Abscission Agent Application and Canopy Shaker Frequency Effects on Mechanical Harvest Efficiency of Sweet Orange

This study was conducted to determine the relationship of 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP) concentration and canopy shaker frequency on fruit detachment force, pre-harvest fruit drop, and mechanical harvesting fruit removal of 'Hamlin' and 'Valencia' sweet orange cultivars. CMNP was applied at 0, 200, and 300 mg.L-1 in a carrier volume of 2806 L-ha(-1). Four days after CMNP application, fruit were harvested with a canopy shaker that was operated at 3.0, 3.7, and 4.3 Hz at a tractor speed of 1.6 km.h(-1). The experiment was repeated 3x for 'Hamlin' (December, early January, and late January) and twice for 'Valencia' (March and April) during the 2008 2009 harvest season. Fruit detachment force was reduced by at least 50% for all CMNP-treated trees compared with the untreated controls at the time of harvest and was lower for 300 mg.L-1 than 200 mg.L-1 on three of the five dates tested. Pre-harvest fruit drop evaluated immediately before mechanical harvesting was higher for all CMNP-treated 'Hamlin' than untreated controls at all harvest dates, whereas 300 mg.L-1 application resulted in higher pre-harvest fruit drop in 'Valencia' when compared with 200 mg.L-1 or the untreated controls on both application dates. CMNP-induced fruit drop was higher in 'Hamlin' than 'Valencia'. CMNP had a greater effect on fruit removal at lower canopy shaker frequencies. The interaction of total fruit weight removed was not significant on any date as a result of variability among trees in the study. These data indicate that the amount of loosening by CMNP was concentration-dependent and facilitated removal, especially with lower canopy shaker frequencies. Development of viable commercial practices should use the percent of the total crop harvested and not the actual weight of fruit removed in determining efficacy of CMNP and harvest efficiency of the mechanical harvesters.