TUNICA, Miss. — High zone syndrome can reduce cotton yields in fields where production potential is historically variable and where uniform rates of inputs are applied, according to a study by Tim Sharp, precision agriculture professor, Jackson State Community College, Jackson, Tenn.

Sharp’s study set out to quantify what producers have always known about variability in their cotton fields — that with traditional, blanket applications of inputs, only the average parts of a field are managed optimally.

For example, a blanket rate application of plant growth regulator based on the average needs of a field would likely be too light a rate for the most productive parts of the same field. If conditions for rank growth are present, the producer would not be doing a very good job of managing it.

Conversely, the least productive parts of the field may receive a rate of plant growth regulator that is too heavy.

Sharp’s studies used GPS, field imagery and computer software to pinpoint low, medium and high productivity zones in several fields which were then managed conventionally and with variable-rate applications.

The four-year study, which Sharp discussed at Cotton Incorporated’s Crop Management Seminar in Tunica, Miss., indicated that variable rate applications of inputs increased yields across all productivity zones — low, medium and high — compared to blanket applications.

The study was conducted from 2001 to 2004 on a working farm in west Tennessee with no irrigation. Four fields received multi-spectral imagery obtained at 700 to 800 heat units after NAWF 5 for each year of the study. The first image in 2001 was used to establish permanent management zones relating to low, medium and high productivity.

Variable-rate treatments included seeding rate, in-furrow insecticide and fungicide, topdress nitrogen, plant growth regulator, over-the-top insecticides and defoliants.

In the variable-rate fields, low vigor (productivity) zones were managed to maximize vigor expression and reduce crop inputs; medium vigor zones were managed to follow basic practices; and the high vigor zones managed to reduce vigor and maximize earliness.

Four other fields were also selected, divided into vigor zones and managed with conventional uniform rate applications based on standard university recommendations.

NDVI images (which show vigor distribution) were collected each week with a hand-held Greenseeker (made by N-Tech). Information on plant development, height, number of nodes, etc., was also gathered.

Yield maps were generated with a PF 3000 AgLeader cotton yield monitor during a once-over harvest.

In 2003 and 2004, when variable-rate applications was intensified over the previous two years, variable-rate inputs increased yield an average 84 pounds in the low zone, 123 pounds in the medium zone and 290 pounds for the high zone compared to conventionally managed fields.

Across all four years, total crop inputs were reduced by 30 percent and average yield was increased 165 pounds per acre, versus blanket applications.

Sharp noted that reduced nitrogen rates along with increased plant growth regulator applications and intensive insect control improved yield in high vigor zones. Increased nitrogen rates with no plant growth regulator in the medium zones resulted in greater medium zone vigor and yield. No plant growth regulator applied in the low zones increased low zone yields.

Increased rates of crop termination materials were applied in the high vigor zone but increasingly reduced rates were applied to the medium and low zones respectively.

“These data indicated that high vigor zone management is critical to maintain maximum field cotton yield,” Sharp said. “Variable-rate management techniques offer a large opportunity to increase cotton yield via better high vigor zone management.”

Sharp said that the value of precision farming to the grower is a 15 percent to 35 increase in input costs, and a 5 percent to 15 percent increase in yield.

The amount would depend on the amount of variability in a field. For example, some fields may have only two zones, while others may have seven or more.

In addition, cotton yields in high zones can vary according to the variety is planted there, according to Sharp.

Sharp also observed that high productivity zones showed the highest variability when blanket applications were made.

For example, in year one of the study, conventionally managed low zones produced the lowest yields, followed by the medium and then high zones. In the second and third years, the medium zones produced the highest yields, followed by the high zone and the low zone.

In the fourth year, the medium zone produced the highest yield, followed by the low zone and the high zone. The latter is a good example of high zone syndrome, according to Sharp.

Another big advantage of variable-rate techniques comes from reduced input costs, or more efficient use of inputs, according to Sharp.

Variable-rate seeding reduced costs about 30 percent; in-furrow insecticide and fungicide, 30 percent; plant growth regulator, 50 percent to 70 percent; topdress nitrogen, 15 to 45 percent; and defoliants, 20 percent to 30 percent.

e-mail: erobinson@primediabusiness.com