Cotton Variable Rate Application


Using GreenSeeker to Drive Variable Rate Application of Various Products in Cotton

In the United States, cotton is grown in 17 states and is a major crop in 14 of those states. The Cotton Belt spans the southern half of the United States, stretching from Virginia to California. Over the last three years, the area planted to cotton ranged from 12 to 16 million acres. Cotton is an intensively managed crop which requires careful nitrogen applications to prevent rank growth, plant growth regulators (PGRs) to maintain a balance between vegetative and reproductive growth, and defoliants at the end of the season to allow for mechanized harvesting. Additional inputs are needed for pest management.

Recent research has documented the uneven distribution of plant biomass in cotton fields as a result of variability in soil nutrients, moisture, pH, texture and variability in microclimate and disease and pest pressures. Yet many cotton producers still apply agrochemicals at uniform rates across the entire field. Common sense as well as recent research suggests that variable rate application (VRA) of nitrogen, PGRs, and defoliants compensates for the uneven distribution of plant biomass and is a good management practice.

Spraying uniform applications of agrochemicals results in over-application or under-application and uneven growth or defoliation.  Variable Rate Application of agrochemicals results in uniform growth and defoliation and conservative chemical usage.

For example, applying more PGR or defoliant to a section of the field with high biomass and less to a section with low biomass will result in more uniform plant growth or defoliation. In contrast, constant rate applications frequently result in over-application or under-application and subsequently uneven growth or defoliation. Uniform growth and defoliation results in higher harvesting efficiency, higher fiber quality, and an earlier harvest with higher recoverable yields.

Nitrogen, PGRs and defoliants are major expenses for cotton producers and inefficient use can significantly drive up production costs. VRA has the potential for improving the efficiency of application and production. VRA also has environmental benefits as chemicals are applied where needed at the rates needed, reducing the threat of nonpoint source pollution. These issues have raised American cotton producers’ interest in precision farming as a means of reducing production costs and improving profitability.

Sensors mounted on the spray boom of a hi-boy sprayer take NDVI readings based on the biomass, or leaf density, of the cotton plants.

VRA on cotton can be implemented using various techniques. The studies highlighted here use sensors mounted on a hi-boy sprayer to create biomass maps, separate the map into management zones with similar biomass, creating appropriate agrochemical prescriptions for the zones, and then using a variable rate controller to apply the prescriptions. The technique relies on using vegetation indices, or VIs, to quantify biomass.

Sensors emit light waves and read the amount of Red and Near Infrared light reflected back to collect NDVI data.

Although dozens of vegetation indices have been developed, the one most commonly used for quantifying biomass is the NDVI or Normalized Difference Vegetation Index. NDVI is calculated as shown below. The equation used red reflected light and near infrared (NIR) reflected light.

NDVI values range from 0.100 – 0.999. A low NDVI indicates low biomass or low leaf density. These values generally occur early or late in the season. A high NDVI indicates a high biomass or high leaf density that typically occurs mid-season.

A low leaf biomass like early or late season cotton will have a low NDVI reading (far left).  A high leaf biomass like mid-season will have a high NDVI reading (far right).

Using NDVI, biomass maps are created using computer software. These maps represent NDVI values throughout the field that have been divided into three ranges: high, medium, and low. Once a biomass has been created, a prescription map is created that can tell the sprayer what rates to spray in certain areas of the field. The sprayer then uses these prescription maps to apply a specific agrochemical.

The goal of these studies is to determine the feasibility of using the GreenSeeker equipment to manage variable rate application of nitrogen, PGRs and defoliants on cotton. Hopefully our work will provide some results that highlight the usefulness and inspire farmers to begin utilization of this precision agriculture technology.

This work has been supported by grants from the Georgia Agricultural Commodity Commission for Cotton, Cotton Incorporated, and Hatch and State funds allocated to the Georgia Agricultural Experiment Stations.

Download the Proceedings paper from the 2010 Beltwide Cotton Conference

Download the Proceedings paper from the 7th European Conference on Precision Agriculture (7ECPA)

Project Leader: George Vellidis
Contact Info:
Affiliation: University of Georgia
P.O. Box 748
Tifton, GA 31793
(229) 386-3170