Product labels offer two rate recommendations. Which should you use?
By J. Ferrell and R. Leon, University of Florida, and E. Prostko, University of Georgia
Acrop oil adjuvant is an essential part of many herbicide applications. When conditions are dry or the weed is a “hard to control” species, a crop oil can often boost herbicide activity. However, when looking at product labels, you will often see recommendations that say, “apply 1 quart per acre or 1 gallon per 100 gallons of water (1% v/v).”
The problem with these two recommendations is that the carrier volume, or the amount of water being sprayed per acre, is not taken into consideration. For example, if you are applying herbicides at 25 gallons per acre (GPA), then a crop oil rate of one quart per acre and one gallon per 100 is exactly the same. But, if your carrier volume is 10 GPA, then one quart per acre will equal 2.5 gallons per 100.
So why are the labels written this way?
Years ago, when most of these herbicides first came to market, farms were smaller and spray applications of 20+ GPA were common. In this scenario, the label provided two ways of calculating how much crop oil should be added to the tank because the per acre and by volume calculations were essentially identical. Since then, farm size has steadily increased and carrier volume has steadily decreased. Today, per acre and by volume measurements don’t always add up.
Why is crop oil used?
The point of a crop oil adjuvant is to improve herbicide uptake. It does this by increasing droplet retention on the leaf AND by partially dissolving leaf waxes so the herbicide can more easily pass this barrier. However, if the crop oil rate is too high, you can see leaf burning simply due to the oil stripping the leaf wax away, causing the tissue below to dry out and die. So it is very important to use enough crop oil to improve herbicide performance, but not so much to cause unnecessary crop injury.
Which rate: per acre or per volume?
Currently, carrier volume varies tremendously from farm to farm. There are growers that spray at 5-8 GPA while others use 15 GPA. With this great variation in carrier volume, a “per acre” rate of crop oil should not be used as a blanket recommendation. Therefore, it is easiest and safest to use a per volume crop oil rate so that carrier volume per acre is not an issue. In our research, we have found that crop oil at 1% v/v (one gallon per 100 gallons of water) performs adequately across a wide variety of herbicides, weed species and environments.
Hopefully switching to a 1% v/v solution of crop oil can simplify this issue and add some consistency to your herbicide programs.