Costs, Comparisons, and Effectiveness Using Chlorophyll Sensing Sprayers in a Chemical Fallow Operation

Final Report for FW06-003

Project Type: Farmer/Rancher
Funds awarded in 2006: $6,950.00
Projected End Date: 12/31/2008
Region: Western
State: Oregon
Principal Investigator:
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Project Information



On farm acreage in north-central Oregon and south-central Washington, erosion from water and wind has been a problem, as soil organic matter depletes. Continuation of full-width tillage summer fallow was not a sustainable practice. The direct seeding system has done a great job of stopping erosion and improving soil quality, and even the organic matter decline has halted and is in the process of slowly reversing. However, in this region annual cropping has met with limited success. Currently there is a general movement by farmers back to summer fallow. If we are to continue to reap the benefits of direct seeding and have a sustainable farming practice, the fallow year must be accomplished without tillage using a chemical fallow system.

Farmers in this region have been working hard to find a cost effective and efficient way to create chemical fallow. The biggest problem to overcome has been the control of summer broadleaf weeds such as Russian thistle, kochia, marestail and prickly lettuce.

Farmers who have been successful with chemical fallow have had to use rates of 50 to 60 ounces/acre of a 4-pound glyphosate product to control these weeds. During the summer months, dust from the wheel tracks of ground sprayers binds with the glyphosate and wrecks weed control under the center of the sprayer. Currently, most operators have switched to aerial applications of chemicals for their mid and late summer treatments. The goal has been to achieve adequate weed control with one aerial application.

Repeated applications with an airplane at these high rates have made chemical fallow more expensive than traditional full-width tillage summer fallow. The key for chemical fallow to be widely adopted lies in a cost-effective way to control summer broadleaf weeds.


I began direct seeding in 1997 and converted the entire farm to a one-pass direct seed system in 1999. Some of my farm now has 11 years of direct seeding history. The positive changes to the soil are tremendous and erosion has been almost eliminated. I believe this farming method is truly sustainable.

Like other farmers, I have had problems controlling summer broadleaf weeds in my chemical fallow. Currently almost all summertime applications in chemical fallow are aerially applied. An option to the airplane has been to use a chlorophyll-sensing sprayer for the summer broadleaf treatments. In 2004, I rented a small 14-foot chlorophyll-sensing sprayer mounted on a four-wheeler. I used this sprayer to spray out weeds in the wheel tracks from excessive dust during the June glyphosate application. I was impressed with how the unit performed.

The sprayer boom heads are spaced at 1-foot intervals. Each head works independently and puts out a beam of infrared and near infrared light. A sensor reads the reflectance, and when the correct green color is detected, a signal is sent to a pressurized solenoid valve, which sprays the weeds. Since summer broadleaf populations are spotty in the fields, this sprayer has the ability to reduce herbicide use by over 90% depending on the weed population density.

In the winter of 2004, I built a 40-foot chlorophyll-sensing sprayer and now have four years of experience and 10,000 acres sprayed using this technology. Through this project, I intended to evaluate the effectiveness and cost of using a chlorophyll-sensing sprayer versus aerial applications for summertime glyphosate applications.


The first step in this project was to select two adjacent 100-acre plots in a single field. The cropping history of the field was chemical fallow in 2004 and winter wheat in 2005. Both parcels were treated identically during the fall and spring herbicide applications. After the middle of June, all applications were aerially applied on one side, and the chlorophyll-sensing sprayer was used on the other.

We accomplished good weed control with both treatments, so yield differences the following harvest were expected to be negligible and were not measured.


In both years, the spray dates were in early November with 12 to 15 ounces/acre of glyphosate. A fall application breaks the green bridge and leaves the field void of live plants over the winter. If a choice to annual crop in the spring is made, another light application of glyphosate can be used. Seeding can commence the day after spraying without waiting for plants to burn down and decay. If chemical fallow is made, it also allows me to delay the first spraying in the spring until early to mid May. By that time the first flush of Russian thistles has usually emerged and another tough weed to kill, rat-tailed fescue, is in a vulnerable stage.

The first aerial application was on July 14 with 48 ounces/acre of glyphosate. There was no summer precipitation after that date and the one aerial spray job provided good weed control for the remainder of the season.

There was a July 14 and an August 17 chlorophyll-sensing sprayer application. After using the chlorophyll-sensing sprayer for four years, it is apparent that standard protocol will be for two applications, one just before our harvest season and one right after harvest.

A November 5 application with 15 ounces/acre of glyphosate was followed by a May 15 spray job with 25 ounces/acre glyphosate. On July 23, the first pass with the chlorophyll-sensing sprayer was made just prior to our harvest season. This year was an exception because of very low weed pressure. Rather than make a second pass with the chlorophyll-sensing sprayer in August, I was able to mop up with a four-wheeler sprayer using a hand wand.

