Yield: Typically, the amount (weight) of fruit over a given spatial area, such as tons of grapes per acre. In some cases, yield refers to the amount of fruit on an individual vine (see Crop load). Usually a generalized target number; it may or may not be related to the vineyard’s ability to produce fruit. As a generalized number, it is often related to contract specifications, not vine physiology.
Crop load: The ratio of reproductive (clusters) to vegetative (exposed, photosynthetically active leaves) development. Understanding crop load ratios allows the grower to determine the optimal amount of fruit that a given vine can ripen. Vegetative growth can either be directly measured by using the leaf area of a vine during the growing season, or indirectly measured using dormant pruning weights. Measuring dormant pruning weights is the most practical means of determining vegetative growth in commercial production. A common name for the ratio of fruit yield per vine to subsequent dormant pruning weight per vine is the Ravaz index (see Dry et al. 2004). Crop load is independent of contract specifications.
Vine balance: The point where crop load is ideally matched with vine growth. Achieving the correct crop load ratio for a balanced vine can optimize the quality of the fruit and lead to consistent production (Skinkis and Vance 2013). Commonly measured as the ratio of fruit weight to pruning weight, the exact number can vary between cultivar, location, training system, management practices, and overall climatic conditions. If using a Ravaz index comparison, every 5–10 lb. of fruit weight (per vine) should have approximately 1 lb. of pruning weight (per vine).
Under-cropped: Used to describe a vine that is carrying less fruit than what it can theoretically ripen. A Ravaz index less than 3 indicates under-cropping in warm environments such as eastern Washington, but might be appropriate in cooler climates such as western Washington.
Over-cropped: Used to describe a vine that is carrying more fruit than what it can theoretically ripen. A Ravaz index greater than 10 typically indicates over-cropping. The standard “appropriate” range falls between an index score of 5 and 10.
Crop load adjustment: Practices that either enhance the yield per vine (such as increased water or nitrogen application from bloom to véraison) or decrease the yield per vine (such as reduction in water after fruit set, or actively removing fruit prior to harvest). Crop load adjustments are done to either help balance vine growth by leaving the appropriate amount of fruit relative to the canopy size, or meet contract specifications from the winery.
Estimating yield is a vital component of commercial grape production. Necessary for many vineyard and winery operational decisions, yield estimation can be a challenging process. There are multiple ways and times to estimate yield. This guide will highlight why yield estimation is important and provide an overview of the various estimation methods (Table 1) currently being used in commercial grape production for both juice and wine.
Yield Estimation Purpose
Yield estimation helps vineyard managers balance their vines and winery managers improve harvest scheduling and space utilization.
Vital to decisions relating to crop load adjustments, yield predictions serve as a base for decisions relating to yield management. The amount of fruit in a vineyard, which is the most common form of yield presentation (that is, tons per acre) is often dictated by contract specifications. However, the amount of fruit that a vine can ripen is dictated by vine health, site capability, weather, and cropping history. Taking both business and viticultural factors into consideration, the appropriate crop load for the same site can vary from year to year. Typically, areas with soils that have high water-holding capacity (such as clay or silt) and organic matter content (>2%, which is associated with nitrogen), along with optimal growing season length (more than 180 frost-free days) and warm average temperatures (daytime highs between 75° and 95°F; 24° and 35°C), result in a greater potential for a large crop load. Vines grown in areas under conditions such as water