Crop desiccation

Pre-harvest crop desiccation refers to the application of an agent to a crop just before harvest to kill the leaves and/or plants so that the crop dries out from environmental conditions ("dry-down") more quickly and evenly.[1][2][3] In agriculture, the term desiccant is applied to an agent that promotes dry down, thus the agents used are not chemical desiccants, rather they are herbicides and/or defoliants used to artificially accelerate the drying of plant tissues.[4] Desiccation of crops through the use of herbicides is practiced worldwide on a variety of food and non-food crops.

Desiccated potato plants prior to harvest

Uses

Crop desiccation can improve the efficiency and economics of mechanical harvesting. In grain crops such as wheat, barley and oats, uniformly dried crops do not have to be windrowed (swathed and dried) prior to harvest, but can easily be straight-cut and harvested. This saves the farmer time and money, which is important in northern regions where the growing season is short.[5][6] In a non-food crop such as cotton, reliance on natural frost may be too late to be effective in some regions. Thus leaves that remain on the cotton plant will interfere with mechanical harvesters and stain the white cotton resulting in a lower quality grade; herbicides which cause both defoliation and desiccation reduce these problems.[7]

Desiccation can improve the uniformity of a crop. It may correct for uneven crop growth which is a problem in northern climates, during wet summers, or when weed control is poor. Plants that have naturally reached the end of their maturation may be mingled with plants in earlier stages of growth; controlled desiccation evens that out. This also increases uniformity of moisture content in grain, which has positive economic benefits in the storage of the grain and the price of the grain.[5][8]

Desiccation can enhance the ripening of a crop. With sugarcane, for example, glyphosate application increases sucrose concentration before harvest.[9] With grains, for example, as a consequence of crop plant uniformity as noted above, grain ripeness can be made more uniform through the same process.

Several additional advantages of desiccation have been cited: harvest can be conducted earlier; weed control is initiated for a future crop; earlier ripening allows for earlier replanting; desiccation reduces green material in the harvest putting less strain on harvesting machinery.

Examples of crops that may be subjected to pre-harvest desiccation include:[5][7][9][10][11][12][13]

Active agents

In agricultural parlance, desiccation is divided into two distinct groups: "true desiccants" and pre-harvest systemic herbicides.[14][15] True desiccants are not chemical desiccants, rather they are contact herbicides which kill only the parts of the plant they touch. They induce plant death/defoliation rapidly and dry down occurs within a few days. True desiccants do not often provide good weed control because killing only the top growth may allow plants to begin re-growing again. In contrast, systemic herbicides are absorbed by foliage or roots and translocated to other parts of the plant. They poison metabolism throughout the plant thus the process is slower, with die off and dry down taking up to a couple weeks.

"True desiccants"

Most of these kinds of contact herbicides are cell membrane disruptors that are either "PPO inhibitors" or "Photosystem I inhibitors." Plant cells have chloroplasts, which contain the protoporphyrinogen oxidase (PPO) enzyme complex. PPO inhibitors poison that enzyme, causing a build up of Protoporphyrin IX (Proto). Normally Proto is present in very low amounts, but when there is too much Proto it interacts with light to form singlet oxygen radicals (1O2). These interact with the fatty acids of membranes, causing disruption of membrane integrity and leakage of cell contents. The plants soon begin to wilt and quickly dry out in the sun. Plants can burn within hours of exposure to these herbicides.[16] In contrast, Photosystem I inhibitors such as Diquat and paraquat work by entering plant cells and immediately diverting electrons away from photosynthetic chain, poisoning photosynthesis. In addition, hydroxyl radicals (•OH) are formed which interact with the fatty acids of membranes, causing disruption of membranes, leakage, plant wilting, and drying out in the sun.[17]

Contact herbicides used for desiccation include: carfentrazone-ethyl, cyclanilide, diquat, endothall, glufosinate, paraquat, pelargonic acid / ammonium nonanoate, pyraflufen-ethyl, saflufenacil, sodium chlorate, thidiazuron, and tribufos. [1][5][4][7][11] The most common and widely used contact desiccant is diquat (Reglone®).

