Though it’s early in the cotton growing season, the factors to be considered when deciding when and how much plant growth regulator to apply are already at play in many fields.
Past research has shown that there’s no interaction between plant growth regulator and nitrogen application strategies on yield and fiber quality, says Dale Monks, Alabama Cooperative Extension System cotton specialist.
“Those are two separate issues, but you’ve got to consider the variety you’re growing, and you’ve got to know your nitrogen status before you can begin your PGR application. If we wait until first bloom before we decide how we’re going to manage that plant, it’s too late if it’s an aggressively growing plant. It’s hard to slow down,” says Monks.
But in parts of the Cotton Belt, some growers are putting extremely high rates of PGR on taller, aggressive cotton varieties, he says. “You have to start early, and the nitrogen level of the crop does come into play. But it’s not a good idea to attempt to control plant height with nitrogen. Put out nitrogen for yield, and then deal with plant height based on the variety and conditions. Look at the status of the plant and how it’s growing,” says Monks.
Plant growth regulators include traditional mepiquat or mepiquat chloride products. “Whenever you use a generic product in an effort to save money, check the formulation. Is the formulation 4 pounds per gallon or 2 pounds per gallon? It my be cheaper, but then you have to use twice as much. Some generics are not formulated quite the same,” he says.
Most growers, says Monks, know where they will have problems with rank cotton, and they try to control it.
The amount of nitrogen applied to cotton certainly is a factor, he adds. “The problem last year is that we had a lot of sidedress applications that went out really late. We just couldn’t get into the field. In the Wiregrass region of Alabama, it was rainy and cloudy for 59 days straight, and we all know that cotton responds to sunlight.
“And it’s not just daylight – it’s the quality of sunlight that we’re looking for. Last year, we ended up not being able to get into the fields, and some of those nitrogen applications went out late. Where some went out early, we weren’t sure what was left after all the rainfall. When you’ve had water standing for a few days, do you assume it’s all gone? Do you put out the full load again or part of it?”
Monks notes that even in a year of excessive rainfall in areas, Alabama growers still made good state yield average in 2013 of about 820 pounds.
“We’re in an era now where the new cotton varieties are in the ‘elite’ level. In Alabama, if we get the rainfall, it’ll take our yields up. If we have had the rainfall with clear skies in 2014, we could easily hit over 2 bale/acre yields, just like in 2013.”
PGR's primary purpose
The primary purpose of a PGR in cotton, says Monks, is to manage excessive growth and set more bolls.
It’s difficult to predict whether a plant growth regulator increases yield, decreases yield, or if it’s yield neutral, he says.
“We can look at data going back many years, and according to the season and according to the variety, a PGR may help you harvest your crop. It may not increase your yield but you may be able to harvest it better because the picker did not have to deal with as much plant material. Maybe it allowed you to get more insecticide down into the canopy. As a result of using a PGR, you’ve got more control over how that plant looks and how it goes through the picker, and you can count that as increasing the yield. It’s a crapshoot as far as what it does for yield on a plant to plant basis.”
It’s for certain that cotton treated with a Pix-type product is darker green than a non-treated plant, he says. In addition, the leaves are smaller and a little bit thicker.
“The chlorophyll is concentrated a little bit more, so you’ll end up with a darker green plant than you would otherwise. Also, it’s more compact. We know that if we put out PGR in the middle of a drought, we’ve done harm to cotton. There’s no doubt about that. There are certain things we have to avoid.”
In addition, late applications don’t help much, says Monks. “The idea is to load the plant as you go, and there are different strategies for that. We used to apply 8 ounces at early bloom and then another 8 ounces. But then we went with 10 to 12 ounces at early bloom and maybe another 10 to 12 ounces 10 to 14 days later. Some varieties need more than that.”
You have to keep a certain level of concentration of PGR in the plant to keep it under control and to prevent it from growing excessively, he says.
No correlation to fiber quality
Researchers cannot find a consistent correlation between PGR use and fiber quality, according to Monks.
“Our study showed that there’s no way to know if it makes fiber quality better or worse. The one thing that has the greatest effect on fiber quality is variety. Fiber quality is inherent to the individual variety, and and yield is a combination of variety and environment. So it’s the genetics in the plant and the environment during the growing season.”
The fact that cotton is a perennial makes it difficult to predict what the plant will do, he says – it’s blooming, growing and laying down fiber quality and yield all at the same time.
Up until the time cotton reaches square, the demand for nitrogen is fairly low, says Monks. “We put out a low rate of nitrogen when we plant the crop to carry us through that time, and then sometime before it blooms, we’ll put on our sidedress. Nitrogen rates usually fall somewhere between 90 and 120 pounds, and, more than likely, are split into two applications.”
Fertilization rates and timing can vary by region, he says. “In the Wiregrass region of southeast Alabama, we’ve said you can wait until first bloom, but you want to put out a sidedress of potash and nitrogen at the same time, you’ve waited too late because nitrogen will get into the root system, but potassium is not moving much. The only way to get it out and into the root system is to put it out a little earlier.”
Generally speaking, if you plant on May 1 in Alabama, you’ll see the first white bloom on July 1, says Monks.
When nitrogen demand peaks
Sometime in late July or the first part of August, the demand for nitrogen from the cotton plant is at its highest. When cotton is at its peak demand, the root system starts aging out for that year. The cotton plant is designed to live for years in the right environment. It’s blooming, fiber is making, yield is making, and quality is making all at the same time.”
Growers have to think of their nitrogen applications separately from their PGR applications. “Where the two are connected is that every variety has its own aggressive tendencies,” says Monks. “Some are more aggressive vegetatively than others. We had a variety a few years ago that if you put on 4 ounces of PGR at match-head square, it was almost too short to pick by the end of the season. That variety was extremely sensitive.
“Other varieties are inherently aggressive, and it’s important to take note of this before or as the season starts so we’ll know if we need to start multiple PGR applications early – do we need to start before first bloom at pinhead or matchhead square and slowly load the plant.”
Concerns about nitrogen application enter the picture when a grower has poultry litter in the field and then adds additional nitrogen, says Monks.
“So chicken litter is being releasing N, we’ve got half a load of nitrogen in the field, and the cotton is growing like crazy. That’s when we start looking at the plant and making those PGR applications based on what the plant is doing.”
Foliar N applications, says Monks, probably won’t do much to push cotton. “The leaves aren’t really made to take up nitrogen – the roots are. We can possibly carry the crop a little bit longer and put on a few more bolls. But we can’t make a crop of cotton with only foliar applications unless we want to be out in the field every two to three days.”