The decision to defoliate a crop is an important management practice and like much of farming, timing is everything. When plants are defoliated, the fiber and seed development essentially stops. If a crop is defoliated too soon it can affect yields, quality and profits. If defoliation is delayed, it could increase the potential for insect infestation and encounters with bad weather.
Defoliation in a plant is the loss of foliage and a natural process that is accelerated by the application of chemicals. The zone located near the base of the leaf petiole is referred to as the “abscission zone”. Enzyme activity within the abscission zone is regulated by hormones produced by the leaf. Young, quickly growing leaves produce an abundance of hormones which hold the petiole tight to the main stem. As the leaf matures, hormone production decreases and the leaf becomes more susceptible to hormonal imbalances and water and cold stresses.
Abscission is the shedding of leaves, flowers, or fruits following the formation of the abscission zone. It’s a physiological process where the living tissue, in this case the leaf, separates from the living plant. The chemical application of a defoliant is designed to inflict enough injury to the leaf to upset the hormonal balance at the abscission zone and allow enzymes to begin the abscission process.
Whether applied by ground or by air, several factors will directly impact the effect of a defoliant; the condition of the plant, air temperature and overall weather conditions at the time of and several days following the application, and the chemical itself. If too many bolls are immature at the time of application there can be a reduction in yield and quality. Plant and field inspection is critical in timing the application of a defoliant to a crop.
Defoliated crop in Graham County, Arizona with some bolls still closed
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