Crop Estimation in Vineyards
Methods of Crop Estimation
Grape growers rely on a number of different systems for estimating wine grape yield including the prebloom method and lag-phase method. The pre-bloom method is based on historical records of cluster weights at harvest. The lag-phase method is based on cluster weights during the growing season, when berry growth slows momentarily (around 50-60 days after bloom).
Pre-Bloom Method
The pre-bloom method is used to provide a crop estimate early in the season. The pre-bloom method utilizes the number of vines per acre, the average number of clusters per vine before bloom or as late as pre-véraison, and the average historical cluster weight at harvest to predict the yield in tons per acre for the current season. The following formula can be used to estimate crop with reasonable accuracy:
Number of Producing Vines per Acre
The number of vines per acre is determined by row and vine spacing. For example, a vineyard with an 8 x 9 feet (vine x row) spacing will have 605 vines per acre.
Average Number of Clusters per Vine
This number varies with the level of canopy management each year. For example, increasing the severity of pruning, shoot thinning, or cluster thinning would decrease the average number of clusters per vine.
Average Historical Cluster Weight at Harvest
Cluster weight is the component of yield that varies the most from year to year and from vineyard to vineyard. Cluster weights are affected by environmental conditions. For example, a spring frost kill of primary buds will reduce both cluster number per vine and the average weight of those clusters.
Lag-Phase Method
Some wine grape growers use the “lag-phase” method for determining crop estimates, especially Pinot Noir grape growers in Oregon. The grape berry has three basic growth phases during the season. The berry grows very rapidly during the first and third periods but grows very little during the middle one, which is called the lag-phase. This method is based on average cluster weight at lag phase multiplied by an “increase factor” as a predictor of final cluster weight. For crop estimation, mean cluster weight at the lag-phase is multiplied by a factor of two. In other words, this method is based on the premise that cluster weights will double from the lag phase to harvest since, at lag phase, the berry has attained approximately 50 percent of its final weight.
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