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Measure and Manage Crop Load to Meet Your Vine Balance Targets

By Terry Bates, Rhiann Jakubowski, and James Taylor.

Crop load is a measure of the crop size relative to the vine's vegetative growth. The Ravaz Index, which is the ratio of yield to pruning weight, is a common way to estimate crop load.  Measuring and managing for crop load can help growers achieve vine balance appropriate to their production goals.

Decades of vineyard studies on Concord in the Lake Erie region have established clear relationships between yield, juice soluble solids, and vine size as measured by grown pruning weight. Crop load – the relationship between vegetative growth and fruit production – provides a framework to evaluate management practices that aim at vine balance for sustainable production. The Ravaz index – the ratio of yield to pruning weight – is a key metric.

To meet processor standards and maintain economic viability, Concord growers need to manage vineyards so that they maximize tonnage, while reaching soluble solid levels of at least 16° Brix at harvest. To maintain vineyard health, they need to maintain adequate vine size. Our studies indicate Concord growers should strive for a yield: pruning weight ratio of 15 to 20 in an average season to maintain vine size and meet the 16° Brix standards. Attaining this goal often means leaving abundant buds during dormant pruning and adjusting crop load by shoot thinning or mechanical crop thinning during the growing season. New precision management tools will allow spatial crop load maps to guide management.

Key Concepts

  • Vine balance is a sustainable management concept aimed to control vine vegetative and reproductive growth to achieve both high fruit maturity in the current season and high fruiting wood maturity for the next season.
  • Crop load, the relative relationship of crop size to vine size, is commonly measured as the Ravaz Index or yield to pruning weight ratio (Y:PW).
  • Crop load management, evaluated through the Y:PW, determines if vines are undercropped, overcropped, or balanced.
  • For Concord juice grapes, a Y:PW approximating 15 results in fruit reaching processor juice standards (16 °Brix) and maintains vine size in an average year.
  • Varying retained nodes with vine size through pruning, such as retaining 40 buds per pound of pruning weight, is one method of crop load management to achieve Y:PW = 15.
  • Light and non-variable pruning, such as with mechanical systems, often leads to high Y:PW in Concord which can be adjusted to more appropriate crop load values through shoot or fruit thinning.
  • The response of Concord to crop load is the same whether the Y:PW is achieved through pruning alone or through light pruning with mid-season fruit thinning.
  • Spatial crop load mapping will allow growers to respond to variable vine size and cropping levels within a vineyard.

Introduction

There is certainly power in completing a phrase. In 1973, the evolutionary biologist Theodosius Dobzhansky wrote, “Nothing in biology makes sense…” My comparative anatomy professor at St. John Fisher, Dr. Crombach, joked that many biology students thought the phrase should end there. The complete phrase is, “Nothing in biology makes sense except in the light of evolution.” The phrase was used in an argument in favor of teaching biological evolution in public schools because evolution was the concept where all other facts, theories, and hypothesis in biology could find a home.

In our grape production world, I would re-write the phrase to read, “Nothing in viticulture makes sense… [awkward pause] except in the light of crop load.”

In my humble opinion, the one thing any New York grape producer should do is measure vineyard crop load because it provides a framework to evaluate all other viticulture studies and management practices. Crop load informs vineyard managers on vine-focused or crop-focused management practices and is usually the main explanation for why management, like fruit thinning or leaf pulling, worked in one vineyard and did not work in another vineyard.

One problem, however, is that there can be a bit of confusion around how to define and measure crop load. From viticulturists to wine writers, many tend to play fast and loose with terms like vine balance and crop load, and use these terms to subjectively describe the quality of the vineyard, fruit, and/or wine.

Here are some of my working definitions so we are speaking the same viticulture language:

Crop load: ‘Crop load’ is the crop size relative to vine size (estimated as pruning weight or leaf area) and is a measure of the sink:source ratio” (Keller 2010, pg 169). Crop load should not be misused to only mean fruit yield or crop size. Correct use: “Block A had a mean Ravaz Index of 25, and the vineyard manager is concerned with the high crop load.” Incorrect use: “The producer is shooting for a crop load of 6 tons/acre after thinning.”

