History of Mechanical Citrus Harvesting
1990 to 2008
Abscission Research and Tree Health Studies
The Citrus Harvesting Research Advisory Council funded research on abscission chemicals to loosen oranges or reduce their fruit detachment force. Prosulfuron (Peak, Novartis) was tested in 1997-98 in Valencia oranges using trunk shakers for fruit removal (8-1). The chemical was phytotoxic to young Valencia fruit when applied before late May, reducing subsequent mature fruit yields, & was minimally effective in reducing mature fruit detachment force & increasing trunk shaker mature fruit removals. In the 1996-97 & 1997-98 seasons, Whitney et al., 2000 (8-2) showed that prosulfuron & metsulfuron methyl (Ally, DuPont Co., Wilmington, DE) loosened Hamlin oranges & significantly increased total fruit removals (which included preharvest fruit drop) by trunk shaker, but were phytotoxic to the trees. In Valencia oranges, metsulfuron methyl & CMN-pyrazole (Release, Abbott Laboratories, Chicago, IL) both generally provided mature fruit loosening & increased total trunk shaker fruit removal, but metsulfuron methyl was phytotoxic to the trees.
Because of grower concerns about long term tree damage, health, & yield of trunk shakers & abscission chemicals, a field study was initiated in the 1997-98 season to apply the chemical & shake the same Hamlin, Pineapple, & Valencia (latest Valencia harvest was usually mid-May) orange trees for 5 consecutive years (8-3). Abscission chemicals prosulfuron & metsulfuron methyl were phytotoxic to the trees & reduced yields whereas CMN-P was not phytotoxic, did not reduce yields, & increased total trunk shaker fruit removal by an average of 9%. Tree trunk growth was not affected by trunk shakers &/or abscission chemicals. Trunk shakers did not reduce yields compared to handpick checks. In addition, later physiological studies (8-4, 8-5, 8-6) confirmed that similar trunk shakers had minimal or no effect on tree health & productivity. In other field studies (8-7, 8-8) with trunk shakers, CMN-P (the most promising abscission chemical on which registration is still being pursued in 2008) was applied at a constant active ingredient quantity per acre in a range of spray volumes, 25 to 450 gallons/acre. It was demonstrated that even though chemical deposition quantities were lower at higher spray volumes, the efficacy of CMN-P was increased - lower fruit detachment forces, preharvest fruit drop & trunk shaker fruit removal increased – as spray volume increased. This somewhat contradictory result might be explained in that higher volumes prolong fruit chemical wetness & increase the fruit stem area exposure to the chemical. Tests (unpublished data) with dipping oranges in a CMN-P spray solution have shown that chemical deposition on the stem end surface half of the fruit results in greater loosening than on the blossom end surface half.
Growers were also concerned about yield losses from tree skirting (removing tree limbs to a ~ 36 inches height at the dripline & maintaining this height) required to allow the shake catch systems to operate efficiently. In two 5 year field studies, Whitney et al. 2003 (8-9, 8-10) showed that yield was reduced the first year, particularly in small trees, but in subsequent years the yields were not reduced so that the 5 year yield average was not reduced compared to unskirted trees.
As with other tree fruit & nut crops, commercial citrus trunk shakers used multidirectional shaking to remove oranges from trees. However, bark slippage/damage was a problem, particularly in Valencia. A linear shaking pattern (perpendicular to clamps pads) offered potential for reducing bark slippage but its effect on fruit removal was unknown. Whitney et al., 2001 (8-11) showed that the linear shake pattern removed 1 to 6% more oranges than did the multidirectional shake pattern. Most commercial trunk shaker manufacturers began using the linear pattern & found that wear & heat buildup in shaker clamp pads were also significantly reduced.
It has long been known that one of the greatest impediments to acceptance of mechanical harvesters using shakers is the inability to harvest Valencia late in the season without significant reductions in future yields. Research has verified this numerous times with various types of shakers since the 1960s, & has usually attributed the reductions to removing too many immature fruit (next year’s crop) greater than 1-2 cm in diameter while removing the mature fruit. Most recently with trunk & canopy shakers currently in use, Roka et al. 2005 (8-12) showed that yield reductions ranged from 20 to 50% the first season after shaking when compared to conventional hand harvesting. Thus the interest in abscission chemicals to loosen the mature Valencia enough to reduce the shaking aggressiveness required & minimize the loss of immature fruit. Burns et al. 2006 (8-13) has reported that low frequency trunk shaking & mature Valencia loosening with abscission agent CMNP minimized the effect on yield the first season after shaking when compared to conventional hand harvesting. Registration of CMNP is currently being pursued & an Experimental Use Permit is anticipated in 2010. However, its feasibility to extend the late season Valencia harvesting still needs to be demonstrated.
Referenced Articles
8-1 Prosulfuron Affects Trunk Shaker Efficiency and Yields of Valencia Oranges
8-2 Abscission Chemicals Affect Trunk Shaker Orange Removal
8-3 Trunk Shaker and Abscission Chemical Effects on Yields, Fruit Removal, and Growth of Orange Trees
8-4 Mechanical Harvesting of Florida Citrus Trees Has Little Effect on Leaf Water Relations or Return Bloom
8-5 Defoliation After Harvest With a Trunk Shaker Does Not Affect Canopy Light Interception in Orange Trees
8-6 Li, K-T and J.P. Syvertsen. 2005. Mechanical harvesting has little effect on water potential and leaf gas exchange in citrus trees. Journal of American Society for Horticultural Science 130(5):661-666.
8-7 Spray Variable Effects on Abscission of Orange Fruit For Mechanical Harvesting
8-8 Effects of Abscission Chemical Spray Deposition on Mechanical Harvest Efficacy of 'Hamlin' Orange
8-9 Whitney, J. D., Wheaton, T. A., Castle, W. S., & D. P. H. Tucker. 2003. Canopy hedging, topping, and skirting effects on yield and quality of Valencia oranges. Proc. Fla. State Hort. Soc. 116: 215-218
8-10 Whitney, J. D., Wheaton, T. A., Castle, W. S., & D. P. H. Tucker. 2003. Tree skirting effects on yield and quality of Valencia oranges. Proc. Fla. State Hort. Soc. 116: 236-239
8-11 The Effect of Trunk Shaker Patterns on Florida Orange Removal
8-12 Mechanical Harvesting Without Abscission Agents- Yield Impacts on Late Season 'Valencia' Oranges
8-13 Late-season 'Valencia' Orange Mechanical Harvesting with an Abscission Agent and Low-frequency Harvesting
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