CROP ESTIMATION
Adapted from: Crop estimation and crop thinning, by Dr. Tony Wolf, Virginia Tech
A. Why perform a crop estimate?
1. Avoid over-cropping. What is over-cropping?
- retaining more crop than the vines can support to the desired degree of maturity by the desired time frame. This maturity is measured by: Brix, pH, TA, color, flavor and aroma
Figure 1.Chambourcin shoot with three clusters. Given that 12-14 leaves are needed to ripen one medium size cluster, this shoot is ‘over-cropped’. The shoot’s leaf area will not support and ripen the amount of fruit present.
- ** in other words, a heavier crop delays maturity …to the point that the crop never ripens given the amount of time before the first fall frost.
2. Determine how much crop to remove to reach a target crop level (if currently above target)
3. Realize available crop for sale/use in your winery, schedule labor, equipment and supplies
B. Requirements:
1. Record of previous harvests and lag-phase average cluster weights, if lag-phase used
2. Accurate count of sampling unit per acre (or missing units per acre)
- Units: bearing vines per acre or ‘panels’ of canopy per acre, if non-uniform vine spacing. ‘panel’ = row distance between two consecutive line posts
3. Accurate count of clusters per sampling unit
C. Source of variability:
Components of yield is divided into two major pools:
1. Components set in previous year, or at dormant pruning, determines clusters per acre:
i. vines per acre
ii. nodes retained per vine
iii. shoots per node
iv. clusters per shoot (fruitfulness)
2. Components determined in current season, determine average cluster weight:
i. flowers per cluster
ii. berries per flower (percent set)
iii. berry weight
All components shown above are variable, which collectively reduces crop estimate accuracy.
For example:
Vines per acre:Attrition due to disease, predation, mechanical damage, water stress,
First step: accurate count of bearing units per acre. Calculate % missing “units” (vine- or panel-basis) and deduct this % from crop estimate, assuming 100% vine stand or trellis-fill.
Nodes per vine: Varies dependent on pruning uniformity and from variety-to-variety, depending on crop expectation; for example, Seyval (hybrid) often has no count nodes retained, and Cabernet Sauvignon typically has 3 to 4 nodes per foot of cordon.
Shoots per node: Varietal difference: Seyval> 1 shoot/node, bud necrosis or winter injury can = <1 shoot per node. GDC (Geneva Double Curtain) training has led to >1 shoot per node (due to enhanced light environment)
Clusters per shoot:Varietal differences, cane vs. spur pruning, light and temperature regime of developing buds in previous season, bud necrosis and previous season’s crop level, all affect this variable.
Berry weight: Affected by fruit set and degree of crop thinning, i.e., crop thinning increases size of remaining berries, and is strongly affected by moisture conditions (rain or irrigation) after fruit set.
· Aim to thin crop in the 2-week period before veraison to limit berry size compensation
· Thin earlier if you need to stimulate vine size and vigor
D. Working equation:
Tons/acre = 1/2000 lbs. X vines/acre X clusters/vine X average cluster wt. (lbs.)
Average cluster weightcan vary significantly, and will likely be major source of variation of error in accurately predicting yield. Note actual numbers below from a vineyard of
the hybrid, Vidal.
Vines/acre clusters/vine (2013-2017) average cluster wt. (lb.) tons/acre
691 (9’ x 7’) 29 – 48 (avg. = 38) 0.49 6.43
691 (9’ x 7’) 29 – 48 (avg. = 38) 0.58(~ 15% greater) 7.61
E. Improving the working equation:
Crop prediction model improved by using anhistorical, average lag-phase cluster weight,plus the current season lag-phase cluster weightto adjust the predicted harvest cluster weight:
Predicted yield = (vines/acre) X (clusters/vine) X (S/Ax H)
where: S= lag-phase cluster weight for current season
A= historical, average lag-phase cluster weight (several years’ data)
H= historical, average harvest cluster weight (several years’ data)
Recommend a destructive sampling of allclusters from 5 to 15 vines per block, to arrive at mid-season cluster weights. To sample otherwise, will introduce bias – you will tend to sample only the largest clusters of the vine. Sample early July, about 45 to 50 days post bloom – be consistent each year.
Be aware of extremes of precipitation after lag-phase that can skew results
Alternatively (less precise); use multiplier pre-harvest to predict harvest cluster weight. Start of lag-phase
What constitutes an acceptable crop level?
Tons per acre is too broadly affected by
plant density and subsequent missing vines.
