grenache, suggesting that damage to the canopy was minimal. Phenolic concentration (both berry and berry fresh weight basis) was higher in fruit from the mechanically thinned vines compared with the control (unthinned) in Tempranillo in 2007 and only on a per berry basis in Grenache in 2006. Mechanical thinning conducted with a machine harvester has the potential to be a cost-effective technique to regulate yield in vineyards with vertical shoot-positioned canopies.
The beneficial effects of crop regulation are well established in Europe through production limits that are imposed as part of the appellation system. The economic regulation of yield is becoming more important as pressure increases to consistently produce high-quality fruit and manage production costs. Yield reduction might lead to an increase of sugar, color, and flavor in fruit at harvest (Bertamini et al. 1991, Guidoni et al. 2002, Tardaguila et al. 2005, Petrie and Clingeleffer 2006, Reynolds et al. 2007). However, other studies have not proved the hypothesis that cluster thinning accelerates ripening and improves fruit composition (Ridomi et al. 1995, Keller et al. 2005).
Hand thinning is a common cultural practice in European viticulture but can be expensive because of high labor requirements. In Spain, vineyards trained to a vertical shoot-positioned (VSP) canopy can require 40 to 50 hours per hectare for hand thinning (Martinez de Toda and Tardaguila 2003), at a cost of $520.00–650.00 US per hectare. Mechanization in European viticulture offers the opportunity to reduce these production costs.
Mechanical thinning has been successfully used to decrease yield and improve grape quality. It was developed in northeastern United States on Concord (Vitis labrusca) vineyards to reduce yield and improve sugar content of grapes at harvest for juice production (Pool et al. 1993, Fendinger et al. 1996). More recently, different methods of mechanical thinning have been successfully trialed with minimally pruned vines in Australia (Clingeleffer et al. 2002). Australian researchers have further refined the mechanical thinning system on Cabernet Sauvignon (Vitis vinifera L.) minimally pruned vines, trained to two vertically divided cordons, with very high yields (28,000–57,000 kg/ha) (Petrie and Clingeleffer 2006). Instead of targeting the whole canopy, beater bars were positioned in the harvester to target only a portion of the fruit, allowing specific yields to be targeted without the fear of overthinning (removing too much or all of the fruit), which can easily occur when attempting to evenly remove fruit from the whole canopy. European conditions are very different than those in regions where mechanical thinning has been used previously, with much lower yields and vines traditionally spur- or cane-pruned with vertically positioned shoots. This canopy type adds further challenges to mechanical thinning with a grape harvester, as the fruit in spur- or cane-pruned vines is concentrated within a far smaller zone, which makes it difficult to selectively thin some fruit from the canopy without damaging the remaining clusters or overthinning. The use of mechanical thinning in VSP vineyards will offer great potential as a management practice in Europe, the United States, Chile, and New Zealand, where VSP is widely used.
The purpose of this study was to investigate the effects of mechanical thinning on yield components and grape composition of Tempranillo and Grenache grapes trained on vertical shoot-positioned canopies.
Materials and Methods
The experiments were conducted in two commercial Tempranillo and Grenache (both Vitis vinifera L.) vineyards situated in separated locations of the Rioja appellation (Spain) over two consecutive seasons, 2006 and 2007. Treatments were arranged in a completely randomized design that consisted of five replicates of 20-vine plots for each treatment. Replicates were spaced along one row. Three treatments were applied at site 1: control, mechanical thinning at 470 beats per minute (low thinning intensity), and mechanical thinning at 500 beats per minute (high thinning intensity). Two treatments were applied at site 2: control and mechanical thinning at 470 beats per minute.
Site 1 was located in Haro (northwestern Rioja). Tempranillo vines (Vitis vinifera L.) were spur-pruned (2 buds per spur) in a single cordon, and trained to a vertical shoot-positioned (VSP) trellis system. Twelve buds per vine were left after pruning. The trellis consisted of a bilateral cordon at 70 cm, two wires at 100 cm above-ground for protection against wind damage, and a pair of movable shoot-positioned wires at 145 cm. Vines were spaced at 2.4 m x 1.2 m, equivalent to a density of 3,300 vines/ha. Row orientation was north-east to south-west. The Tempranillo vines were grafted onto 110R rootstock and planted in 1996 in a loam to clay-loam soil. Vines were not irrigated and shoots were trimmed in August. All shoots were positioned twice between the wires and no leaf removal was conducted.
