High-Yield Wheat: The Science Behind Optical Crop Sensors

January 24, 2014 08:21 PM
Variable wheat field

One of the biggest challenges I have each season is determining the appropriate nitrogen rates across each wheat field. Different soil types; topographies; crop and yield histories; and productivity zones affect nitrogen requirements. Not knowing how the growing season will play out further complicates matters. Both moisture and temperature influence the amount of nitrogen released from the soil or lost via denitrification or leaching.

phil needham

Nitrogen is almost always the nutrient that provides the greatest yield response per unit applied, but it’s also the most expensive. Too much nitrogen can also result in lodging and increased disease pressure, which can indirectly reduce yields and profits. By comparison, too little nitrogen can result in reduced yields.

In recent years, we have improved the way we refine nitrogen recommendations, with soil and tissue tests being the most common. Chlorophyll meters and aerial imaging have also helped. The challenge with these tools is gathering enough spatial data in a timely fashion to make sound variable-rate applications across each field.

Targeted information. In 1998, while managing Opti-Crop Consulting, we had the opportunity to evaluate the first Hydro N-Sensor in the U.S. We found the top advantages of an optical crop-sensing system include:

  • Help in determining the optimal nitrogen rates across different regions of the field.
  • Increased fertilizer efficiency and reduced input costs.
  • Increased yields and grain quality.
  • Reduced lodging, which is almost always associated with lower yields and profits.
  • Reduced risk of nitrogen losses to the environment.

Throughout the years, several universities, including Virginia Tech, Oklahoma State University and the University of Kentucky, have conducted research to create algorithms to determine nitrogen application rates when using optical crop-sensing systems in wheat.

In 2008, I started working with Lloyd Murdock, a University of Kentucky Extension soil fertility specialist, to help refine the GreenSeeker algorithm for winter wheat. Across five replicated field trials and four growing seasons, we found a 2 bu. to 5 bu. per acre yield advantage in 80 bu. to 100 bu. per acre fields.

It’s also important to add that the GreenSeeker system almost always recommended slightly lower nitrogen rates, compared with the standard fixed-rate applications. The increased profit generated from the slightly higher yields and lower nitrogen rates was around $20 per acre. We also saw improvements in crop standability across the fields, especially in higher yielding situations, which results in less variability in residual nitrogen.

Nitro Greenseeker

This Miller-Nitro sprayer is equipped with a 120' boom with six GreenSeeker sensors. The Capstan PinPoint system allows the liquid nitrogen application rate to be adjusted up or down while maintaining a constant pressure.

How they work. Optical crop-sensing systems determine a crop’s nitrogen demand by measuring its light reflectance every second. The sensors shine light of specific wavelengths at crop leaves, then measure the type and inten­sity of the light wavelengths reflected back. The reflectance characteristics for visible and near infrared (NIR) are used to develop vegetative indices to compare the health of crops and create a basis for adjusting nitrogen rates up or down.

The Normalized Difference Vegetation Index (NDVI) is calculated using the reflectance of red and NIR wavelengths. A typical plant-sensing system gives values from 0.6 to 0.95 in most wheat canopies around the jointing stage. Because the newer crop-sensing systems, which include GreenSeeker (marketed by Trimble) and OptRx (sold by AgLeader) use an artificial light source, this allows them to function day or night and with minimal influence from shadows created by passing clouds.

Optical crop-sensing systems can be mounted on nitrogen applicators equipped with a variable-rate controller to adjust the rate of nitrogen across a field. Crop-sensing systems are calibrated using an N-Rich strip, a small region in each field or variety where we maximize plant health and chlor­ophyll content. In order to achieve this, we apply a non-limiting rate of nitro­gen to a small area of the field, typically 1.5 times the highest rate commonly recommen­ded. The N-Rich strip has nitrogen applied early in the season, often as part of a split spring application strategy, where the first pass is made as a flat-rate application to adjust tiller populations up or down.

Put it into practice. Once the wheat has had time to take up the first appli­cation, a second application is made around the jointing stage, also known as Feekes growth stage 6.

When making the variable-rate pass with the GreenSeeker, an algorithm determines the nitrogen application rate, using the difference between the NDVI of the N-Rich strip and all other regions of the field.

For example, if the N-Rich strip reads 0.85 and another region in the field reads 0.81, the difference is 0.04; this value is used to adjust the nitrogen rates based on response curves generated from research trials. A high and low rate is also set, based on the average rate being applied, sprayer speed and the application equipment.

It’s amazing to watch the rates automatically adjust up or down as the sprayer moves across the field. Just by adjusting pressure, most sprayers can vary from a base rate of 10 gal. per acre up to a high of 20 gal. of liquid nitrogen.

For a wider range of rate adjustments, consider a sprayer rate control system such as Case IH AIM or the Capstan PinPoint, which use electronic solenoids on the nozzle bodies that open and close 10 times per second. These systems can vary liquid nitrogen application rates from 5 gal. to 40 gal. per acre, while maintaining the same operating pressure.

While different algorithms are available for different classes of wheat and regions in the U.S., some refining on a local basis is likely required. At a typical speed of 10 mph to 15 mph, the sensors signal the rate controller each second, which adjusts the application rates up or down as the applicator moves down the field every 20' to 30'.

Setup varies for optical crop sensors. For example, the current recommendations for a GreenSeeker system are: four sensors equally spaced across a 60' boom, five for a 90' boom and six for a 120' boom. The readings are aver­aged across the boom to get one reading for the entire width. Therefore, it is not making adjustments in small areas but changes nitrogen appli­cation rates as the soil, topography or plant health changes in the field.

While optical crop-sensing systems are a huge improvement from flat-rate applications, they are only designed for adjusting nitrogen rates. If other nutrients are limiting, the crop isn’t likely to reach its full potential, even with the correct amount of nitrogen.

At Agritechnica in 2011, I learned that German researchers at the University of Bonn were evaluating an optical crop-sensing system that could detect other nutrient deficiencies, inclu­ding phosphorus, potassium and sulfur. If such systems can be refined, the applicators of the future might be able to carry different nutrients and apply them as necessary across the field, based on the specific requirements of the growing crop.

Agronomist Phil Needham brings European wheat management tips to the U.S., mixing an English accent with a Kentucky drawl. Contact Phil:

E-mail: wheatcollege@farmjournal.com

YouTube: Youtube.com/Needhamag

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