Ravaz Index (Y:PW): A quantitative measurement of crop load calculated as the yield (Y) to pruning weight (PW) ratio. The units of measure should always be the same for yield and pruning weight, such as pounds of fruit/vine divided by pounds of pruning weight/vine, so that the resulting index is unitless. Example: A vine with 30 pounds of fruit and 2 pounds of pruning weight has a Ravaz Index of 15.

Crop size (Yield): This is the fresh fruit weight or “yield” measured on a per vine or per land area basis, such as pounds/vine or tons/acre. Yield level affects crop load as a component of the Y:PW. Likewise, vineyard management practices such as shoot and cluster thinning to control yield level affects vineyard crop load. Crop size, however, is not synonymous with crop load.

Vine size (Pruning Weight): Dormant cane pruning weight (PW) of one-year-old wood. Viticulture studies over the years have shown PW to be a sort of “Jack of all trades, master of none.” Pruning weight is a straightforward measurement to collect, but it is only an indirect indicator of several vegetative growth parameters. In high-wire cordon trained and undivided Concord grapevines, PW is directly related to total vine leaf area. When PW is combined with vine spacing measurements, exposed leaf area, shaded leaf area, and general light interception can be estimated. Whole Concord vine excavation studies show that PW is also directly related to total vine biomass, roots and all.

Vine Size–Yield Relationship: Crop load and the vine size-yield relationship are interrelated, but they are not quite the same thing, which can cause some confusion. The vine size-yield relationship term is forward-thinking in that it predicts yield potential for the next season; however, realizing that yield potential is influenced by management, such as pruning severity. It is an expected relationship not a real value.

In contrast, Crop load is a quantitative backward-looking term in that it is measured at the end of the season.

Vine balance: A qualitative term based on region, variety, viticulture production goals, and grape market destination (Howell 2001). Acceptable levels of vegetative and reproductive vine growth for any given market are up to the vineyard manager; however, quantitative crop load (Y:PW) measurement and management can assist managers in targeting their definition of vine balance.

Our subjective definition of “vine balance” for Lake Erie AVA Concord grown for the juice grape market is growing the largest possible crop reaching 16 °Brix by commercial harvest (30 to 40 days after veraison) and having no net change in vine size.

Investigating Concord Crop Load

To get a better understanding of how Concord responds to crop load management, we compared pruning severity and fruit thinning level over four years with a range of vine sizes in Concord vineyards and measured the effect on yield, juice soluble solids (JSS), and the seasonal change in pruning weight (Bates et al. 2021). In addition, we tracked the interaction of seasonal weather conditions and thinning level over eleven years.

Although multiple treatments were applied over multiple years, the different experiments could be easily merged because the results were always interpreted in the light of crop load effects. This allowed the merged data to reveal clear trends on the effect of crop load on grape maturity in that season (JSS) and its effect on the seasonal change in vine pruning weight (Figure 1).

The big picture: On average, the industry standard of 16 Brix was achieved at a Y:PW of 20 and no seasonal pruning weight change was observed at Y:PW of 17.5 (Figure 1). Given some seasonal variations and my tendency to be crop load conservative, we put the target Y:PW for balanced Concord vines in the Lake Erie region at “15” (i.e. 15 pounds of fruit for each pound of pruning weight). As the Y:PW increased above 15, there was a reduction in harvest juice soluble solids (JSS) and a net decrease in PW from the beginning to the end of the season. As the Y:PW decreased from 15 to 7.2, there was an increase in harvest JSS but there was no additional increase in JSS below a Y:PW of 7.2. The annual change in pruning weight also increased below Y:PW of 15 but did not plateau like JSS.

