It has been said for years that things change whenever you cross the river from Alabama into Georgia, and that's certainly true for cotton nitrogen recommendations. Extension specialists in Alabama prefer a base rate of 90 pounds per acre while Georgia recommends differing rates according to yield goals.
But University of Georgia Extension Agronomist Glen Harris contends the differences in the two states' recommendations are very small, and there are very good reasons for the differences that do exist.
“Our recommendations are not that different — their base rate is 90 pounds per acre,” says Harris. “Our old base rate was 60 pounds of nitrogen for the 750-pound yield goal. There were a number of reasons for going with a yield goal concept, and we have research data to support our recommendations.”
While Georgia doesn't have long-term studies comparable to those in Alabama, a nitrogen study was begun in 1993 by now-retired Georgia Cotton Breeder Shelby Baker, says Harris.
“He was using different nitrogen rates to grow high-yield cotton, and that's when we determined that our base rate of 60 pound of nitrogen probably was too low for higher yields. In this study, he was growing three-bale cotton and it was requiring 80 or 100 pounds of nitrogen. Alabama's base rate is 90 pounds of nitrogen, so it's not as if we're recommending excessive amounts,” he says.
Part of the reasoning for yield goal nitrogen recommendations can be found in the precision agriculture concept of treating each field differently, notes Harris. “Fields with a higher yield potential probably need more nitrogen,” he says.
Recommendations also are based on how much nitrogen is removed by the cotton, notes Harris. “It's true enough that cotton doesn't remove much nitrogen. But when we harvest cotton, we also remove the seed, and the seed contains significant amounts of nitrogen.”
Georgia's current nitrogen recommendations for cotton call for 60 pounds of nitrogen per acre for 750 pounds of lint, 75 pounds of nitrogen for 1,000 pounds of lint, 90 pounds of nitrogen for 1,250 pounds of lint, and 105 pounds of nitrogen for 1,500 pounds of lint.
“We do have more irrigation in Georgia than in Alabama, and that helps to increase our yield potential. The 1,250 and 1,500-pound yield goals assume that you have irrigation. There are a number of factors that make our recommendations different from those in Alabama and other states. There's not one easy answer.”
Field conditions also are different in Georgia, says Harris. “Look at the Bainbridge area in southwest Georgia. Those irrigated deep sands consistently make high yields, and those fields require more nitrogen.”
Nitrogen recommendations for cotton can be modified, he says, when cotton follows a legume — such as peanuts — or when cotton is strip-tilled following a cover crop. “Basically, we give legumes a 30-pound nitrogen credit. And, if you're using a small grain cover crop and strip-tilling, we'd like to see growers increase their nitrogen rates by about 25 percent, which usually works out to be 20 to 25 pounds.
“Legumes can fix and release nitrogen and small grains can immobilize it. We've completed on-farm studies to verify that. We've done studies with cotton following crimson clover, legumes, and we've had number of nitrogen studies with strip-tilled cotton following a small grain cover crop.”
As for the timing of nitrogen applications on cotton, Harris says there are no “hard and fast” recommendations. “We do recommend split applications. The official line is that one-fourth to one-third should be put on at planting and the remainder at side-dressing. That normally translates into about 20 or 30 pounds at planting and 60 or 70 pounds at side-dressing.”
The problem, he says, is that many Georgia growers no longer do any preplant applications. “I definitely would like to see them put out at least 20 to 30 pounds at planting. When should they make up that extra amount for small grains? That's a good question. I don't want to overdo early on, although I think that's when you get some of your worst tie-up. I'd almost like to see them spread out that 25 percent increase between the preplant and the side-dress.”
Looking back at the 2003 season, Harris says Georgia growers were coming off several drought years and had been cutting back on all fertilization.
“Along comes 2003 — a good year with plenty of rain — and many growers had problems getting into the fields to side-dress, so some were caught short. There was a lot of foliar feeding because of that. They couldn't get in the fields with their tractors so they tried catching up with foliar feeding. The growers who had decent fertilizer rates and who didn't receive excessive rainfall did very well this past year.”
Some growers might wonder if they should fertilize for a normal year or a good year, says Harris. “In the big scheme of things, these things usually run in cycles, and you have to make it in a good year to get you through the bad years. I'd want to fertilize for a good year — we're talking about maybe 15 pounds or more of nitrogen, and that probably won't break the bank.”
In-season adjustments also can be made, he adds. “If you go ahead and put out your 20 or 30 pounds of nitrogen preplant, you might already have an idea of the kind of year you're having by side-dress time. You also can make adjustments with foliar applications. We probably can make up 10 to 15 pounds of nitrogen with foliar applications alone. Of course, if it turns off dry after side-dressing, you might decide against foliar fertilization.”
Research conducted in the mid-1980s showed that the best combination for fertilizing cotton was a preplant application followed by side-dressing and then foliar feeding, says Harris.
“Once Bt and Roundup Ready varieties came along, we weren't doing as much spraying, and we didn't have as many opportunities for doing foliar feeding. We recommend that you foliar feed if a petiole sample shows the need for it. If you detect a deficiency after side-dressing, make a foliar application.”
Georgia growers capped off an excellent growing season in 2003 with an average yield of about 800 pounds per acre, he says, and high-yielding varieties like DP-555 have gotten a lot of attention.
“Varieties such as DP-555 have a tendency to grow tall, and some growers are asking if they should cut their nitrogen rates on these varieties.
“I don't think so. We should continue to follow these recommendations that are supported by data. If you cut back, you could be cutting your yield potential. Even 105 pounds per acre isn't too much for three-bale cotton. On this taller cotton, it's more important that we follow a good growth regulator program. With good insect control, a good Pix program and boron, we can handle higher nitrogen rates.”
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