For potatoes, sulfuric acid is sometimes used as a non-herbicide chemical desiccating agent.[18]

Systemic desiccants (glyphosate)
Main article: Glyphosate

Glyphosate (Roundup®) is the principal pre-harvest systemic herbicide used for desiccation of a wide variety of crops. As a systemic herbicide it is not a true desiccant as it can take weeks rather than days for the crop to die back and dry out after application.[15][19]

Glyphosate works by poisoning the shikimate pathway which is found in plants and microorganisms but not in animals. Specifically, it inhibits the EPSP synthase enzyme which is required for plants to make certain amino acids.[20] Without these, metabolism in the plant collapses. In addition, shikimate accumulates in plant tissues and diverts energy and resources away from other processes, eventually killing the plant over a period of days to weeks.

In the UK, glyphosate began to be applied to wheat crops in the 1980s to control perennial weeds such as common couch which was very effective and meant that sowing of the next crop could occur sooner. Use as a harvest aid in the UK increased after the introduction of strobilurin fungicides which prolong the longevity of the leaves,[21] and by 2002, 12% of UK wheat crops were treated in this way.[22]

The timing of application is crucial as the moisture content of the grain must be below 30% for the yield of the crop to be unaffected and to minimize uptake of glyphosate by the grain.[23] Yield may be affected and residues increased if applications are made to uneven fields in which some areas have a moisture content over 30%. Although used in weed-free and evenly maturing crops with the aim of reducing the grain moisture content more rapidly to hasten the harvest, there is little or no advantage in doing so.[21]

The application of glyphosate differs between regions and countries significantly. In North America, for example, its use on wheat crops is uncommon in the United States but more common in Canada which has a colder climate and shorter growing season. In the UK where summers are wet and crops may ripen unevenly 78% of oilseed rape is desiccated before harvest, but only 4% in Germany.[24]

Questions over practice

Herbicide residue in food has been raised as a concern. Residue quantities are regulated by Codex Alimentarius of the Food and Agriculture Organization of the United Nations.[25]

In July 2013 Austria banned the use of pre-harvest glyphosate citing the precautionary principle.[26] In April 2015 an oat buyer in Western Canada announced that it was refusing oats in which pre-harvest glyphosate had been used.[27][28]

Glyphosate was found in 5–15% of cereal crop samples tested in the UK between 2000 and 2004, although never exceeding the Maximum Residue Level of 20 mg/kg.[22] A survey of British wheat in 2006-2008 found average levels of 0.05–0.22 mg/kg with maximum levels of 1.2 mg/kg.[29]