Two graphs showing (top) a measurement of juice soluble solids (y axis) and crop load (x axis) and the relationship between these factors and pruning weight in Concord grapevines.
Figure 1. The relationship between crop load (Ravaz Index, Y:PW) and harvest juice soluble solids (top) or the seasonal change in pruning weight (bottom) in Concord grapevines. To achieve the wide range in crop load values, vines were either pruned to different levels (30+10, 60+10, or 100 nodes/vine) or pruned to 120 nodes/vine and then fruit thinned 30 days after bloom to retain 25%, 50%, 75% or 100% crop. Black dots indicate a result from a thinning treatment and white dots a result from a pruning treatment.

The specific treatment effects: While Figure 1 shows the big picture for crop load, Figure 2 provides some more detail on specific pruning and thinning approaches that are common in Concord and their ability to achieve a ‘balanced’ production system.

A graph showing yeild (y axis) and pruning weight (x axis) across a variety of vines pruned to a specific number of nodes.
Figure 2. The relationship between pruning severity, the vine size-yield relationship, and crop load over four seasons in Concord (own-rooted, single cordon trained, cane pruned, standard 9’ row x 8’ vine spacing). The four curves indicate four pruning severities, the response of each curve represents the vine size-yield relationship at each pruning severity, and the shaded regions represent relative crop load. The blue arrows indicate the level of fruit thinning that would be needed to bring high node number vines into balance at a given vine size.

Conservative balanced pruning at 30+10 (i.e. retaining 30 buds for the first pound of pruning weight and 10 additional buds for each additional pound of pruning weight) consistently undercropped the vines (mean Y:PW = 7 and quartile range = 4.6 to 8.6) by limiting yields, even at large vine sizes. This is a possible pruning strategy for building vine size and improving the long-term productivity potential of young or stressed vineyards, but it is not a viable option for sustainable economic Concord production, at least not at the current prices. It is also consistent with other cool climate crop load studies which describe excessive canopy growth and the need for canopy management at Y:PW below 5 (Figure 3).

Liberal balanced pruning (60 + 10) had a mean Y:PW of 11 (quartile range = 8.3 to 13.0) with juice soluble solids between 16.6 and 17.0 and a seasonal increase in vine size. This pruning management is appropriate for conservative producers who want to ensure a ripe crop in any given season and do not want to adopt fruit thinning as a crop control strategy. Since retained nodes and yield increased with increasing vine size under this treatment, economic revenue is maximized at high vine size > 2.5 pounds/vines at 9’ row x 8’ vine spacing.

Fixed node pruning at 100 nodes/vine without additional crop adjustment had a mean Y:PW of 17 (quartile range = 12.0 to 20.0) and tended to be on the high end with small vines and the low end with large vines. In commercial Concord operations in NY, the term “grower pruning” typically refers to pruning standard spaced vines (9’ row x 8’ vine) to between 80 nodes on small vines and 100 nodes on large vines. This study indicated that 100-node pruning management reasonably satisfies the crop load goal for achieving 16 Brix fruit while maintaining vine size in NY Concord. The disadvantage of high node number “pruning only” crop management is that the crop potential is set during the dormant season and does not allow for in-season crop load adjustments. In the event of frost or poor fruit set, limiting bud number limits potential yield by limiting the number of secondary shoots, leading to undercropped vines and limited revenue.

High 120 node pruning, similar to what is done in mechanical pruned systems, is almost always overcropped except for at very high vine size. Vines pruned to 120 nodes without additional crop adjustment had Y:PW values > 20, with reductions in juice soluble solids and vine size. In Lake Erie Concord production, there is a trend to retaining relatively high node numbers because of mechanized pruning and the need to mitigate frost risk. Mechanized pruning reduces production costs, but is less precise in managing retained node quantity and quality; therefore, producers err on the side of retaining too many buds. In this study, fruit thinning one month after bloom was used to reduce Y:PW, increasing harvest juice soluble solids and the seasonal change in pruning weight. The Concord crop load response was the same whether a given Y:PW was achieved through pruning severity or by retaining additional fruiting nodes followed by mid-season fruit thinning. This is clearly shown in Figure 1, where crop load effects on JSS and change in PW could equally be achieved via either thinning (white dots) or pruning (black dots) treatments.

