Pre-harvest crop desiccation (also siccation) refers to the application of a herbicide to a crop shortly before harvest. Herbicides used include glyphosate, diquat and glufosinate. For potatoes, carfentrazone-ethyl is used. Other desiccants are cyanamide, cinidon-ethyl, and pyraflufen-ethyl.[unreliable source?]
Desiccation corrects for uneven crop growth which is a problem in northern climates during wet summers or when weed control is poor. Several additional advantages of desiccation have been cited: more even ripening is achieved and 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. Some crop may be mechanically destroyed when crop desiccation machinery moves through the field.
The application of glyphosate differs between countries significantly. It is commonly used in the UK where summers are wet and crops may ripen unevenly. Thus in the UK 78% of oilseed rape is desiccated before harvest, but only 4% in Germany.
Residue quantities are regulated by Codex Alimentarius of the Food and Agriculture Organization of the United Nations. Several technical recommendations by governmental agricultural services suggest guidelines regarding sprays. Pre-harvest desiccation is used in agriculture in crops including:
- Cereals such as barley, wheat, and oats
- Oilseed rape
- Legumes including lentils, garbanzos, and soybeans
Glyphosate is used as a pre-harvest herbicide and harvest aid on cereal crops including wheat and oats and on oil seed rape. As a systemic herbicide it is not a true desiccant as it can take weeks rather than days for the crop to die back after application.
In the UK, it 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, and in 2002, 12% of UK wheat crops were treated.
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 minimise uptake of glyphosate by the grain. 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.
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. A survey of British wheat in 2006–8 found average levels of 0.05–0.22 mg/kg with maximum levels of 1.2 mg/kg.
In July 2013 Austria banned the use of pre-harvest glyphosate citing the precautionary principle. In April 2015 an oat buyer in Western Canada announced that it was refusing oats in which pre-harvest glyphosate had been used.
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Media related to Crop desiccation at Wikimedia Commons