Targeting 1.0 to 2.0 pounds of crop per foot
canopy is a generally acceptable, albeit
somewhat broad benchmark.
Example: consider Cabernet Sauvignon vines
Planted 9’ x 5’, thus, 968 vines/acre. If
cropped at 1.5 lb. /foot of canopy
(VSP trained), = 1.5 lb. x 5’ x 968 vines/acre =
7,260 lbs. (3.63 tons/acre).
A reduction in fruit maturity
(less color density) is common when this
crop level is exceeded. White fruited
varieties @ 2.0 lbs. /foot, OK;
use the lower end for high quality red
wine potential.
 |
Figure 1.Diagram showing relative size and color of berries at 1- day intervals after flowering, passing through major developmental events (rounded boxes). Also shown are the periods when compounds accumulate, the Brix and an indication of the rate of inflow of xylem and phloem vascular saps into the berry. Illustration by Jordan Keutreumanidis. Winetitles. |
Complications and considerations:
1. Drought / excess moisture
2. oBrix is not a good indicator of crop level, especially with large vines that are not balanced. There is example of Cabernet Sauvignon vineyard at a 9 tons/acre crop, that did not differ from 4.5 tons/acre crop vineyard…with respect to soluble solids accumulation rate or values at harvest. BUT… what was difference in color density? Remember that consumers ‘drink with their eyes’…color is important.
When is best time to thin crop?
1. Early (within 30 days of fruit set) result in increased berry size – potential for > ROTS
2. Less compensation (berry size increase) occurs if crop thinned near veraison
3. Less benefit to remaining crop after veraison
4. “green thinning” = preferential removal of clusters that exhibit delayed color formation at onset of veraison – tends to mechanically reduce variability of maturity at harvest
5. Timing is not as critical with large vines, as it is with small vines
Table 1. Average cluster weight data for selected varieties
grown at the AHS Jr AREC in Winchester, VA
Variety | Ave. cluster wt. (lbs) |
Cabernet Franc | 0.38 |
Cabernet Sauvignon #8 | 0.28 |
Chardonnay #4 | 0.41 |
Chardonnay #95 | 0.32 |
Chardonnay # 96 | 0.30 |
Chardonel | 0.43 |
Mourvedre | 0.61 |
Petit Manseng | 0.22 |
Petit Verdot | 0.24 |
Syrah | 0.40 |
Tannat | 0.58 |
Traminette | 0.32 |
Vidal | 0.49 |
Viognier | 0.41 |
Table 2.Selected average cluster weight data from Winchester vineyard.
Year | Ave. cluster weight (lbs) at harvest | ||
| Chardonnay #4 | Cabernet Sauvignon #8 | Vidal |
1991 | 0.35 | 0.23 | 0.19 |
1992 | 0.49 | 0.31 | 0.38 |
1993 | 0.47 | 0.32 | 0.36 |
1994 | 0.33 | 0.22 | 0.41 |
1995 | 0.52 | 0.28 | 0.42 |
1996 | 0.32 | 0.31 | 0.34 |
1997 | 0.45 | 0.29 | 0.42 |
1998 | 0.41 | 0.27 | 0.49 |
Mean | 0.42 | 0.28 | 0.38 |
Table 3.Percent (%) increase of average berry weight of Cabernet Sauvignon and Chardonnay, in North Carolina.
vine | variety | avg. berry wt. - | avg. berry wt.- | % increase | avg. berry wt. – July 15 | % increase | avg. berry wt. – July 21 | % increase | avg. berry wt. – July 28 | % increase | % increase July |
BW | Cab Sau | 0.309 | 0.484 | 56.6% | 0.666 | 37.6% | 0.740 | 11.1% | 0.779 | 5.2% | 152% |
RP | Cab Sau | 0.466 | 0.712 | 52.9% | 0.754 | 5.9% | 0.936 | 24.2% | 0.958 | 2.3% | 106% |
SCC | Cab Sau | 0.437 | 0.716 | 63.7% | 0.807 | 12.7% | 0.846 | 4.8% | 0.840 | -0.7% | 92% |
BW | Chard | 0.518 | 0.671 | 29.6% | 0.754 | 12.4% | 0.922 | 22.3% | 0.960 | 4.1% | 85% |
RP | Chard | 0.754 | 0.920 | 21.9% | 1.046 | 13.7% | 1.183 | 13.1% | 1.239 | 4.7% | 64% |
SCC | Chard | 0.595 | 0.748 | 25.7% | 0.883 | 18.0% | 0.910 | 3.1% | 0.983 | 8.0% | 65% |
* Vineyards in Surry Co., NC., within a ~15-mile radius. All clones of Cabernet Sauvignon
were likelydifferent. The Chardonnay clones at RP and SCC (Surry Community College) were likely
the same clone: Dijon clone 96.
