The price of exported fruit from Chile is a function of fruit size and shipment date with larger and earlier shipped fruit receiving the highest price per kilogram at the farm gate. For processing peaches, fruit size is not the most important determinant of price, because all fruit greater than 55 mm in diameter receive the same price. To improve fruit size, hand thinning is normally done in most peach and nectarine orchards. It is widely recognized that fruit size is largely determined by crop load, with larger fruit size obtained as the crop load is reduced. However, reducing crop load also reduces total yield (Blanco et al., 1995; Inglese et al., 2002; Johnson and Handley, 1989; Ojer et al., 1996, 2001; Reginato and Camus, 1993; Rowe and Johnson, 1992). Although larger fruit size results in higher prices, the true economic effect of thinning on fruit grower returns must take into account the reduction in yield that is required to obtain large fruit size. The optimum economic crop load is a function of fruit size, total yield, and price. However, because different cultivars also have different yield and fruit size potentials (Inglese et al., 2002; Johnson and Handley, 1989), additional factors such as harvest date are also important in determining the farm gate crop value. Attempts to calculate optimum orchard crop load to maximize crop value by computer models have incorporated crop load and other additional factors with kiwi (Atkins, 1990) or peaches (Johnson and Rasmussen, 1990). The kiwi report considered crop load expressed as fruits per meter squared of canopy surface area, whereas the peach report used fruits per tree. Also, crop value has been determined for economic evaluation of thinning practices (Byers et al., 2005; Marini, 2000; Stover et al., 2001).
Several approaches have been used to measure crop load, including fruit number normalized by tree size based on trunk cross-sectional area (TCA; cm2), tree size based on canopy volume (m3), canopy leaf area (m2), fraction of light energy intercepted, or weight of wood pruned off (kg/tree) with TCA being the most widely used normalizing unit as was proposed by Lombard et al. (1988). On the other hand, light interception of between 70% and 80% has been suggested as a target for orchard canopies to obtain desired yield under field conditions. This is because orchard yield is largely explained by photosynthetically active radiation (PAR) interception (Robinson and Lakso, 1991) independent of training system (Iannini et al., 2000; Nuzzo et al., 2002) and is linearly related to it up to 50%; after this light interception level, yield varies because other factors become limiting (Wünsche and Lakso, 2000).
The objective of this project was to predict the economic performance of nectarines and cling peaches of different harvest dates based on a standardized measure of crop load that is based on a more fundamental input variable (light intercepted by the tree) rather than trunk cross-sectional area. This information could provide basic data that could be later integrated in computer models as an aid to growers and researchers in objectively comparing management practices to improve orchard performance, make better thinning decisions, and aid in orchard design.
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