References
  1. Miller, Donnie K.; Boudreaux, Joseph M.; Griffin, James L. (2010). "Herbicides As Harvest Aids". Weed Science. 58 (3): 355–358. doi:10.1614/WS-09-108.1. Retrieved 1 August 2019.
  2. "Crop Desiccation". Sustainable Agricultural Innovations & Food. University of Saskatchewan. 25 October 2016. Archived from the original on 31 July 2019. Retrieved 31 July 2019.
  3. MacLean, Amy-Jean. "Desiccant vs. Glyphosate: know your goals". PortageOnline.com. Golden West. Archived from the original on 31 July 2019. Retrieved 19 August 2016.
  4. Fishel, Frederick M. (February 2018). "Defoliants and Desiccants [#PI-101]". UF/IFAS Extension. University of Florida, Institute of Food and Agricultural Sciences. Archived from the original on 6 December 2018. Retrieved 2 August 2019.
  5. "The agronomic benefits of glyphosate in Europe" (PDF). Monsanto Europe SA. February 2010. Archived from the original (PDF) on 17 January 2012. Retrieved 2 June 2013.
  6. "Pre-harvest Management of Small Grains". Minnesota Crop News. University of Minnesota Extension. 18 July 2017. Archived from the original on 27 May 2019.
  7. "Sodium Chlorate" (PDF). USDA Agricultural Marketing Service. United States Department of Agriculture. 6 November 2000. Archived from the original (PDF) on 1 August 2019.
  8. Fowler, D.B. "Harvesting, Grain Drying and Storage - Chapter 23". Winter Wheat Production Manual. University of Saskatchewan. Archived from the original on 9 December 2018. Retrieved 2017-05-03.
  9. Gravois, Kenneth (14 August 2017). "Sugarcane Ripener Recommendations". LSU AgCenter. Louisiana State University, College of Agriculture. Archived from the original on 20 September 2018.
  10. "Herbicide Options to Enhance Harvesting - FAQs" (PDF). Saskatchewan Ministry of Agriculture. October 2010. Archived from the original on 2 October 2016.
  11. Lofton, Josh (February 2019). "Using Harvest Aids in Grain Sorghum Production [PSS-2183]". OSU Fact Sheets. Oklahoma Cooperative Extension Service. Archived from the original on 2 August 2019.
  12. "Desiccants - a tool for improving harvesting efficiency in lodged rice crops". Rice Extension. AgriFutures Australia. 2 February 2018. Archived from the original on 12 March 2019.
  13. "Desiccation programmes". Potato Council. 2 August 2011. Retrieved 2 June 2013.
  14. "Crop Desiccation". Sustainable Agricultural Innovations & Food. University of Saskatchewan. 25 October 2016. Archived from the original on 31 July 2019. Retrieved 31 July 2019.
  15. Schemenauer, Ian. "Desiccation and pre-harvest glyphosate". Government of Saskatchewan, Ministry of Agriculture. Archived from the original on 22 October 2016. Retrieved 22 October 2016.
  16. "Inhibition of Protoporphyrinogen Oxidase" (PDF). LSU AgCenter Weed Science Research and Extension. Louisiana State University. Archived from the original (PDF) on 14 January 2005.
  17. "Inhibition of Photosynthesis -- Inhibition at Photosystem I" (PDF). LSU AgCenter Weed Science Research and Extension. Louisiana State University. Archived from the original (PDF) on 14 January 2005.
  18. Franc, Gary D. (8 April 2016). "Potato Vine Desiccants". Bugwoodwiki. Center for Invasive Species and Ecosystem Health at the University of Georgia. Archived from the original on 2 August 2019. Retrieved 2 August 2019.
  19. Brian Hall (12 August 2014). "Direct Harvest: Do you need a desiccant or a pre-harvest glyphosate?". Field Crop News.
  20. Steinrücken HC, Amrhein N (1980). "The herbicide glyphosate is a potent inhibitor of 5-enolpyruvyl-shikimic acid-3-phosphate synthase". Biochemical and Biophysical Research Communications. 94 (4): 1207–12. doi:10.1016/0006-291X(80)90547-1. PMID 7396959.
  21. J. H. Orson and D. H. K. Davies (June 2007). "Pre-harvest glyphosate for weed control and as a harvest aid in cereals" (PDF). Home-Grown Cereals Authority. Archived from the original (PDF) on 2015-06-15.
  22. Pete Berry (November 2006). "FSA Pesticide Residue Minimisation Crop Action Plan - Cereals" (PDF). Food Standards Agency.
  23. "RoundUp preharvest staging guide" (PDF). Roundup.ca. Retrieved 3 May 2017.
  24. "Preharvest weed control and desiccation". Glyphosate.eu. 21 November 2013. Retrieved 15 January 2014.
  25. "Archived copy". Archived from the original on 2016-10-19. Retrieved 2017-03-26.CS1 maint: archived copy as title (link)
  26. "Preharvest use of glyphosate:Recent Austrian decision." Glyphosate.eu. 8 January 2014. Retrieved 5 January 2014.
  27. Arnason, Robert (22 April 2015). "Oat buyer says no glyphosate pre-harvest". The Western Producer. Retrieved 8 June 2015.
  28. Daniels, Calvin (30 April 2015). "Oats on forefront in industry news". Carlyle Observer. Retrieved 10 June 2015.
  29. E. D. Baxter, N. Byrd and I. R. Slaiding (November 2009). "Food safety review of UK cereal grain for use in malting, milling and animal feed" (PDF). Home-Grown Cereals Authority. p. 52.

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