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Ed Stover, Ferdinand Wirth, and Terence Robinson

Analysis of apple (Malus×domestica Borkh.) and citrus thinning experiments indicates that the relationships between cropload, fruit size, and total yield can be used to assess optimal cropload for highest crop value. Mean fruit size increased and total yield declined as the cropload (number of fruit/cm2 trunk cross-sectional area) was reduced through the use of chemical thinners. Because crop value is influenced by fruit size and total yield, intermediate croploads gave the highest economic returns in all experiments evaluated. For `Empire' apple, croploads greater than those expected to provide good return bloom often produced the highest crop value for a single year. In citrus, optimal crop values resulted from a broad range of intermediate croploads. A method is described to analyze optimum cropload from thinning experiments.

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Kathy Davis, Ed Stover, and Ferdinand Wirth

Hundreds of fruit thinning experiments have been reported for various fruits including apple (Malus × domestica) and citrus (Citrus spp.). Unfortunately, very few of these reports attempt to evaluate the economic implications of thinning. Researchers routinely report significant cropload reduction accompanied by an increase in fruit size. Although these are crucial responses to thinning, they are not always associated with an increase in crop value, which is the commercial justification for thinning. The few economic studies summarized in this review illustrate that the economic effects of fruit thinning vary widely, and successful thinning often reduces returns to the grower, at least in the year of treatment. It is important to quantify the economic benefits of thinning and identify croploads that balance the trade-off between yield and fruit size to provide optimal crop value. Future thinning research should report total yields and fruit size distributions to permit economic assessments and comparisons of treatments.

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Ed Stover, James Salvatore, and Ferdinand Wirth

Sensor-actuated precision spray systems are designed to prevent pesticide delivery unless canopy is detected in the corresponding spray zone. Where frequent gaps are present in the tree row, using orchard sprayers with these systems is likely to lower pesticide costs and reduce off-target deposition. Pesticide savings from use of a sensor-actuated precision spray system were assessed in 27 grapefruit (Citrus paradisi) blocks selected without prior knowledge of grove characteristics, with nine blocks in each of three age categories: 5-6 years, 10 to 12 years and 20 years and older. The sprayer was optimized for each block by opening only those nozzles appropriate for tree size and furrow depth, so that no spray was delivered under or over the canopy of most trees. The same randomly selected 3.0 to 4.7 acre (1.2 to 1.9 ha) section was then sprayed in each block both with and without activation of the precision spray system. In each block, the precision spray system computer also calculated spray savings based on precision sprayer use with no operator nozzle adjustment. Mean savings in spray material from use of the precision sprayer was 6.6% of total conventional output when comparisons were made with optimal sprayer nozzling in each grove versus 18.6% with no operator adjustment of nozzles. In this study, optimizing nozzling provided a larger proportion of spray savings than use of the precision sprayer on 100% of groves 5 to 12 years old and 44% of groves greater than 20 years old. However, in 70% of groves tested, precision spray systems increased spray savings by more than 2% even when using optimal nozzling. Assignment of precision sprayers to groves with greatest potential for savings will likely provide greatest efficiency, while uniform groves forming hedgerow will offer so little potential savings that even the additional cost of weed management will probably not be recovered.