Table 4.Percent (%) increase of average berry weight and final berry weight of Cabernet Sauvignon and Chardonnay, in North Carolina.
vine | variety | avg. berry weight - harvest | % overall increase, July 1 to harvest | 50% of avg. berry weight - harvest | 50% weight date | GDD (growing degree days, 50o F) | harvest date |
Black Wolf | Cab Sau | 1.194 | 286% | 0.597 | 07/12/05 | 1514.8 | 10/19/2005 |
Round Peak | Cab Sau | 1.659 | 256% | 0.830 | 07/17/05 | 1680.4 | 10/20/2005 |
SCC | Cab Sau | 1.464 | 235% | 0.732 | 07/09/05 | 1554.6 | 10/10/2005 |
Black Wolf | Chard | 1.682 | 225% | 0.841 | 07/18/05 | 1663.4 | 9/21/2005 |
Round Peak | Chard | 1.855 | 146% | 0.928 | 07/08/05 | 1455.1 | 9/14/2005 |
SCC | Chard | 1.617 | 172% | 0.808 | 07/11/05 | 1606.7 | 9/13/2005 |
* Vineyards were all in Surry Co., NC., within a ~15-mile radius. All clones of Cabernet Sauvignon were likelydifferent. The Chardonnay clones at Round Peak and SCC (Surry Community College) were likelythe same clone (Dijon clone 96)
CROP YIELD ESTIMATION: (adapted from article by Oregon State University’s Steve Price)
In the late 1980’s, Pinot Noir production in Oregon was expanding. Being able to estimate the final crop amount early in the season would be an economic advantage to grape growers and winery owners. The Oregon climate (especially the Willamette Valley) is often cool and cloudy during fruit initiation and bloom. These conditions affect clusters formed per vine, flowers per cluster and berries per cluster and vary from year to year. This variability directly impacts the amount of harvestable crop in a given year.
.
The system uses
1. Bearing vines per block
2. Clusters per vine
3. Cluster weight at the lag phase of berry growth
4. Cluster weight at harvest
This information is used to make 2 crop estimates per year: one at pre-bloom, and the second at pre-veraison.
-- Vines per Block
The vine is the basic vineyard unit. An accurate count of vines per block is needed to use yield/vine or clusters/vine in an crop estimate. Remember that young vines come into production at slightly different times and in older blocks, vines succumb to disease such as crown gall, nematodes, tractor ‘blight’ or possibly phylloxera. Vine counts are simple …in a rectangular block multiply the number of vines/row by the number of rows. In an irregular block, a row by row count is required. After you know the total number of vines, you can count missing or non-productive vines each year and subtract that number from the total to arrive at producing vines per block.
Figure 2.Xarello variety at modified E-L stage 12, with pre-bloom clusters, easily visible. NMSU viticulture, experimental vineyard at Fabian Garcia, Agriculture Science Center, Las Cruces, NM
--Cluster per Vine
Nodes per vine changes yearly and is determined by vine size and pruning severity. Shoots/node is determined by pruning and weather during bud initiation the previous year. Clusters/shoot depends on weather in the year prior to cropping. Clusters are visible and easily counted about 2 weeks before bloom (see Figure 2). Not all vines in a block have to be counted, although all clusters on the selected sample vines should be counted. The number of sample vines per block depends on how uniform the block is. Where vines are the same size and age, and pruned to the same bud count, about 4% need to be sampled. This percentage holds for small blocks up to about 3 acres in size. In non-uniform blocks with variable vine size and vine age the percentage counted needs be increased. A vine count of 30% was required in one trial with non-uniform vines.
--Cluster Weights at Lag Phase
Cluster weights are the most difficult component of the crop estimation process to predict, partly because they are sensitive to the environment right up to harvest. The lag-phase occurs about mid-way between bloom and harvest, when berry growth rate slows temporarily. It is generally associated with lignification or hardening of the seed coat. Flowers per cluster, berries per flower or fruit set and seeds per berry (which greatly affects berry weight) all have been determined by this stage, and their effects can be measured by a sample cluster weight. This sample weight correlates well with final cluster weight. Simply, large clusters at lag-phase = large clusters at harvest and small clusters at
lag-phase, are small at harvest. A random sample of 200-400 clusters/block is usually adequate. This total weight, divided by the number of clusters = average cluster weight. There is no need to weigh individual clusters. Timing of mid lag-phase sampling is more difficult (refer to Tables 3 and 4 above) as it takes some trial and error to determine the mid-point of lag-phase.
