Written by Dr. Ting, Compiled and Edited by Jess Trapeni

A summary of two fruit thinning experiments involving shoulder thinning and cluster halving.

Introduction

Fertile Virginia soils can often produce vigorous grape vines with a heavy crop load that struggle to fully ripen. It is common practice to reduce crop load by pruning and cluster thinning to decrease shading and disease pressure, increase sunlight exposure, and encourage ripening. Crop reduction is also thought to improve overall vine balance and prevent stress that can lead to lower fruitfulness in subsequent years1. However, crop reduction comes at a measurable cost. In her landmark crop load study in Oregon, where participants were routinely cropping at 2-2.75 tons/acre, Patty Skinis estimated that a 25-50% crop reduction came at a cost of $700-800 per acre. In Virginia, the commercial cost of reducing premium Merlot crop from 4.5 to 3 tons/acre averages over $37502. The question then arises: what is the right balance, for the vine as well as the bank account?

Previous studies examining the effect of altered crop load in different regions with different grape varieties have shown markedly mixed results. Some studies3–5 reported important differences with crop reduction including faster maturity, better color, higher anthocyanins, and higher wine quality scores while others found little to no differences at all1,6,7. Some studies even described decreases in wine quality with lower crop levels when negative flavor and aromas (such as methoxypyrazine) were concentrated8. Interpretation of these studies is complicated by differences in growing region, grape variety, and whether grapes were harvested on the same day or at the same level of maturity. The method of crop management may also matter, with pruning more effective than crop thinning8. Some studies found few differences until very high crop loads are reached7,8. Many of these studies reported vine phenology and grape chemistry but did not include an examination of the flavor and aroma of wine, either chemically or by sensory analysis.

When taken as a whole, the literature seems to suggest that for any given situation, there is a range of crop loads that will produce comparable quality wine, with lower quality wine at too high and too low a value. The most cost-effective management strategy, then, would be to maintain the vineyard on the higher end of the range of comparable quality. Unfortunately, given the large ranges examined in the literature with differing results, there is no clear formula to determine the right amount of crop to grow on a given site. These values depend on factors such as vine vigor, leaf area, vintage, variety, water availability, length of the growing season, and many others.3,4,7,8 When presenting the results of a crop load study at Virginia Tech in 2000, Bruce Zoecklein stated “The relationships between yield and quality are complex…There may be no exact point when over-cropping begins.” The best approach to determining the right crop load for a given vineyard then is by practical experimentation.

In the 2023 growing season, two WRE studies explored the effects of cluster thinning on grape and wine quality. At Stone Tower, very fruitful Nebbiolo vines were either not thinned, or thinned by removing half of each cluster, at two different time points. At Boxwood, an already judicious cluster thinning protocol was made more severe by removing shoulders from the clusters. In both experiments, quality was assessed with grape and wine chemistry as well as sensory analysis.

Experiment #1: Does the timing of cluster halving impact grape and wine quality in Nebbiolo? (Preston Thomas, Stone Tower Winery)

Stone Tower Winery has several acres of Nebbiolo intended for varietal wine. Without judicious crop thinning, these vines produce a heavy crop load with very large clusters. Typically, the crop is thinned at veraison by removing roughly half of each cluster, allowing better cluster morphology and, presumably, more even ripening. However, this comes at a considerable cost in labor and yield. The purpose of this experiment was to (1) validate the efficacy of cluster halving and (2) test the impact of this treatment at two-time points (post fruit set vs at veraison). Three long rows of Nebbiolo were treated as control (no thinning), T1 (thinning by cluster halving between fruit set and bunch closure), and T2 (thinning by cluster halving at veraison). Fruit chemistry during ripening was very similar between early vs. late thinned fruit. The control fruit lagged behind with higher berry weight, lower Brix, and lower pH at two sampling dates. Finished wine chemistry was also very similar between the two treatments. Both had indications of riper fruit than the control, such as higher alcohol, higher pH, and lower TA. Both treatment wines also had higher concentration of anthocyanins and phenolics, but lower tannins, than the control wine. The control wine and T2 (later thinning) wines had very similar sensory scores. The wine produced from early thinned fruit had significantly higher sensory scores for color intensity, hue, and fruit intensity. Early fruit thinning resulted in a 22% lower yield while later thinning resulted in a 37% lower yield than control. In this case, lower yield correlated with advanced ripening in both treatments and improved sensory results in the early thinned treatment.

