Spring is right around the corner, and soon it will be time to control the weeds in our fields once again. There are so many choices for farmers now in terms of weed control. Twenty years ago, our discussion on weed control would have been pretty limited, but today, there are hundreds of herbicides available on the market, and it seems like with each one of them there are a few little tricks to making the product work perfectly.

Yes, there are a lot of little things that go into the overall performance of each herbicide, but today we'll talk about some of the basic principles to making herbicides work better.

We'll talk first about what it takes for most herbicides to control weeds. Then, we'll look at some of the factors that you must evaluate to improve the performance of your herbicide.

Remember that the ultimate goal is to end up with weed-free fields so you can harvest the maximum amount of bushels in the fall. Plus, you always need to keep dollars and cents in mind, too, so you don't just end up with maximum yields. You need to end up with maximum profitability, too.

Again, we'll talk about how to make herbicides work better on your farm, and that starts by understanding what it takes for most herbicides to control weeds. We often say there are 3 things, intake, translocation, and a lethal dose of chemical By intake, what we're talking about is how much chemical the plant absorbs through its leaves or roots. Obviously, the chemical needs to get inside the weed if you hope to kill it.

Then, there's translocation. That's a term we use to describe the movement of the chemical from wherever it enters the plant to the growing point or points of the weed.

Finally, the most important step is you must have a lethal dose of chemical once the herbicide reaches the growing point. Nothing else matters if the chemical reaches a growing point in the weed and there is not enough herbicide to kill that growing point. If this happens, the weed lives. However, if there is a lethal dose, the plant will die.

Once again, for most herbicides to work, there must be intake, translocation, and a lethal dose of product once it reaches the growing point. With this in mind, we'll move on to some of the factors involved in the success or failure of most herbicides. We'll talk about weed susceptibility, weed size, application timing, and spray coverage.

First is weed susceptibility. In other words, how easily can your herbicide kill a target weed? As you know, certain weeds are more susceptible to certain herbicides, compared to others. For example, Liberty is outstanding on woolly cupgrass, and 1 pint per acre will usually kill it. However, it takes twice as much Liberty to kill yellow foxtail, 2 pints per acre. Woolly cupgrass is therefore much more susceptible to control with Liberty than yellow foxtail is.

Also, each weed has different biotypes. Think about corn and soybeans for example. Is each variety of corn the same as the next? No. Some varieties stand better, some emerge quicker, some are high yielding, and some are more susceptible to injury from herbicides. It is the same thing with weeds, but instead of calling them different varieties, we call them different biotypes. Certain biotypes of a particular weed will stand better, some emerge quicker, some produce more seed, and some are more susceptible to injury from herbicides. Keep these things in mind when you are trying to control the weeds on your farm.

Next is weed size. The smaller a weed, the less herbicide it usually takes to control that weed. Think about it, if the weed is smaller, yet it gets the same amount of chemical as a big weed, the concentration of chemical inside the plant would then effectively be higher. However, keep in mind that small plants aren't always more easy to control. For example, if a broadleaf weed is so small that all you see are the cotyledons, you may not get much herbicide into the plant, as the cotyledons are very waxy and usually repel a good percentage of spray solution. Another thing to consider with smaller weeds is they typically have less total surface area of leaves with which to absorb herbicide. For these reasons, a small plant may take a little lower rate of herbicide, but it probably won't be much lower than normal.

Next is one of the biggest factors of all, application timing. As we said earlier, the only way a weed will die from most herbicides is if a herbicide can be taken into the plant, moved to the growing point, and finally, there must be a lethal dose of that herbicide when it finally reaches the growing point. Application timing can have an effect on all 3 of these things, so let's start by talking about intake.

In order for a weed to take in a lot of herbicide, the pores on the leaf must be receptive to the spray solution. If the weed is suffering for some reason, intake will be reduced. For example, if the weather is extremely hot or extremely dry, the pores on the leaves will close up. This happens because in hot and dry conditions, the leaves are trying to conserve as much moisture as possible. If the pores were open, moisture would be allowed to escape, and that wouldn't be good.

Another example of when intake is poor is when temperatures have been very cold. If the temperature has recently dropped below 32 degrees Fahrenheit, some of the leaf tissue may have died. Even if the temperature didn't get quite that cold, the plant will still not be taking in water or spray solution very easily. If you want the best conditions for intake of herbicide, you need a plant to be actively growing. That means that the weather must be warm and humid for some time prior to application. One last example of a weather condition affecting intake is dew. If there is much dew on leaves, the addition of a herbicide will simply lead to runoff and poor weed control.