Typically, it is accepted that the mid-point occurs about 55 days after first bloom. In the Oregon work, mid-point lag-phase coincided with 75% hard seed tips. This “hard seed tip” determination is done by passing a sharp knife through the grape berry. If the knife pushes the seed, instead of slicing through it, then the berry is said to be at ‘hard tip’ or ‘hard seed’ stage, or mid-lag phase. The Oregon study used Pinot Noir. Chardonnay and Sauvignon Blanc show cluster weight variability due to differences in bud fertility. Merlot and Gewurztraminer have differences in fruit set that impact cluster weight. Obviously, seed hardening test cannot be used with seedless table grapes, and this is where a certain number of days past bloom would be used.
--Cluster Weight at Harvest
Cluster weights at harvest are a key part of yield prediction, and it is crucial that cluster weights be determined each year to provide a record for crop estimation in future years. The long-term record establishes the between cluster size variation and yield variation. The information can be used with early season predictions and with lag-phase sample to predict this year’s harvest cluster weight. To record cluster weights at harvest, it Is easier to sample out of harvest bins as fruit is received at the crush pad. However, a sample randomlycollected directly from the vines correlates better with the lag-phase sample, and is likely closer to the true cluster weight than is a bin sample. In the bin, it is difficult to tell partial from complete clusters. Only about 200-400 clusters collected directly from the vines are needed to determine an accurate representation of average harvest cluster weight.
--Predicting Yield
The grower will need the following information: vines/block, clusters/vine, lag-phase cluster weight, and harvest cluster weight to generate an effective yield prediction. The first prediction uses vine counts and estimates of clusters/vine, just before bloom. Vines/block are multiplied by cluster/vine to estimate clusters/block. Then harvest cluster weight is multiplied by clusters/block for an initial estimate of tons/block.
This first estimate can be revised after the lag-phase cluster sample is taken (~ 55 days after full bloom …or mid lag phase as you determine it). This sample is compared to average lag-phase cluster weight to estimate harvest cluster weight. For example, if the lag-phase sample is 20% larger then the average lag-phase cluster weight, the harvest clusters are estimated to be 20% larger than average harvest cluster weight. This revised cluster weight is then multiplied by clusters /block for an estimate of tons/block. These two predictions can be put in formula form as shown below:
First prediction: V x C x H – estimated yield
V = vines/block, C = cluster/vine, H = average cluster harvest weight
Second prediction: V x C (S/A x H) = estimated yield
where S = lag phase cluster weight, A = average lag phase cluster weight
This formula assumes that cluster weight will always increase the from lag-phase to harvest. This won’t always happen…take a look at what we found in North Carolina when we tracked berry weight development over the course of the season in Chardonnay and Cabernet Sauvignon (Tables 3 and 4).
Final cluster weight is influenced by environmental effects on berry size. During the final part of the season growers should be aware and adjust estimate accordingly. A hot dry season will reduce berry sizes, so crop estimates should be reduced. Conversely, late rains can increase berry size and increase crop estimates. Bird damage, heat and sunburn that shrivel berries, botrytis and animal predation or disease can affect cluster size and yield. So, no estimate is final …until the crop is in.
.
--Conclusion
Growers need a record of harvest cluster weights and lag-phase cluster weights in order to apply these formulas and generate a crop estimate. The better the data (records)… the better the estimate. Accuracy depends on long-term records and how weather/climate and your practices affect yield variation. The systems summarized above worked well with Pinot noir in the Willamette Valley of Oregon and with certain varieties in Virginia.
Accurate crop estimation is still very much a work in progress in New Mexico, but the concepts and the procedures explained here are sound. Overall, crop estimation is meant to reduce confusion and streamline processes and work in the vineyard and winery. This season…I will be collecting average cluster weights from cooperating growers. I plan to share the results at the annual NM Wine Educational Conference tentatively scheduled to be held at Santa Fe in 2021. If you would like to work with me to develop records of harvest cluster weights in 2020, please contact me:
Gill Giese, Extension Viticulture Specialist
ggiese@ nmsu.edu
505 865-7340
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