At site 2 (Aldeanueva de Ebro, southeastern Rioja) Grenache vines (Vitis vinifera L.) were spur-pruned (12 buds per vine) in a cordon and trained to a VSP system. The trellis was a double cordon trained to a single wire at 65 cm, two wires at 95 cm for protection against wind damage, and a pair of movable shoot-positioned wires at 130 cm. Vines were spaced at 2.8 m x 1.2 m, equivalent to a density of 2,800 vines/ha. Row orientation was north to south. Grenache vines were grafted onto 110R rootstock and planted in 1993 in a loam-sandy soil. Vines were not irrigated during the growing season and shoots were not trimmed. All shoots were positioned twice between the wires and no leaf removal was conducted.
At site 1, mechanical thinning was conducted on 18 July 2006 and 19 July 2007. Berries were at pea size as stage 32 (Coombe 1995), ~10 days before the beginning of veraison. At site 2, mechanical thinning was conducted on 24 July 2006 and 19 July 2007. Berries were at the beginning of veraison, with 10–15% colored berries, as stage 35 (Coombe 1995). At both sites, mechanical thinning was performed by a New Holland VL610 grape harvester (Coex, France), fitted with two pair of bow rods operating at 470 and 500 beats/min and driven at ~3.0 km/hr. The height of the harvester and the position of the bow rods were adjusted so that the fruiting zone of the canopy was not hit by rods. The clusters were mostly located between 0 and 0.25 m above the cordon, and both pair of rods struck the vine trunks below the cordon, causing thinning to occur due to canopy vibration. At both sites, after mechanical thinning, some damaged clusters and berries were not completely removed, but remained on the vines and desiccated over the following 5 to 7 days.
Yield components were assessed at harvest. The number of clusters and total vine yield per vine were determined in 10 vines per plot. Crop weight and number of clusters per vine were used to calculate cluster weight. Yield per hectare was then calculated according to vine density at both sites. Moreover, the exposed canopy surface area for each treatment was determined using a published method (Smart and Robinson 1991). Canopy surface area/yield ratio was also calculated at both sites over seasons 2006 and 2007. In order to determine several yield components and berry compositional parameters, 15 clusters per plot were collected and stored overnight in a cool room (4°C). In the lab, each cluster was weighed and the visual presence of Botrytis bunch rot was determined (% incidence) Cluster compactness was also rated according to OIV 204 standard (OIV 1983). A 20-berry sample was separated from the rachis and weighed for the estimation of berry weight. Berry weight and cluster weight were used to calculate berries per cluster.
All clusters were frozen and stored at −18°C for fruit composition analysis. Fruits were defrosted overnight before composition analysis. A berry subsample (~300 berries) was hand crushed and filtered. Total soluble solid concentration was determined using a temperature-compensating digital refractometer (Atago, Tokyo, Japan). A 50 mL must sample was used to determine titratable acidity, pH, and tartaric and malic acid according to the method of OIV (OIV 1990). An automatic titrator, coupled to an autosampler and control unit (Titroline 96, Schott, Germany) was used to determine titratable acidity (expressed as tartaric acid) and pH. The remaining berry subsample was homogenized using a knife mill (Retsch, Grindomix, Germany) at high speed (10,000 rpm for 30 sec). Anthocyanin and phenolic concentrations were determined according to a published method (Iland et al. 2004). Total anthocyanins were expressed as mg per berry and mg per gram of fresh berry mass, whereas total phenols were expressed as absorbance units at 280 nm (AU) per berry and per gram of fresh berry mass.
Statistical analyses were conducted using InfoStat (2007 edition; Cordoba, Argentina). A combined analysis of variance over years (two-way ANOVA with replications) was performed on the Tempranillo and Grenache data. Year was considered as a random variable and the error term for the mechanical-thinning treatments was the year x treatment interaction mean square. Mean separation between mechanical-thinning treatments was accomplished with the Student Newman–Keuls test and t-test for Tempranillo and Grenache, respectively. The year x treatment interaction was tested over the pooled error and considered only if significant.