The aerial spraying was performed on July 6, and by August there were just a few weeds on the entire 100 acres. These weeds were also sprayed using the four-wheeler sprayer and hand wand.


Weed control with both chemical fallow treatments was very good for both years. The calculated cost is more complicated. In the first year, glyphosate costs varied from $10.34/gallon to $12.41/gallon. When you couple the low glyphosate price with two ground rig broadcasts and one aerial application, there was virtually no difference in total cost/acre compared to the chlorophyll-sensing sprayer treatment.

In the second year, the price of glyphosate skyrocketed to $30/gallon. The aerial applicator charged $33/gallon for the glyphosate. The increased cost of glyphosate made a dramatic increase in the overall cost on the aerial side. The weed pressure the second year was also very low and allowed me to skip my second chlorophyll-sensing sprayer treatment.

The four-wheeler sprayer mop up was an inexpensive alternative. When the total costs of both treatments were calculated, the chlorophyll sensing sprayer side had a $7.83 advantage for the second year. Both treatment years had slightly below average summer precipitation. On years with wetter summers, I have seen two aerial broadcasts needed for good control. On the same wet years, it is probable that an extra ground rig broadcast or an extra chlorophyll-sensing sprayer treatment may be needed.


Positives of chlorophyll-sensing sprayer attributes:

1) Tremendous savings in chemicals. In fields with low weed populations I have saved up to 90% of the cost of a similar broadcast treatment.

2) Lethal rates can be used. Hard-to-kill summer broadleaves such as Russian thistle and prickly lettuce require high rates of glyphosate. With the chlorophyll-sensing sprayer, when one head fires on one weed, I am applying 25 gallons/acre total spray with 1 gallon/acre of glyphosate.

3) Environmentally friendly solution. This tool reduces pesticide use.

4) Alternate chemicals may be used. Our mainstay chemical for fallow is glyphosate. We rely too heavily on this product, and alternate chemicals need to be found. There are new chemicals that may work in chemical fallow. So far, all of these chemicals are too expensive to use in fallow on a broadcast basis. With the chlorophyll-sensing sprayer, these chemicals may still be cost effective. If a major glyphosate-resistant weed shows up in chemical fallow, the chlorophyll-sensing sprayer may be the only solution for adequate cost-effective control.

Negatives of chlorophyll-sensing sprayer attributes:

1) Uneven calibration. When the calibration switch is activated, each head independently looks at the ground below it and stores what it sees as zero. If there is tall stubble or a green weed under the head, the unit is much less sensitive than the other heads in detecting weeds.

2) Calibration decay. With changes in temperature, the calibration levels will drift. It is important to calibrate often when spraying.

3) Small weeds. Even with proper calibration, the sprayer will miss an occasional small weed. If there are many weeds fist-sized or smaller, the sprayer can’t get them all.

4) Dust. This sprayer is still a ground rig and even with high rates of glyphosate, the weeds can be slow to die in the wheel tracks. To counter this problem I do most of my spraying at 5 miles/hour or slower and do most of the spraying early in the morning when there is less dust.

5) Ghost ring on large dead weeds. On the second pass usually in August with the chlorophyll-sensing sprayer, there may be large dead weeds. This happens when the first spraying is a little late, and the weeds such as Russian thistle become woody and keep their original size after death. When this occurs, the sprayer heads will see the dead skeletons and fire on them.

6) Tall stubble. Whenever any material is thrust closely to the sensor, it will overwhelm the head, which will fire.

7) Inability to watch units fire. A spray pattern is hard to see unless the sun is angled just right. To properly adjust sensitivity and monitor the firing heads, we need a method to see the heads fire.

8) Cost. The chlorophyll sensing sprayers cost approximately $1,000/foot to build.


During the project, I had one demonstration farm tour organized by our cereal crops extension agent. I spoke at the Pacific Northwest Direct Seed Association Annual Conference on chemical fallow and the chlorophyll-sensing sprayer in 2007.

Now that the project is complete, I am in the process of writing an article for our local extension newsletter and for the Pacific Northwest Direct Seed Association’s Direct Link newsletter. In January, I spoke at the Pacific Northwest Direct Seed Association Annual Conference on this subject. In addition, I spoke at our county level winter grower meeting sponsored by Oregon State University Extension.


I recommend designing a bright LED into the front of the head so the operator can see each unit fire. Currently each head acts as a separate unit. The connecting cables use 12 wires that send electrical signals from the control box to each unit. If the units were redesigned to have a central processor in each head, then all communications could be routed through a CAN BUS cable. This would allow the ability to calibrate all the heads equally based on an average background across the whole sprayer. This system would allow for the control box to keep track of firing and alerts could be made when units were firing more or less than the rest of the sprayer.


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  • Larry Lutcher


Participation Summary

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Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.