By Brian Hefty

One of the most popular questions I get every year is, “Can I cut my herbicide rate?”  Here is how I usually answer this question.

Yes, you can cut your herbicide rate in some situations.  However, in other cases you may actually need a higher rate.  Below are some of the factors you need to consider when answering this tough question.

  1. Weed susceptibility.  If your herbicide is great on a particular weed, then your ability to cut the rate is much improved.  On the other hand, if you’re dealing with a difficult-to-control weed, you probably need to bump your rate.
  2. What’s the weather like?  Herbicides perform best when weeds are actively growing.  If you have had temperatures in the 70 to 80-degree range during the last few days, that’s great.  If you have good soil moisture and good humidity, that’s even better.  Most herbicides perform poorly when the weather is too cold or too hot or when the weather is too wet or too dry.
  3. Weed size.  Most weeds are easiest to kill when they are very small.  By that, I mean less than one inch tall.  The bigger weeds get, the higher dose it usually takes to control them.
  4. Spray coverage.  If you are using drift reduction nozzles, you are creating bigger droplets.  That’s great for keeping your product from moving off-target, but the downside is reduced spray coverage compared to flat fan nozzles.  Many herbicides move well in plants, but if you don’t get as much herbicide into the plant as you need to, you will not have a lethal dose to control that weed.
  5. More concentrated droplets.  This is not true for all herbicides, but with products like Roundup I actually prefer less water in many cases.  When you have the same amount of herbicide in two different tanks, but one has far less water, that means that each droplet you spray will now be more concentrated.  If you have waxy-leaved plants or very small plants where you can only get a little bit on them before your spray runs off, the net result of using less water means those concentrated droplets deliver more herbicide into the plant.
  6. Spray adjuvants.  Some spray adjuvants can improve herbicide performance dramatically, but they may burn your crop.  For example, MSO (Methylated Seed Oil) helps weed control with many herbicides because it allows the herbicide to better penetrate through waxy leaf cuticles.  If you want less leaf burn, you may go with NIS (Non-Ionic Surfactant) in some situations, but the weed control usually suffers slightly.
  7. Tankmix partners.  Some herbicides have antagonism when mixed together.  This is common when combining grass and broadleaf products.  For best results using lower rates, spraying separately is generally the way to go.

As you can see, there are a lot of factors that go into this.  I also have to say that using labeled rates is generally a good idea if you want good weed control.  While it is certainly possible to cut your herbicide rates, you need to consider the risk vs the reward.  If you are trying to save $1, it’s probably not worth it.  If you can save $10 per acre, I can certainly understand why you may want to try a lower rate.  Before you cut the rate of any herbicide, we encourage you to visit with your agronomist to make sure you can still get the weed control and yield you expect.  Saving $5 on herbicide and losing $20 in yield doesn’t make sense, but if you can save $5 and give up no yield, that’s a home run.