Figure 1: Anthocyanin concentration (mg/L) in wine made from three cluster halving treatments of Nebbiolo (ETS Labs)

Experiment #2: Exploring the effect of shoulder thinning in Cabernet Franc (Joyce and Stephen Rigby, Boxwood Winery)

A rich mouthfeel is a hallmark of quality red wine. The mouthfeel of a wine is influenced by its astringency, fruit concentration, alcohol content, and acidity. Both astringency and fruit concentration are a function of grape quality and maturity and are difficult to correct in the winery. At Boxwood, vines are planted in closely spaced vines and rows, with judicious crop thinning to fewer than 12 clusters per vine. The purpose of this experiment was to determine if removing the shoulders of the remaining clusters would lead to improved mouthfeel in the finished wine. A single block of Cabernet Franc clone 214 on 101-14 rootstock was divided into six sections of three rows each. All sections were thinned according to the standard protocol. In three of the six sections, shoulders were removed from the remaining clusters. Fruit samples were collected from each replicate just prior to harvest. Three treatment replicates were combined in a single harvest lot to produce production-scale wine. There were no differences in fruit chemistry, berry weight, or cluster weight between treatments at harvest. Anthocyanin and tannin concentrations were higher in samples from fruit with shoulders removed, but this difference was not found in the finished wine. The wines were not different in a triangle test, and there were no significant differences in descriptive scores for fruit intensity, fruit character, herbaceous/green character, or the intensity of astringency. When asked to compare the feeling of astringency to the tactile sensation of 4 different fabric standards, the wine produced from shoulder-thinned fruit was most often described as having suede-like tannin texture while the astringency of wine made from fruit with standard thinning was most often described as “sandpaper”.

Figure 2: Fabric standards were used to describe the texture of tannins in wine made from control fruit vs. fruit that had the shoulders removed.

Conclusions

Fruit and wine quality improved with cluster halving at Stone Tower, but shoulder removal at Boxwood did not produce significant differences. Cluster halving (Stone Tower) removed much more fruit than shoulder removal (Boxwood), possibly leading to more pronounced effects. But these differences may also be due to the different starting points. Stone Tower started with very fruitful (potentially overcropped) vines while the control for the Boxwood experiment already had judicious crop thinning in place. These vines may already be within that range of crop load that produces the highest wine quality possible at that site, such that further removal may have no effect. The 2023 vintage was marked by warm temperatures, dry conditions, and low disease pressure. These results may differ in a cooler, wetter vintage where accelerated ripening would be advantageous.

For full reports from these experiments and a more thorough review of crop load studies, please visit the Learn section of WRE Website (Module: Effect of Crop Load on Wine Quality)( https://winemakersresearchexchange.com/learn/effect-of-crop-load-on-wine-quality)

References

(1) Moreno Luna, L. H.; Reynolds, A. G.; Di Profio, F. Crop Level and Harvest Date Impact Composition of Four Ontario Winegrape Cultivars. I. Yield, Fruit, and Wine Composition. American Journal of Enology and Viticulture 2017, 68 (4), 431–446.

(2) SMS Research Advisors. 2019 Virginia Commercial Grape Report. 2020.

(3) Bravdo, B.; Hepner, Y.; Loinger, C.; Cohen, S.; Tabacman, H. Effect of Crop Level and Crop Load on Growth, Yield, Must and Wine Composition, and Quality of Cabernet Sauvignon. Am J Enol Vitic. 1985, 36 (2), 125–131.

(4) Wolf, T. K. Wine Grape Production Guide for Eastern North America; Plant and Life Sciences Publishing: Ithaca, New York, 2008.

(5) Sinton, T. H.; Ough, C. S.; Kissler, J. J.; Kasimatis, A. N. Grape Juice Indicators for Prediction of Potential Wine Quality. I. Relationship Between Crop Level, Juice and Wine Composition, and Wine Sensory Ratings and Scores. Am J Enol Vitic. 1978, 29 (4), 267–271.

(6) Matthews, M. A. Terroir and Other Myths of Winegrowing; University of California Press: Oakland, California, 2015.

(7) Skinkis, P. The Low Down on High Yields: Challenging Yield-Quality Standards for Oregon Pinot Noir, 2017. https://industry.oregonwine.org/resources/workshops/2017-oregon-wine-symposium-low-high-yields-challenging-yield-quality-standards-oregon-pinot-noir/.

(8) Chapman, D. M.; Matthews, M. A.; Guinard, J.-X. Sensory Attributes of Cabernet Sauvignon Wines Made from Vines with Different Crop Yields. Am J Enol Vitic. 2004, 55 (4), 325–334.

(9) Zoecklein, B. W. Update of Ongoing Projects, 2000.