Some defoliants, when used in hot, dry weather can cause leaves to stick. Cool weather can slow the physiological processes within the plants and reduce the performance of some harvest-aid products. And if there’s too much rain – as was the case in Mississippi last August and early September – growers can encounter problems with boll deterioration, seed sprouting in the field, regrowth, and reduced lint weight, quality and seed.
“Weather is the wild card when it comes to defoliation,” says Charles E. Snipes, plant physiologist and area cotton specialist at Stoneville’s Delta Research and Extension Center. “We like to apply what we’ve learned from the previous harvest seasons. But the fact is you can’t plan on much until you see what the prevailing weather conditions will be.
“With last season’s unusually warm and extremely wet weather, we saw a lot of growth regulators used to help remove excessive juvenile regrowth. But in 1999 and again in 2000, we had hot, dry conditions before and during harvest that hardened off our crop and turned the leaves leathery. Under those conditions, we tend to use more contact-type materials.
The Mississippi State University researcher says it often takes a combination of materials with two different modes of action, like Def/Folex and Prep or Harvade and Dropp, to defoliate a drought-stressed crop. Plus, the crop oil in the latter combination helps both the Dropp and the Harvade to penetrate the hard waxy layer.”
Ordering the weather
If cotton growers could custom order their weather at defoliation, it would be just like Goldilocks’ porridge. Not too hot and not too cold. Just right.
“I like to see a warm, humid climate at defoliation,” says Snipes. “When we have moderate temperatures … 80 degree days and night-time temperatures in the high 60s and low 70s … it kind of opens up the full arsenal of harvest-aid materials for our growers.”
Warmer temperatures help growers save money by reducing rates of some harvest-aid products. High humidity also reduces moisture evaporation from the leaves and permits harvest-aid chemicals to remain on the leaf surface for a longer period, which aids in absorption of the chemical into the leaf.
But extremely hot weather can also cause defoliation problems, says Bobby Phipps, Extension cotton specialist with the University of Missouri’s Delta Center. “Unless you decrease your rates, some chemicals can be too effective under hot temperatures,” he cautions. “If you kill the leaves before the abscission layer develops, that can cause the leaves to stick. I’d say that’s a pretty common problem here in the Missouri Bootheel.”
The arrival of unseasonably cool weather, or a delayed harvest, can also hamper defoliation efforts, and render some harvest-aid products less effective. That’s a good reason to plan well ahead. Weather data shows there’s a 10 percent probability of frost in some areas of north central and central Mississippi as early as October 12, and a 50 percent chance by the last week of October.
“Here in the Missouri Bootheel, our harvest temperatures probably range 4 to 5 degrees cooler than in Mississippi,” says Phipps. “While a product like Dropp works very well in the Delta and extreme southern Missouri when temperatures are warm, it can be very temperature sensitive once our night-time temperatures drop into the lower 60s.
Harvade can be mixed with the most widely used harvest-aid products to improve defoliation and boll-opening consistency under all temperatures. “Harvade is a good tank-mix partner, especially in weedy fields and in fields where cotton has reached peak maturity,” says Snipes.
“In my experience, Harvade is a good choice no matter what the temperature is at harvest,” adds Phipps. “Where some products miss the mark, Harvade picks up the slack year in and year out. In fact, we have research data showing that a combination of Prep/Def and Harvade is more consistent under all temperatures than a Prep/Def combination.
“We were fortunate to have an ideal harvest last season,” Phipps concludes. “Now that our boll weevil eradication program is underway, we had a top crop like we hadn’t seen for years. Our average for the Bootheel is 630 pounds, and last fall we set a state record with 864 pounds.
“Plus, we had beautiful fall weather last season. But we can’t count on that kind of harvest weather every year. That’s why it’s so important for growers to check the weather before they set their defoliation program in concrete.”