Of the 193,000 ha of sweet oranges [Citrus sinensis (L.) Osbeck] grown commercially for juice in Florida (Anonymous, 2008), 12,153 ha or ≈7% are mechanically harvested (Florida Department of Citrus, 2008). The most common mechanical harvesting systems use a canopy shaker composed of a vertical axis with 12 sets of free-floating tines 2 m long that radiate out from the vertical axis (Ebel et al., 2009; Whitney and Hedden, 1973). The 12 sets of tines span 3.5 m from the lowest to the highest set along its vertical axis. Where the tines connect to the vertical axis, a mechanism vibrates each set of tines independently at frequencies between 3 and 5 Hz during harvest, yet each set of tines floats freely around the axis so that as the harvester moves along a row, the tines swivel through the canopy. The vertical axis and tines can be manually moved into the tree canopy to a depth determined by the operator. Fruit are “harvested” by the vibrating mechanism causing the tines to impact fruit directly or by impacting fruit-bearing branches. The percent of the total crop removed at any given time during the harvest period is a function of depth of placement of the shaker within the canopy, the frequency of shaking of each set of tines, tractor speed, and fruit detachment force (Whitney, 1997).
There are two kinds of canopy shakers currently used in the commercial citrus industry in Florida. “Pull-behind” canopy shakers are pulled behind a tractor and drop fruit to the ground (Whitney, 1997), which are picked up by hand laborers or pick-up machines (Bora et al., 2006; Hedden et al., 1983; Whitney, 1999). The self-propelled canopy shakers contain decks located horizontally under the shaker that can be moved perpendicular to the long axis of the machine and positioned under the canopy to catch fruit that drop during shaking. Self-propelled canopy shakers work in pairs on opposite sides of a tree row and position the decks under the tree until they meet. The two units move together down the tree row shaking both sides of each tree until the decks are full, at which point they stop and unload the fruit through a conveyor into the bed of a truck that remove the fruit from the grove.
There are no abscission agents that are currently registered for use on sweet oranges that would aid mechanical harvesting. An effective abscission agent would enhance predictability and removal of sweet oranges, which is currently a limitation of mechanical harvesters (Whitney, 1975, 2000, 2003; Whitney et al., 2000a, 2000b, 2001). An abscission agent would be especially useful during late-season harvesting of ‘Valencia’, in which mechanical harvesting has to be ended early as a result of excessive removal of the newly developing immature fruit (Burns et al., 2006b). Furthermore, an abscission agent would allow lower harvester settings that would presumably reduce canopy injury (Buker et al., 2004; Li and Syvertsen, 2004; Whitney, 2003). Canopy injury has led commercial growers to express concerns about long-term productivity, although research has shown that properly managed trees show no long-term reductions in yield or tree mortality (Hedden and Coppock, 1968; Li and Syvertsen, 2005; Li et al., 2006; Whitney et al., 1986; Yuan et al., 2005).
A commercial label for the abscission agent 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP) is being actively pursued by the commercial citrus industry in Florida. CMNP has been shown to increase fruit removal and reduce the force necessary to remove the fruit (Burns et al., 2005; Ebel et al., 2009a, 2009b; Freeman and Sarooshi, 1976; Koo et al., 1999; Whitney, 1975, 1976; Whitney et al., 2000a, 2000b; Wilson, 1973). CMNP is currently being studied to understand factors that affect efficacy. Research has shown that CMNP efficacy is largely a function of concentration, coverage, post-spray precipitation, and air temperature (Alferez et al., 2005; BenSalem et al., 2001; Burns et al., 2006a; Ebel and Burns, 2008; Farooq et al., 2003; Kender and Hartmond, 1999; Koo et al., 1999, 2000; Salyani et al., 2002). CMNP efficacy is especially sensitive to temperature, with 15.6 °C considered a critical minimum (Ebel and Burns, 2008; Yuan and Burns, 2004).
This study was conducted to determine the relationship of CMNP concentration and cycling frequency of canopy shakers on harvest efficiency of ‘Hamlin’ and ‘Valencia’ sweet orange. The study was conducted on multiple dates for each cultivar but with climate conditions and CMNP application and canopy shaker frequency treatments standardized.
Alferez, F., Shila, S., Umback, A.L., Hockema, B. & Burns, J.K. 2005 Citrus abscission and Arabidopsis plant decline in response to 5-chloro-3-methyl-4-nitro-1H-pyrazole are mediated by lipid signaling Plant Cell Environ. 28 1436 1449
Anonymous 2008 Citrus summary, 2006–2007 Florida Agricultural Statistics Service, Fla. Dept. Agric. Consumer Services Tallahassee, FL
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Buker, R.S., Syvertsen, J.P., Burns, J.K., Roka, F.M., Miller, W.M., Salyani, M. & Brown, G.K. 2004 Mechanical harvesting and tree health Electronic Data Information Source, Institute of Food and Agricultural Sciences, University of Florida. Publication #HS961
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