Defoliation “rules of thumb” revealed

03 December, 2002

___Defoliation of cotton crops at the correct stage is vital to ensure optimum yields and quality, according to the latest information from CSIRO’s cotton research unit and CSD._

A new CSD grower information brochure on defoliation for quality cotton, notes that crops are safe to defoliate when the youngest boll expected to be harvested is physiologically mature.

CSD extension and development agronomist, Greg Kauter, said boll maturity occurs when the fibre is well developed, the seeds are solid and cotyledons fully developed, and seed coats are turning brown.

“The cut out boll will generally be between 6 and 4 nodes from the terminal, and is the last harvestable boll on the plant.

“The crop can be defoliated safely after 60-65 per cent of the bolls are open, but under moisture stress, a higher percentage of open bolls is desirable,” Greg Kauter said.

__Alternatively, he noted that when there are 3-4 nodes of first position bolls above the highest cracked boll in the first position, the cut out boll will be mature and unaffected by defoliation.

As a final check, he suggested it is safe to defoliate when 98 per cent of mature bolls become difficult to cut, the seed is well developed, and the seed coat begins to tan.

He warned that premature defoliation can damage both yield and fibre quality by forcing immature bolls to open and lowering the fibre quality of the overall sample.

He noted also that chemical defoliants are most active under high temperatures and relative humidity, and the addition of an adjuvant, such as crop oil, is important in assisting defoliants through the thick cuticle of mature leaves.

“Indeterminate varieties may need to be managed more actively to cut out evenly and minimise rank growth, which can make defoliation difficult.”

Greg Kauter said defoliation of dryland crops, Roundup Ready crops, Ultra Narrow Row crops and Pima varieties offer keen challenges to growers, and may require additional personal advice from consultants or the CSD extension and development team.

Further Information: Robert Eveleigh, John Marshall, Greg Kauter or Craig McDonald