Crop Estimation In Vineyards
July 1, 2009 
One of the most difficult tasks for any grape grower during the growing season is trying to estimate their final crop yields. While not easy to do, it is an important task to undertake for a couple of reasons. First, it will help the winery that will be using the fruit, whether it’s the grower’s own or somebody else’s, to plan for harvest. Secondly, estimating a crop can determine whether or not the vines are overcropped, and if and how much crop thinning needs to be done to bring the vines into balance.
Most growers probably rely on a vineyard’s historical yields as the basis of their crop estimation. If the vines look like they are carrying more or less crop than usual, then the grower makes a mental adjustment and that’s about the end of it. In some years, this best guess at yield will be fairly accurate. But by collecting just a little bit of information from the vineyard during the growing season, grape growers can greatly improve their chances of predicting their final yields with better accuracy than the eyeball method.
Yield Components
There are many factors that influence the final yield at harvest for grapes (see “Yield Factors” chart). These variables are called the yield components, and they each can influence the tonnage that is harvested to various degrees. Fortunately, all of these yield components can be captured in three simple measurements that are easy to take:
• Vines per acre. This is the easiest of the three pieces of information to collect. If the number is not known already, simply multiply the distance between the vines by the distance between the rows, then divide 43,560 (number of square feet in an acre) by that number to get the number of vines per acre.
• Clusters per vine. It is easier to do this earlier in the season when shoots are at about 10 to 12 inches long and the clusters are easier to see. Another option is to do the counts after fruit set, which allows for a basic assessment of berry set (high, normal, poor) at the same time, but it can be done anytime during the season. Count the clusters on random vines in the block, making sure to account for the level of variability that is present, and avoiding vines on the perimeter of the vineyard. If there is high variability in the vineyard, more vines should be counted in order to have a more representative sample. Calculate the average number of clusters per vine based on these counts.
• Average cluster weight at harvest. While there are some published values for this information based on research trials, estimates will be much more accurate using site-specific data. This will require growers to collect this data each harvest for several years in order to develop average cluster weight information for their own vineyards. Separate averages should be developed for each variety/rootstock combination and blocks with significantly different soil types.
Harvest all of the clusters from randomly selected vines, counting them as they are picked, into picking bins or some other container. Weigh the bin (remember to subtract the weight of the empty bin) and divide by the number of clusters to get the average. This data will need to be recorded over several years in order to develop a fairly reliable average.
Crop estimation in grapes is never an exact science. Even with lots of good data, it is difficult to estimate the exact size of a crop prior to harvest. Many factors can influence the final harvest yield after the data is gathered, such as predation, disease, or weather. Experienced growers are generally satisfied with their estimates if the difference is less than about 15% to 20%, but in some years, even this level of precision is difficult to achieve. But with practice and good site-specific data, growers can improve the accuracy of their crop estimates over time.
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