Tests at Clemson University indicate variable rate application of side-dress nitrogen on cotton can save growers 30-50 percent on nitrogen use without loss of yield.

The spike in nitrogen fertilizer prices in 2008 and the continued volatility of both supply and cost, combined with continued low prices for cotton, has placed many growers in dire need of improving the efficiency of the fertilizer they use.

For the past three years Clemson researchers at the Edisto Research and Education Center in Blackville, S.C., have been testing three different options for variable rate application of side-dress nitrogen on cotton. In 2007 and 2008 they reduced total nitrogen use by 30 percent with no yield loss. In 2009, they upped the ante to saving 50 percent on nitrogen use.

The proof of the 50 percent reduction will come after cotton is harvested and yield and quality numbers are in, but so far tests on the research station and with cooperating farmers look good.

“If you look at cotton prices, they are about the same as in 2003. However, if you look at the price of nitrogen, despite a drop in price this year, it is still near record highs. The recession has kept prices low, but we don’t know where nitrogen prices will go next year,” says Clemson Researcher Wes Porter.

Porter is a graduate student working with Ahmad Khalilian, a Clemson University Professor of Agricultural & Biological Engineering and guru of precision agriculture for a number of years.

For the past three years Khalilian, Clemson Extension Precision Ag Specialist Will Henderson and a group of dedicated graduate students have tested variable rate technology for use in applying nitrogen and other fertilizers on cotton.

Porter says there are three routes a farmer can go to apply variable rates of nitrogen to cotton. The first is the simplest and least costly: A nitrogen ramp calibration strip (N-RCS) can serve as a simple guide to nitrogen use.

The N-RCS strips, usually 100-150-feet long, begin at a low rate of nitrogen in as close to an average soil type as possible and increase nitrogen rates to a rate higher than a cotton plant can use. The response of cotton to nitrogen rate will increase to a certain level of nitrogen, after which there will not be any significant increase in crop performance with increasing nitrogen rate. On a 150-foot test strip, the Clemson researchers upped nitrogen rates by 10 pounds per acre every 16 feet of the test row.

These strips should be applied early in the growing season (at planting or soon after planting). As the season goes along, growers can check the strips and visually determine which rates are producing optimum growth. Based on color and height differential of cotton at various nitrogen rates, the grower can make decisions as to how much nitrogen to apply in side-dress applications.

The obvious problem with this system is the ability to visually determine which rates are best. Each year is different and each year requires establishing a new N-RCS. Simple and inexpensive, the N-RCS will give veteran cotton growers a visual guide to determining how much side-dress fertilizer needs to be applied.

The second option takes the N-RCS a little farther, using a hand-held GreenSeeker. “When you get ready to apply the first side-dress application of nitrogen, you walk over the cotton, hold the device 36-40 inches above the cotton. Measure the ramp strip to the highest NDVI number — that’s where you want to apply nitrogen.”

Demonstrating the hand-held GreenSeeker, Porter says the optimum amount of nitrogen for that particular field for 2009 was just over 80 pounds per acre. “In this field, we reduced nitrogen amount by 10 pounds per acre. Multiply that over hundreds or thousands of acres of cotton, and it will be a huge savings,” Porter says.

GreenSeeker provides useful data to determine Normalized Difference Vegetative Index (NDVI = NIR - Red / NIR + Red, where NIR and Red are Near Infrared and Red wavelengths, respectively). These NDVI readings can be used in conjunction with other agronomic references to measure crop nutrient response and condition, yield potential, stress, pest and disease impact in a quantitative objective manner. This unit can be used to monitor changes in plant conditions during the growing season.

Cost of the hand-held Greenseeker is about $3,000.

The NDVI also can be read with a six-row system on a Hi-Boy sprayer, or similar system. Clemson researchers built a six-row system, which is used to apply variable rates of nitrogen to cotton at the Edisto Station and with growers in cooperative tests.

The Greenseeker is height sensitive, so the Clemson researchers attached a height sensor on the Hi-Boy sprayer to insure maximum accuracy from the sensors.

The Clemson rig has an on-board controller with built in algorithms that ask for maximum crop yield for the region and number of days after planting in the field. The on-board computer takes the NDVI rating and the high rate of the ramp (N-RCS) and calculates the optimum rate of nitrogen needed at different areas of that field of cotton in that particular year.

Using the same controller on a variable rate nitrogen applicator, you can apply map based or real time. Over the past three years of testing, Porter says they have seen a 30 percent reduction in nitrogen totals, with no yield loss. In 2009, using an algorithm designed just for South Carolina cotton, they got a 50 percent reduction in nitrogen use. Once cotton is picked and data is processed, they will determine whether there was any yield loss at the 50 percent reduction rate of nitrogen.

The Clemson variable rate application rig also incorporates GPS technology. The GPS system provides location at the field as well as ground speed of the fertilizer applicator, adding even more precision to the application process.

A commonly used Farm Works software program, with a touch screen computer is used to change rates as the rig passes through the field. The program tells the operator the rate of nitrogen applied, rpm of motor, acres applied, and speed of the rig as it passes through the field.

Porter notes this machine is an ideal way to apply field-specific nitrogen. If you have two fields five miles apart, one gets an inch of rain and one gets none, you will need different rates of nitrogen — just based on that one parameter. This application system solves the problem of getting maximum yield potential for both fields, he adds.

The variable rate system also helps growers manage different cropping systems. For example, a cotton crop following soybeans and wheat will have different nitrogen requirements, since these crops leave different residual amounts of nitrogen in the field. Using the variable rate system, nitrogen is applied based on what is needed for that field under that year’s conditions. So, it takes into account how much residual nitrogen is left in a field.

Using the Clemson rig, Khalilian explains how it can be a one-pass system for applying varying rates of nitrogen. “First you go through the nitrogen-enriched strip and it puts the data into memory automatically. When you go over the rest of the field, the algorithm in the controller calculates how much nitrogen should go to every part of the field.”

The cost of a six-row GreenSeeker system is in the $20,000-$25,000 range, according to Khalilian.

Regardless of the variable rate application system used, there is clear evidence that Southeastern cotton growers can reduce the cost of secondary application of nitrogen on cotton and reduce their overall fertilizer bill without jeopardizing yield.

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