Results and Discussion
Yield components of Tempranillo and Grenache were significantly affected by mechanical thinning (Table 1⇓). Significant differences in cluster compactness, berry number per cluster, and cluster weight were observed among thinning treatments in both varieties. Mechanical thinning successfully reduced the crop yield in both varieties over the two seasons. The reduction in yield was ~65%, which is within the range reported in other studies of mechanical thinning (Pool et al. 1993, Fendinger et al. 1996, Clingeleffer 1993, Clingeleffer et al. 2002, Petrie and Clingeleffer 2006). The yield of the thinned vines was lower than in previous studies conducted on wine-grapes; however, in these trials, thinning was carried out on larger machine- or minimally pruned canopies, with irrigated vines, which also had a higher unthinned and target yield (Clingeleffer 1993, Clingeleffer et al. 2002, Petrie and Clingeleffer 2006).
Effects of mechanical thinning on yield components of Tempranillo (Haro, La Rioja) and Grenache (Aldeanueva de Ebro, La Rioja). Pooled data for 2006 and 2007 seasons.
In Tempranillo, mechanical treatments led to a reduction in yield, with 25% fewer clusters per vine and berries per cluster compared to unthinned (control) vines. On the other hand, yield reduction in Grenache was caused by a significant decrease in berry number per cluster (41% reduction), thus resulting in lower cluster weight. Where cluster and berry parameters have been reported in other studies of mechanical thinning in winegrapes, thinning has had a greater effect on cluster number than berry number per cluster or cluster weight (Clingeleffer et al. 2002, Petrie and Clingeleffer 2006). In these previous studies, the beaters of the harvester impacted directly on the canopy and the clusters, whereas in this work the beaters were positioned below the fruiting zone. When beaters hit the fruit zone, it is likely that whole clusters are removed, compared with individual berries when beaters hit the trunk and shake the whole vine. As this thinning method removed a lower proportion of whole clusters, it would be difficult to use counts of cluster numbers in the canopy or on the ground to estimate the proportion of fruit that has been removed.
In Tempranillo, a significant year-by-treatment interaction was found for berry weight, cluster weight, and cluster compactness (Figure 1⇓). Average berry weight was not significantly affected by mechanical thinning in 2006 in Grenache, and cluster compactness decreased significantly only in 2007 (Table 2⇓). Petrie and Clingeleffer (2006) noted that mechanical thinning increased berry size, but less than hand thinning; however, that was largely due to impact damage (bruising) of the fruit. Other authors have observed an increase in berry size in response to hand thinning (Kaps and Cahoon 1989, Reynolds et al 1994).
Effects of mechanical thinning on yield components of Grenache (Aldeanueva de Ebro) over two seasons.
Effect of mechanical thinning intensity on berry weight (A), cluster weight (B), and cluster compactness (C) of Tempranillo (Haro, La Rioja) in two seasons, 2006 and 2007. Compactness rated according to OIV 204 standard (OIV 1983). (Low-intensity thinning: 470 bpm; high-intensity thinning: 500 bpm. Vertical bars indicate SE.)
The incidence of Botrytis was significantly reduced by mechanical thinning in Grenache in 2007 and unaffected in other sites or seasons (Table 1⇑, Table 2⇑). Weather conditions in the Rioja appellation during ripening in 2006 and 2007 were typical for the climate. The results suggest that mechanical thinning did not cause any health reduction of grapes at harvest. It is possible that the reduction in cluster compactness that was seen as a result of mechanical thinning reduced disease incidence.
The intensity of mechanical thinning (470 and 500 beats per minute) did not significantly modify yield components in Tempranillo. Research on Concord (Vitis labrusca), where the whole canopy was struck by the harvester beaters, showed effective beater speeds of between 175 rpm (10% fruit removal) and 275 rpm (100% fruit removal) (Pool et al. 1993). The high frequency and relatively small difference between the treatments (6%) used in the current trial would have contributed to the lack of a significant effect.
