Pecan trees benefit from thinning technique

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Photo courtesy of Dr. M. Lenny Wells

Pecan trees, like many fruit trees, have a tendency to bear fruit in cycles, producing a large crop in one or two years, followed by one or two years with little or no crop. This cycle, called "alternate bearing", is the most profit-limiting biological problem facing pecan producers; the inconsistent production pattern creates supply and marketing challenges that can have severe negative effects on the pecan industry. Producers have experienced success using mechanical fruit thinning as a way to minimize the effects of alternate bearing on several pecan cultivars. While the protocol for mechanical crop thinning has been established for some pecan cultivars grown in Oklahoma and Kansas, little research has been conducted regarding the economic value and potential profitability of fruit thinning of cultivars found in the southeastern United States. To address this issue, researchers at the University of Georgia's Department of Horticulture studied the effects of mechanical fruit thinning on pecan yield, nut quality, and profitability using 'Sumner' and 'Cape Fear' pecan trees, two important cultivars prevalent in areas of the southeastern U.S. The research study was published in a recent issue of HortTechnology.

Ten 20-year-old trees of both 'Sumner' and 'Cape Fear' were used for the study. Trees grown in Tifton loamy sand soil in a commercial pecan orchard in Irwin County, Georgia, were used for the study. The trees were spaced 40 x 40 feet and were drip-irrigated. Treatments were replicated five times in a randomized complete-block design. Five trees of each cultivar were mechanically thinned using a tree shaker with a hydraulic shaker head—a process called trunk shaking—to remove 30% to 40% of the fruit on each tree, and five trees were not thinned.

Crop load was estimated by counting 50 random terminal branches on each side of the tree at midcanopy level for each tree and dividing the number of fruiting terminals by the number of terminals counted. At harvest, yield was estimated by shaking trees and measuring the weight of nuts in a wedge-shaped grid consisting of 1/100 of the area beneath the tree.

According to the study, yields from thinned and nonthinned 'Sumner' trees were almost identical in 2007, the year of thinning. According to Dr. M. Lenny Wells, lead author of the study, this was probably the result of an increased incidence of water-stage fruit-split on the nonthinned trees. Wells noted; "Because it occurred after flower induction, the apparent loss of fruit from water-stage fruit split did not affect the return crop of 'Sumner' in 2008. Visual observations indicate that water-stage fruit split was less severe on mechanically thinned trees, and there did not appear to be a proportional yield reduction resulting from water-stage fruit split on thinned trees compared with nonthinned trees. Thus, it appears likely that the amount of nuts removed during mechanical thinning was not in addition to those lost to water-stage fruit split later in the year."

The OFF year return crop and return crop value of 'Cape Fear' and 'Sumner' was increased by mechanical thinning in the ON year, thus enhancing the total 2-year value and 2-year average value of both cultivars. The researchers concluded that increased profitability of 'Cape Fear' and 'Sumner' pecans using mechanical fruit thinning results primarily from higher yields and prices in the OFF year of production, which offset any loss in yield and/or crop value generated by fruit thinning in the ON year. They noted that "mechanical fruit thinning is a highly valuable practice, leading to increased profit potential for 'Cape Fear' and 'Sumner' pecan."

Source: American Society for Horticultural Science