Yield Estimation
Knowing how much crop is hanging on the vine is one of the more important items in the yearly production of wine and juice grapes, yet it's a bit like looking into a crystal ball. There are a number of reasons why you should do yield estimation. These include:
• Scheduling deliveries to the winery/processor
• Allowing the winery/processor to make room for the harvest
• Make adjustments per contract agreements
• Allows buyer to make the necessary payment schedule
In a normal situation, grapevine yield can be variable year to year, thus yield estimation is essential to help make decisions about management. Yearly variation is not due to biennial bearing, as with apples and some other tree fruit crops, but to other weather-related events or physiological imbalances. Nutrient deficiencies, hail, wet, cool conditions can all lead to a reduction in fruit set.
Sampling Techniques
Whenever you go out to sample – there are a couple of rules to follow. Be consistent. It's important to make sure that the same person or group of people makes the necessary measurements from year to year, if possible. It's also important to conduct random sampling, so that one portion of the vineyard is not favored over another. This is especially important if your vineyard is non-uniform in soil, nutrition, or growth, of which very few vineyards that I know are uniform.
Sampling size is another question that I often get from newer growers – how many do you sample to get an accurate number? There are a number of ways to determine how many vines should be sampled, but in statistics, it comes down to sample error. How much error can you tolerate? 5%? 10%? 25%? Most scientific studies only tolerate a 5% error rate, so if you have about 800 vines in an acre, you should sample 260 vines. Now, that isn't possible for most people, unless you have a lot of time (or family members to help!) So, the decision must be made to work out how many vines can be sampled in a reasonable amount of time, with the monetary resources that you have budgeted for this particular task in the vineyard. If you are curious about calculating sample size, I found a calculator online: Sample Size Calculator
Collecting Data
There are a lot of data that must be collected before you sit down to calculate your yield estimation. These are:
• Vines per acre/block - (don't forget to subtract missing vines!)
• Row spacing
• Vine spacing
• Planting density = vine spacing x row spacing
• Total area per acre = 43,560 ft2
• Number of vines per acre = 43560/planting density
• Acres in a block or field
• Average clusters per vine
• Average berry weight
• Average berry diameter
• Average cluster weights
• Multiplier (= about 2 at lag phase)
• Target yield in tons/acre or lbs/vine
• Number of clusters per vine
• Small vineyard = ~25%
• Large vineyard = ~2-5%
When calculating yield estimates, you may or may not need every single bit of information here that is listed. There are several ways to estimate yield in the vineyard, which uses all or a portion of the information that is listed above.
Yield Estimation Techniques
Historical Cluster Weight (HCW)
This technique is used early in the season, when you have a database of cluster weights from previous years. It does not account for any changes in fruit set that may occur due to adverse weather conditions, overspray of herbicides, or burning that can occur with certain fungicides. Since this is any early season estimate, an idea of the tonnage can be had before bloom. In order to use this method, use the following equation to get tons/acre:
Lag Phase Estimates (LPE)
Many growers use the weight of clusters at lag phase to estimate their tonnage, which uses a multiplier to find the final tonnage. The multiplier number will be dependent upon the time of the season that you are collecting the data (i.e., the earlier you are before lag phase the multiplier would be >2, while later in the season after lag phase it would be <2). If you are sampling at lag phase, then use 2 as a multiplier, as research has shown that approximately half of the crop weight is present on the vine at lag phase. When using this method, be sure to wait until shatter has passed to estimate cluster weights, with the optimum time at lag phase. Use the following equation to get tons/acre:
Average Berry Number (ABN)
This method depends upon the average number of berries in a cluster. In order to use this method, data should be collected for berry number and berry weight over a number of years. Be sure to sample each block of the vineyard, rather than each variety, as vineyard blocks can vary significantly from one end of the vineyard to the other. As with the other methods, determine your sampling size based upon the sampling error that can be tolerated and time available. It is best to count all of the berries on the cluster and subsequently weigh each one to determine berry number and berry weight.
Conclusions
Overestimation or underestimation of tonnage in the vineyard can lead to discontent between vineyard managers and winemakers who have to deal with the logistics of harvest. There are a number of different yield estimation methods, but the most important point in all methods is to be consistent, in sampling technique and method of estimation. Good communication between the winemaker and viticulturist can only lead to one thing – high quality grapes which will be a good start to premium wine production.