Crop Management for Defoliation
(PDF file version, 27KB)
by Jeffrey C. Silvertooth,
Extension Agronomist - Cotton
By the time final irrigations are made, the harvestable yield potential is on the plant, and management efforts should focus on protecting the crop in terms of both yield and quality. As we are all very well aware, the quality of the lint we harvest is very important to the marketability of Arizona cotton. Effective defoliation is an essential step in the overall process of harvesting high quality lint. The program for an effective defoliation will often change from year to year due to crop and environmental differences. It is important therefore to remember that we are trying to speed up and control a natural process (senescence) when applying defoliants to a cotton crop. To develop an effective defoliation program for a farm, there are several factors which need to be taken into account:
A general rule of thumb which can be applied to timing a defoliation is to wait for a period equal to twice the late season irrigation interval, following the last irrigation, before applying defoliants. For example, if late season irrigation intervals were about 12 days for a field, then we would allow about 24 days from the time of the last irrigation to the time when the defoliants should be applied. This is only a "general rule of thumb" and not at all exact because crop dry-down times can vary depending on a number of factors including boll load, weather, soil water holding capacity, and the amount of water applied in the last irrigation.
A better, but perhaps a more cumbersome approach, would be to estimate soil water depletion and then target defoliant application when approximately 70% of the plant-available soil water (PAW) is depleted from the soil. To do this you need to know: 1) the soil texture, 2) the water holding capacity of the soil, 3) the depth of the soil profile which is filled with the last irrigation, and 4) the evapotranspiration rates (ET) from the crop. The ET rates for the crop can be obtained from the Arizona Meteorological Network (AZMET) or from the weekly cotton advisories distributed within each cotton producing county in Arizona. Refer to the UA Extension Agronomy Bulletin "Water Management for Defoliation", by J.C. Silvertooth for more details and an example case. It is also important to point out that we commonly irrigate a soil at about 50% depletion of PAW to prevent water stress in-season. Accordingly, extending the PAW depletion level to 70% will impose a distinct and visible level of water stress on the plant.
Nitrogen Fertility Status
Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director Cooperative Extension, College of Agriculture and Life Sciences, The University of Arizona.
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Information provided by Jeffrey C. Silvertooth, firstname.lastname@example.org
Extension Agronomist - Cotton, College of Agriculture, The University of Arizona.
Material written February 2001.
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