The use of defoliation, desiccation, and senication in modern crop production

The physiology of defoliation: how to accelerate ripening and facilitate harvesting

When crops fail to ripen fully before the autumn weather turns bad, the agronomist loses out on both yield and harvest quality. Defoliants, desiccants, and senecants help accelerate ripening in the pre-harvest period. Defoliation is the artificial removal of leaves using special chemical agents, which triggers processes in the plant analogous to natural aging and leaf fall.

During defoliation, valuable nutrients flow from the dying leaves into the ripening fruits and seeds. At the cellular level, an abscission layer forms at the base of the leaf petiole, breaking the connection with the vascular bundles. Before the leaf falls, active elements of the protoplasm are evacuated from it:

• amino acids and protein hydrolysis products;
• vitamins;
• enzymes;
• organic acids.

In cotton farming, defoliation solves the problem of fiber spoilage: green leaves clog combine harvesters, and their moist sap stains the seed cotton, necessitating additional drying costs. In viticulture, the method is used on fruit-bearing bushes before mechanical harvesting, in nurseries before digging up nursery plants, and on scion mother plantations for the convenience of preparing cuttings before frost. Defoliated grape nursery plants are easier to dig up and store, while the risk of fungal diseases developing on them is reduced.

Defoliation shows high effectiveness for castor beans. According to field trial results, this agronomic practice allows the crop to be harvested in an optimally short time. At the same time, seed moisture and heap contamination are significantly reduced.

• Magnesium chlorate application rate — 15 kg/ha
• Seed moisture without treatment — 12.8 %
• Seed moisture after treatment — 5.7 %
• Number of leaves per plant after treatment — 3.4 pcs.

Defoliant products and rules for their use

For chemical leaf removal, calcium cyanamide, magnesium chlorate, calcium chlorate-chloride, and butifos are most often used. When choosing a product, it is important to consider the specifics of its effect on the plant organism. One of the classic defoliants is calcium cyanamide (CaCN2) — a dark gray, dusty powder with the smell of acetylene, which is poorly soluble in water.

The commercial form of the product contains 50–55% calcium cyanamide, 18–20% cyanamide nitrogen, and 0.5–1.5% mineral oil. The chemical reaction and the penetration of the active ingredient into leaf tissues occur only in a moist environment. Without this, the product will remain inactive and will not provide the necessary effect.

1. Contact of the powder product with dew drops or atmospheric moisture on the leaf blade.
2. Formation of mono-substituted calcium cyanamide and calcium hydroxide.
3. Hydrolysis of mono-substituted calcium cyanamide with the release of free cyanamide.
4. Dissolution of free cyanamide in water and its rapid penetration into leaf tissues.

If the leaves are well moistened by dew, the entire reaction cycle and the penetration of the product take about an hour. After this, the process of forming an abscission layer on the petioles is triggered. The leaves fall, clearing the plant for the passage of machinery.

Calcium cyanamide and magnesium chlorate: mechanisms of action and regulations

The effectiveness of defoliation depends directly on the temperature conditions at the time of treatment. For example, the rate of penetration of calcium cyanamide into the leaf blade increases by more than three times as the air temperature rises from 10 to 20 °C. Once inside, the substance binds copper, iron, zinc, and other metals into sparingly soluble salts, blocking photosynthesis enzymes and destroying pigments. As a result, the level of carbohydrates drops, proteins are destroyed, the intensity of respiration decreases, and protein breakdown products are redirected to the generative organs of the plant. On cotton, calcium cyanamide is applied by dusting 10–15 days before harvesting, and if the timing is observed, the product does not impair fiber quality.

• Temperature for cyanamide penetration — from 10 to 20 °C
• Calcium cyanamide consumption for cotton — 35–60 kg/ha
• Temperature threshold for magnesium chlorate — 9–10 °C
• Magnesium chlorate consumption for cotton — 10–12 kg/ha

Magnesium chlorate is a water-soluble hygroscopic powder or colorless crystals containing 58–62% magnesium chlorate hexahydrate. The product is fire-safe, non-explosive, and moderately toxic to warm-blooded animals (LD50 for mice — 620 mg/kg). Its action is based on strong redox properties: it sharply disrupts the water regime of the leaves, suppresses photosynthesis, and activates hydrolytic processes. The leaves wilt, an abscission layer forms at the base of the petioles, after which leaf fall occurs.

On cotton, an aqueous solution of magnesium chlorate is applied 6–12 days before harvesting. The preparation has also proven itself excellent on castor bean crops. Its defoliation technology includes the following steps:

1. Wait for the browning phase of the capsules on the central raceme, when the accumulation of dry matter and oil in the seed is complete.
2. Perform spraying of the crops with magnesium chlorate at an application rate of 15 kg/ha.
3. Begin mechanized harvesting 10–15 days after treatment.

Timely defoliation of the castor bean does not reduce yield, preserves the sowing and commercial qualities of the seed, and also significantly facilitates combine operation. The results of production trials are presented in the table.

Calcium chlorate-chloride and Butifos: application rules for cotton

Calcium chlorate-chloride is supplied as a light-gray liquid with a 30–32% content of calcium chlorate. It is highly soluble in water and has low toxicity to humans and livestock animals (LD50 for mice is 1112 mg/kg, MPC in the air of the working zone is 2.5 mg/m3). The preparation acts more gently than magnesium chlorate but works on a similar principle. It is intensely absorbed by leaves, leading to their dehydration and fall within 2–3 days after treatment.

On cotton, calcium chlorate-chloride is used for spraying crops at an application rate of 20–30 kg/ha. The optimal treatment time is the period when about 1/4 of the capsules have opened on the majority of the bushes. Defoliation with this preparation must be completed at least 6 days before the start of cotton harvesting.

Butifos (active ingredient — tributyl trithiophosphate, a.i. content — 70%) is a light-brown liquid with a sharp, unpleasant odor. The preparation is highly toxic to humans and warm-blooded animals (LD50 for rats is 177 mg/kg, MPC in the air of the working zone is 0.2 mg/m3). It is a highly active contact defoliant that penetrates only those leaves onto which it is directly applied. The preparation is equally effective in both dry and humid weather but requires warmth — its maximum effectiveness is manifested at an air temperature of 18–20 °C.

On cotton, Butifos treatments begin when 1/4 of the capsules on a bush have opened. The optimal dosage of the preparation is 1.5–3.0 kg/ha. For safety reasons, operations with the preparation on cotton crops must be stopped strictly 10 days before the start of harvesting.