As a crop consultant, Bob Griffin sees the value of a good crop rotation program every day. And with today's high input costs, he sees the value of limiting nitrogen applications on cotton and corn through steady shifts from one to another, along with soybeans or wheat.
“We cannot afford to put on fertilizer whether it's needed or not,” the Jonesboro, Ark., consultant said while serving on a rotation-aimed panel during a Beltwide Cotton Conference session recently in San Antonio, Texas.
“Soil tests are a must when different rotations are used. Corn may need extra nitrogen, phosphorus or potassium. But cotton may not need as much fertilizer when it follows corn.”
Griffin uses what he calls “smart sampling” to make sure soil tests are made at the right place in a field to determine which fertilizer or chemical combinations are needed.
“Known areas of low yield, high yield, poor drainage or soil texture can be chosen to determine where soil tests may be needed the most,” he said, adding that aerial imagery through infrared photography, Veris soil sensing zone maps, yield maps and grid soil sample maps can be used to determine sample locations.
With aerial imagery, growers can develop a variable-rate prescription for fertilizer and other chemical applications. The practice can reveal differences in fields that might otherwise be overlooked.
“You cannot assume that zone colors (from imagery photographs) always represent the same differences,” said Griffin. “Zones can represent many things such as drainage problems, fertility levels, disease, insect management, etc.
“The biggest benefit of aerial imagery is allowing us to discern problem areas of fields so that we may sample these areas and address the problem. After identifying the problem, we can often use the imagery to create a variable rate map used to apply products at rates that are best suited to each area of the field.”
Growers rotating cotton after corn or soybeans can often reduce nitrogen applications by 25 percent or more because of the residue remaining in the field. Griffin noted that soybeans have less demand for potassium and phosphorus, however, soybeans can still decrease P and K to levels not acceptable for cotton or corn.
“We can also see benefits of rotation in helping control weeds that hurt cotton,” he said.
Another panelist in the Beltwide forum, Auburn University agronomist Charles Burmester, said cotton yields are often higher with a rotation program.
At the Alabama Tennessee Valley Substation, cotton yields were observed with different rotations between 1995 and 2005. The cotton/corn rotation showed an 11 percent increase in cotton yields. Cotton following soybeans had a 9 percent yield increase.
Cotton rotated with wheat showed a whopping 17 percent higher yield, while cotton following soybeans double-cropped with wheat had an 11 percent increase. Cotton planted no-till in stubble showed a 10 percent increase, and cotton no-tilled into wheat saw yields jump by 16 percent.
Yield increases were likely boosted partially by the amount of organic matter left in the field, said Burmester.
In Auburn research, organic matter in soil reached 1.48 percent in a continuous cotton program; but increased to 1.65 percent with a cotton/soybean rotation; 1.7 percent with a cotton/corn rotation; 1.98 percent in a cotton/wheat/soybean rotation; 2.23 percent in cotton planted in no-till stubble; and 2.26 percent with cotton no-tilled into wheat.
Different rotations can also help prevent soil compaction, says Burmester. But while rotations can enable growers to improve yields, take advantage of particular markets and enhance soil fertility, they can disrupt time management.
He said growers who harvest wheat in July, then double-crop it with soybeans can face harvest-time headaches.
“Double-crop soybeans, corn and cotton may need harvesting at the same time,” he noted.