By Jacy Lewis (June 2017)
Decisions made in the spring can dramatically impact the crop you harvest months down the road. The ability to accurately estimate the potential yield of a block enables you to determine how much fruit you will likely have at harvest and plan accordingly. Most importantly, it can help you assess the ability of your vines to ripen the crop they are set to produce. This is the only opportunity to reduce crop load in a way that will change the quality of the fruit you harvest and potentially reduce the stress that a large crop can place on young or weak vines. The decision to reduce yield by crop thinning can be a complex one, taking into account a variety of factors.
How to make the decision to thin is beyond the scope of this article. Here we will detail the basics of how to estimate yield, the first component of how you will manage your crop yield for the rest of the season. Additional more time-consuming techniques can be used to get higher precision in your estimates that are also beyond the scope of this article.
Estimating next year’s crop starts at harvest this year. Estimating yield in a given season requires that you have available historical records of cluster weights at harvest. Because cluster weights vary year to year, it is important that you collect data yearly in order to eventually obtain a reliable average that you can work with in subsequent years. You will find that this is likely the single largest variable in your equation, so good record keeping here is essential. In young blocks, you may rely on averages from other blocks of the same variety that you have collected data on in the past, but there is no substitute for collecting data from each variety and block to get the best prediction of average expected cluster weight.
When collecting average cluster weights, it is tempting to pull clusters from a harvest bin for weighing. There are a number of problems with this practice. First, it is too easy to introduce sampling bias when one is forced to look at a bin and choose clusters. There is the tendency to choose either large or small clusters inadvertently. Additionally, at this point clusters may have become damaged and/or have lost berries. A better method is to randomly select vines from your vineyard map then hand harvest these individual vines. One caveat here is that it is best to avoid using vines on the edge of your vineyard as they are commonly not representative of the vineyard as a whole. So choose from vines not at the end of a row or from the outside rows of the block. Count the total number of clusters per vine. Weigh all of the clusters together, then divide that number by the total number of clusters. This will give you the average cluster weight for that vine. You can then average the (mean cluster weight) for all of the sampled vines. This will give you the average cluster weight for that vineyard block.
How many vines need to be sampled depends on the number and uniformity of vines in the block. In a block with little variability, a sample size representing 1%-2% of the total number of vines may be sufficient. In an established block that has a mix of non-bearing and bearing vines, or vines of multiple ages and skips, it is necessary to increase this number. Any increase in the number of vines sampled will increase the accuracy of your predictions.
When spring yield prediction is done, you must determine the number of bearing vines per block as well as the average number of clusters per vine. Be sure to make adjustments to your estimates by assessing the vineyard for vines that may have been lost in the previous year, and tracking the number of replants, or retrained vines that will not be expected to bear in the current growing season. It is important that this count be done accurately, as even a small over or under estimate of the number of bearing vines can result in a large decrease in the accuracy of yield. The smaller the block, the greater the importance of the accuracy of this number.
Estimating clusters per vine is done in a similar fashion to estimating cluster weight at harvest. A minimum of 1%-2% of the vines in the block, provided it is uniform, should be selected randomly and the number of clusters on each vine carefully counted. You must be sure you wait until developing clusters are all exposed. Waiting until berry set will help ensure better accuracy if set is low.
Finally, the equation for estimating crop yield is as follows:
Yield in tons/block= 1 / 2,000lbs X Vines/block X Clusters/vine X Average historical cluster weight