In practice, Concord crop estimation and adjustment are done by mechanical fruit thinning with a harvester at approximately one month after bloom. Deciding how much fruit to thin, or retain, can be additionally adjusted by seasonal climatic conditions. The 11-year thinning data in our study indicated that warmer than average seasons achieved 16 Brix at higher Y:PW than cooler than average seasons. Assuming GDD accumulation one-month after bloom is reflective of total season GDD, high GDD accumulation at fruit thinning means the vineyard can be managed to higher crop load levels and may require less thinning or no thinning at all. Low GDD accumulation at fruit thinning would warrant more fruit thinning to maintain higher sugar accumulation rates from veraison to harvest.

The Fruit Thinning Controversy… in Light of Crop Load

Nothing about crop thinning makes sense… [awkward pause] except in the light of crop load. Viticulturists typically have strong opinions about crop size and fruit quality without the context of crop load. For example, a Riesling grower may insist on 4 shoots/foot of row and thin to one cluster per shoot with no more than 2-3 tons/acre for quality production. A Concord grower, in contrast, may prune to more than 15 shoots/foot of row and will only think about fruit thinning in years when the crop sets more than 10 tons/acre and there is a late bloom. We argue that a reasonable measurement of crop load is missing from both situations.

Several cool-climate crop load studies in both juice and wine grapes, have been published that mostly investigate the effect of cluster thinning on vine crop load and fruit quality. When it comes down to the physiological vine response to crop load, most studies show both high fruit and wood maturity at Y:PW between 8-10, whether the variety is Riesling or Concord (Figure 3). However, our definition and acceptable crop load range for balanced Concord vineyards (Y:PW 15-20 depending on season) considers the economic impact of higher yield, acceptable JSS, and sustainable return crop potential.

The main point here is that there are more similarities in crop load response between varieties and regions than differences. In the Riesling example, the grower could potentially increase crop size towards a more balanced crop load (Y:PW = 8) and focus on canopy management, thereby increasing yield and maintaining or improving quality. In a perfect viticulture world, Concord crop load management should also target Y:PW of 8, and we have shown that the level of impact of fruit thinning on both JSS and PW is influenced primarily by where the vines sit on the crop load spectrum before and after thinning and secondarily on seasonal weather differences. We cannot discount, however, the influence of economics on crop load management. What if Riesling sold for $200/ton and Concord sold for $1200/ton? Or if the target quality range (e.g. JSS) was smaller or larger for either variety?

Chart showing publications on crop load studies along with a chart showing crop load (Ravaz Index).
Figure 3. Classic crop load studies and more recent regional fruit thinning research, when combined, span a wide range of crop load measured by the Ravaz Index. The color bar and response descriptors illustrate the “physiological” balance or imbalance of vine vegetative and reproductive growth.

Spatial Measurement of Crop Load

Spatial crop load mapping identifies the pattern in the vineyard and provides valuable management information to the producer. Using technology developed in the Efficient Vineyard project (efficientvineyard.com), spatial yield maps can be generated from grape yield monitors at harvest. In addition, bloom NDVI spatial sensor data has also been used to direct in-field crop estimation samples to generate predicted yield maps. These maps open the door for in-season variable-rate crop adjustment (Bates et al. 2018). For pruning weight estimates in Concord, veraison NDVI measurements have been used to direct in-field pruning weight samples. Validated yield and pruning weight maps can then be processed to give spatial vineyard crop load maps (Taylor et al. 2019) (Figure 4).

In the example Concord vineyard in Figure 4 and using our Concord economic crop load definition, 25% of the vineyard was slightly to severely undercropped. We would predict these regions to have high fruit maturity and gain vine size leading to increased crop potential the following season. Roughly 50% of the vineyard was balanced according to our Concord crop load model. The remaining 25% was slightly to severely overcropped and we would predict low fruit quality and reduced vine size in these regions. The overcropped regions would have benefited from fruit thinning. Having this spatial map, the grower can make spatial management decisions about pruning severity, crop prediction, and the potential for fruit thinning in the next season.