In terms of berry composition and fruit quality, mechanical-thinning treatments led to significantly higher sugar concentration (Brix) in both Tempranillo and Grenache vineyards (Table 2⇑, Table 3⇓). The advancement in fruit maturity of the mechanically thinned vines over the two seasons in Tempranillo and in 2006 in Grenache suggests that damage to the canopy was minimal, with no removal of leaf area in any treatment (Table 1⇑). In fact, the enhancement of sugar accumulation could be linked to the increase of canopy surface area/yield. Higher sugar concentration in the fruit at harvest have been seen in response to mechanical thinning in a number of other trials on both wine (Clingeleffer 1993, Clingeleffer et al. 2002, Petrie and Clingeleffer 2006) and juice grapes (Pool et al. 1993, Fendinger et al. 1996). According to Petrie and Clingeleffer 2006, the observed increase in Brix caused by the mechanical-thinning treatments would be mostly due to the advancement of berry maturation rather than to the variation of the sugar accumulation rate. In 2007 the Grenache in Aldeanueva was harvested very late, which led to unusually high sugar concentration for both treatments (Table 2⇑). As fruit matures, sugar accumulation can plateau (e.g., McCarthy 1999). If the Grenache had been harvested earlier, differences in sugar concentration among thinning treatments might have been apparent.
Effects of mechanical thinning on berry composition of Tempranillo (Haro) and Grenache (Aldeanueva de Ebro). Pooled data for 2006 and 2007 seasons.
Other must parameters (pH, malic and tartaric acid) were unaffected by mechanical thinning in both varieties. Only titratable acidity showed significantly lower values due to mechanical-thinning treatments as compared with the control in Tempranillo (Table 3⇑). Very little effect on acidity parameters, such as titratable acidity and pH, have been reported in other studies of mechanical thinning in either wine (Clingeleffer 1993, Clingeleffer et al. 2002, Petrie and Clingeleffer 2006) or juice grapes (Fendinger et al. 1996).
Other indicators of fruit quality were anthocyanin and total phenolic content. Mechanical thinning did not significantly alter anthocyanin content, either per berry or per gram of fresh weight in the two varieties studied. Mechanical thinning led to significantly higher concentrations of phenolics per berry in Tempranillo in 2007 (treatment by year interaction) (Table 3⇑). When the results from 2007 were considered separately, mechanical thinning also increased the phenolic content per gram of fresh berry weight (Figure 2⇓). In Grenache the total phenol concentration (mg/g berry weight) was significantly higher for the mechanical-thinning treatment during 2006 (Table 3⇑). Unfortunately, this parameter was not reassessed during the 2007 season.
Effect of mechanical-thinning intensity on the concentration of total phenolics per berry of Tempranillo (Haro, La Rioja) in two seasons, 2006 and 2007. (Low-intensity thinning: 470 bpm; high-intensity thinning: 500 bpm. Vertical bars indicate SE.)
Mechanical thinning has the potential to lead to increased levels of total phenols as compared to the control in the two studied varieties. Mechanical thinning has also been shown to increase phenolic content in fruit in studies on Cabernet Sauvignon (Petrie et al. 2003, Petrie and Clingeleffer 2006). A strong relationship between grape and wine phenolics has been demonstrated (Jensen et al. 2008). While the relationship between young wine anthocyanin concentration and quality is better recognized (Somers and Evans 1974), phenolics can positively influence wine color stability during aging (Cheynier et al. 2006) and potentially affect wine aroma and mouthfeel (Preys et al. 2006). These results may be especially relevant to Grenache, as it was shown to have one of the lowest levels of total phenolics in both grapes and wines of eight varieties that were assessed (Jensen et al. 2008).
Finally, it could be attested that mechanical thinning is a technique to regulate crop yield quickly and cost effectively, with an estimated operation time of 1.5 hr/ha and a cost of about $220.00 US per hectare (data not shown).
Conclusion
This study is the first report of regulation of yield using a machine harvester in vineyards trained to a vertical shoot-positioned canopy. All mechanical-thinning treatments successfully reduced yield and improved fruit quality as measured by an increase in sugar and total phenol concentration in Tempranillo and Grenache. Mechanical thinning offers the potential to regulate yield in VSP vineyards quickly and cost effectively, especially where labor is expensive or unavailable.
Footnotes
Acknowledgments: The authors acknowledge Agrupación Bodegas Centenarias y Tradicionales de Rioja (ABC), Viñedos de Aldeanueva, ADER and New Holland for their assistance and help during mechanical thinning at the vineyards.
- Received April 2008.
- Revision received July 2008.
- Copyright © 2008 by the American Society for Enology and Viticulture
Vol 59 Issue 4
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