A new crop sensing device — Crop Circle — being used at the Pee Dee Agricultural Research and Education Center near Florence, S.C., indicates growers may benefit by waiting until cotton reaches first square stage to apply nitrogen.

USDA researcher Phil Bauer used the new crop sensing system to test the response to nitrogen by cotton at three different growth stages. He notes his findings are in the preliminary stages, but so far his tests indicate that slow-growing cotton definitely responds to fertilizer differently and at different growth stages.

Bauer, who is a research agronomist with USDA’s Agricultural Research Service’s Coastal Plains Soil, Water and Plant Research Center, says Crop Circle can be mounted on the front of the tractor or sprayer — whichever one is used to apply nitrogen. Crop Circle will take a reading of crops and via research-driven equations will determine how much nitrogen needs to go on the crop.

For growers already equipped with variable rate application systems, Crop Circle can be teamed with the applicator and nitrogen rate can be determined on the fly as the applicator moves through the field.

Bauer notes there is a great deal of work being done on corn and wheat using Crop Circle in the Midwest. Working at the Pee Dee Research Center, Bauer’s research team is looking at the new technology on cotton.

“We know cotton is very slow growing, especially early in the year. Often early season nitrogen differences — where there is nitrogen in the field and where there is none — is often not apparent,” Bauer says.

Cotton yield can be very responsive to nitrogen fertilizer. However, over-application of nitrogen can result in excess vegetative growth, which can delay maturity and increase the need for growth regulator, defoliant, and insecticide, in addition to wasting money on fertilizer that does not produce a return.

In the Pee Dee test, Bauer put out three rates of nitrogen (zero, 30 and 100 pounds per acre) on 1,000 foot strips in a five-acre test field. Crop Circle was run over the crop at four different times during the crop season.

Cotton in the test was planted after soybeans to provide some carryover nitrogen. Bauer, speaking at a recent field day at the South Carolina research facility, stressed that his results are preliminary and need to be replicated several years for verification.

“We found at 16 days after cotton emerged we saw no difference in response on cotton to which 100 pounds of nitrogen was applied and where no nitrogen was applied.

“At 31 days after emergence, or shortly after first square, we found differences among 0, 30 and 100 pound nitrogen rates, but not between 30 and 100 pounds.

“We really didn’t see any significant differences among all three rates until 36 days after emergence,” Bauer explained.

He said this indicates, if you have nitrogen in the soil, growers may need to put nitrogen on a little later than is commonly done. The Crop Circle system indicates at first square or a little later than first square might be more efficient.

Crop Circle sensors can be configured to output either raw IR and VIS data so that you can examine crop biomass and nitrogen status separately or it can calculate and output several different crop reflectance indexes automatically.

Unlike some other sensing systems used to develop variable rate application programs, Crop Circle uses multiple sensory data, including:

• Normalised Difference Vegetative Index (NDVI).

• Wide Dynamic Range Vegetative Index (WDRVI).

• Simple Ratio Index (SRI).

• Infrared band reflectance (RNIR).

• Visible band reflectance.

USDA researchers in Missouri used three different sensing systems (Greenseeker, Cropscan and Crop Circle) to monitor cotton response to nitrogen at different growth stages of the crop — early-square, mid-square and first bloom.

Six different nitrogen rates were used, ranging from zero nitrogen to 200 pounds per acre. These rates were compared to a uniform application of 100 pounds of nitrogen per acre. Had a grower used one of the sensing systems and variable rate application, he would have increased profitability by $43 per acre.

In the Missouri test, all three sensing systems were efficient in predicting the most profitable rate of nitrogen when cotton was at the mid-square and early bloom stage (20-40 inches tall), but not at the early square stage.

Bauer says his research is promising, but that more years of replication on cotton is needed. However, the Crop Circle system he used, the Greenseeker system that has been on the market for a while, Cropscan, a new crop sensing system all have the potential to put technology to work for growers to reduce crop and improve crop efficiency.

e-mail: rroberson@farmpress.com