A spacial map of crop load in a Concord Vineyard in western New York.  Dark orange indicates overcropping, while dark green indicated undercropping.
Figure 4. Spatial crop load mapping in a commercial Concord vineyard in western New York. Yield data was collected from an ATV yield monitor, and pruning weight was predicted using ancillary NDVI sensor data coupled with in-field pruning weight samples. Spatial yield and pruning weight data were processed to generate a crop load (Ravaz Index, Y:PW) map.

Conclusion

Measuring and managing vineyard crop load has the dual goal of achieving desired fruit maturity in the current season and providing adequate vegetative growth and fruiting potential for the next season. Vineyard managers need to consider how vines respond to crop load, how the grape market and production goals set their crop load targets, and how seasonal and spatial variation can adjust their management strategies. Based on pruning and fruit thinning studies in Concord, general crop load recommendations have been established with these considerations (Table 1).

Table 1. General crop load descriptions and management recommendations for Concord production in the Lake Erie AVA

Y:PW Category Predicted Brix a Management
0-5 Severely undercroppped 17.2

Juice soluble solids (JSS) maximized and vine size increased by 0.15-0.20 kg/vine. Severe undercropping, generally only observed in frost damaged vineyards, can be managed to increase overall vine size and crop potential for the following season.

5-10 Undercropped 17.0-17.2

JSS > 1.0 Brix above the 16.0 standard and vine size increased by 0.10-0.15 kg/vine. This crop load is not economically viable for long-term Concord production in NY and recommended only when attempting to build vine size in young or stressed vineyards.

10-15

Slightly undercropped

Balanced in cool season

16.5-17.0 JSS 0.5 to 1.0 Brix above the 16.0 standard and vine size slightly increased by 0.03-0.09 kg/vine. This conservative crop load can be achieved with moderate balanced pruning, does not require fruit thinning, and will still mature to 16 Brix in cooler than average seasons.
15-20 Balanced in average season b 16.1-16.5

JSS at or slightly above the 16 Brix standard and vine size maintained +/- 0.03 kg/vine

20-25

Slightly overcropped

Balanced in warm season

15.7-16.0

JSS below the 16 Brix standard and vine size reduced by 0.03-0.09 kg/vine in an average season. Harvest delays and reduced crop potential for the following season are expected; however, vines will maintain balance in warmer and wetter than average seasons. This crop load recommended if mid-season fruit thinning is part of the management strategy. In cool and average seasons, the crop can be moderately thinned to maintain balance. In warm seasons, no thinning would be necessary.

>25 Severely overcropped <15.7 JSS well below the 16 Brix standard and, if left unthinned, will still require a significant period of ripening after harvest has started. Vine size will be reduced by > 0.1 kg/vine (0.25 lbs/vine) with a lower future yield potential and a lower return crop. It requires excessive fruit thinning to achieve vine balance mid-season, which has been shown to cause canopy damage in Concord and negates the positive effects of fruit thinning on vine size/health. This level of crop load stress is not recommended.
     

a Predicted Brix in an average season at a standard harvest of 30 - 40 days after veraison. The given ranges reflect this spread of time.

b An average season = 1455-1723 GDD (+/- 1 st. dev. from the 11 -year GDD mean). Cool season < 1455 GDD, Warm season > 1723 GDD.

Photo of Terry Bates.

Terry Bates is a senior research associate in the Cornell School of Integrative Plant Science and Director of the Cornell Lake Erie Research and Extension Laboratory with Cornell AgriTech in Portland, NY. For the past five years, he has led the Effi­cient Vineyard project, funded by the US­DA's Specialty Crops Research Initiative.

Photo of Rhiann Jakubowski.

Rhiann Jakubowski is a research aide at the Cornell Lake Erie Research and Extension Laboratory in Portland, NY. Her research focuses on vineyard GIS systems, spatial data processing, sampling schemes in sensor validation, and techniques for improved crop estimation.

Photo of James Taylor.

James Taylor is a Research Director in the area of Precision Agriculture for the French National Agriculture and Environment Research Institute (INRAE). He leads the Decision and Modeling research team within the ITAP (Agriculture of Tomorrow) research unit in Montpellier, France.


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