With the finishing line in sight — after pouring money, effort and sweat into a cotton crop — the last thing a farmer wants to do is mess his crop up just prior to harvest. To keep the crop on track as the season winds down, Bobby Phipps, Missouri Extension cotton specialist, says there are some key elements to pay attention to.
First, the water. If it's a hot, dry growing season — like last year was — don't expect defoliants to perform as well as they do during a less heated environment.
“Last year, I saw a field where a farmer had put 50 pounds of nitrogen out in August. This was a worst-case scenario. The crop grew to about 6 feet tall with little cotton. When it gets in that condition, cotton grows like mad and slowing it down is nearly impossible. It takes a lot of defoliant,” says Phipps, who spoke at the Milan, Tenn., No-Till Field Day on July 26.
Temperatures may help determine what defoliant is used. “Many farmers south of Missouri can use Dropp and it works very well. In our area, when it starts getting cold, the performance of that product lessens.”
What about numbers of applications?
Many growers in Missouri use two applications, says Phipps. “For several years, I've tried both one and two applications. As long as the cotton isn't too vegetative, I can't tell a difference.”
On normal cotton, farmers can usually step up pressure and water and that will work fine. Phipps usually goes with 15 gallons. If you drop down to 10 gallons, the performance isn't as reliable, he says.
“Much of the time I like a tank mix. It's hard to predict exactly what's going to work from year to year and field to field. The variables are many. Tank mixes help one product take up the slack for another.”
What about cost?
“This time of year, farmers have spent a bunch of money already and want whatever doesn't cost much. But there's more than one thing to think about in that regard. If you try and save money by cutting back on rates, you'll often have to go back and spray again and lose the money you thought you were saving. Do it right the first time or you may spend more money than is necessary.”
If you do a poor job of defoliating, it may not cost you in the field but it will when you sell the crop, says Phipps. Poor grades and leaf trash will lower your sell price.
Timing of defoliants is always a big question to consider. If a farmer goes too early, he'll reduce yield.
“So you want to go with defoliants late enough to get maximum yield. If you go too late, though, you risk wet weather and exposing your crop to unwanted elements.”
There's several ways to time defoliation. “Some folks use cut bolls. I used 60 percent open bolls when first moving to Missouri. But I noticed we had a zone of bolls missing around nodes 12 and 13 where the aphids had knocked them off.”
Where there are missing positions, the open boll results will be biased. You have to be careful with that, says Phipps.
Another method to time defoliation is when the top open boll is within four nodes of the top harvestable boll. “I have a hard time determining what's harvestable. I've chased a few phantom bolls. So be realistic when using that method.”
There's also the Cotman computer program developed by the University of Arkansas and Cotton, Inc. This relies on 850 DD60's after crop cutout.
“That means farmers have to watch their weather records. If the crop doesn't cutout early, you use a calendar date to determine when the boll has a 50 percent chance of opening. In Missouri, that's August 10.”
The last method was devised by Hal Lewis. It's complicated, but many of the better growers use it. The Lewis method is aimed at maximizing yield and protecting a crop's micronaire.
“This method is based on the micronaire of the bottom four first position bolls. You hand gin them and if the micronaire is high, the crop may need to be defoliated prior to 60 percent open. Doing so will stop further development of the unopened bolls and the mic will be less than 5.0. Farmers can thus avoid a discount of high micronaire count.
“Micronaire counts are like playing blackjack. You want to hit less than 5.0 — or 21 — and not go over. You don't want to bust and have a per pound penalty.”
Phipps has used these methods for three years on several test plots — high nitrogen, low nitrogen, wet and dry. In 1999, defoliation was called for on the same date across all plots. In 1998 and 2000, Cotman was first to say defoliation was needed. It was followed by the Lewis method, four nodes above cracked boll and 60 percent open.
“Of them all, over the three year trial, the Lewis method appears to be superior as it has protected both micronaire and yield the best,” says Phipps.
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