Chapter 30

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).

Harvest Cluster Weight Method

The harvest cluster weight estimate is used to provide a crop estimate early in the season. The harvest cluster weight method utilizes the number of vines per acre, the average number of clusters per vine, and the average historical cluster weight at harvest to predict the yield in tons per acre for the current season.

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. 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 2.

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