That's a little about intake and the effect of application timing. Now let's look at translocation. Again, this term means moving the herbicide from wherever it entered the plant to the growing point. Just like with intake, the weed must be actively growing for translocation to be maximized. If the weather has been warm and humid prior to application, that's a start, but the weather must be good after application, too. You see, it is very important to not only get the herbicide into the plant, it is equally important to move that herbicide to the growing point or points.

If the weather is cool and/or dry, translocation will not be as quick as when the weather is warm and humid. The longer it takes for the chemical to be moved to the growing point, the more chance the plant has to metabolize or break down the chemical. It is very possible for you to get enough chemical into the plant to kill it, but if the weather is adverse, the plant is able to neutralize the chemical and survive.

That brings us to the most important thing to always remember when spraying herbicides…lethal dose. Again, if the herbicide doesn't get taken into the plant or moved to the growing point or points quickly, you may not have a lethal dose of herbicide. If there is not a lethal dose, the plant doesn't die.

Alright, let's talk about some other environmental factors with application timing like wind, sun, and soil moisture.

Wind is usually a bad thing, because it can move a herbicide off-target and lessen control. However, a slight wind can occasionally be good to help move herbicide underneath all the leaves on the plant. As a general rule, though, if there is much wind, your performance will suffer.

Sun is something we've had lots of questions on this winter. Farmers ask, is it better to spray when it is sunny, cloudy, or dark. Well, if you take temperature and humidity out of the equation, weed control is generally about equal. The exception to this rule includes weeds like velvetleaf and morningglory, as they seem to be controlled better during the day, because their leaves droop at night. In the daytime, they seem to catch more herbicide and intake is better.

I said,"take temperature and humidity out of the equation," when talking about weed control in sunny, cloudy, and dark conditions. Let me explain that a little more. Generally, when it is dark, it is cooler and there is a dew on most leaves. When it is sunny, it is usually warmer and dryer. I agree that we want warm and humid conditions for best results, but that's not what we're talking about here. In studies done by South Dakota State University, they have shown virtually equal weed control at noon, midnight, 6 am, or 6 pm, as long as the temperature and humidity was the same at all 4 times. About the only disadvantage we see to spraying when it is very sunny is that evaporation may be slightly higher, but otherwise, we see almost no difference regardless of the sunlight conditions.

One last thing with application timing we'll quickly discuss is soil moisture. Obviously, if a soil is too wet to drive on, you shouldn't be out there spraying herbicides, but other than that, having some soil moisture is good. Some degree of soil moisture is necessary for your weeds to be actively growing, and as we've said several times already, an actively growing weed is easier to kill.

Our last topic of discussion today is spray coverage. An ideal scenario would be for a post-emerge spray to completely saturate every leaf of every weed in a field or for a pre-emerge herbicide to completely cover every particle of sand, silt, clay, and organic matter in a field. If this happened, spray coverage would be maximized.

However, we live in a real world, so we just have to do the best we can to get good spray coverage. Basically this comes down to carrier volume, spray pressure, spray tip selection, boom height, and canopy. Again, the ultimate goal is to saturate either the leaves of the weeds or all particles of soil, depending on whether you are spraying post-emerge or pre-emerge.

As a general rule, to get the best spray coverage, you need more carrier volume, more spray pressure, flat fan nozzles, a lower boom height, and less canopy. Having more carrier volume, usually water, is important, especially as the crop gets bigger. In a burndown situation where all the weeds are half an inch tall or less, 5 gallons of water per acre may be sufficient. If the weeds in your field are a foot tall, though, you will probably need 20 gallons of water per acre in order to get the best coverage possible.

What we feel is more important than carrier volume is spray pressure. As a rule, the more pressure the better. The higher the pressure, the finer the droplets. Think about it, if you can have 1 big droplet land on a leaf or 100 little droplets, which will provide better coverage? Which will have more chance of splashing off or running off the leaf? Well, university numbers for years have backed up this argument that increased pressure leads to smaller droplets, which in turn leads to better coverage and better weed control.

Nozzle selection is important to droplet size, too, and while many custom applicators are switching to turbo teejet or air induction nozzles to increase droplet size and decrease drift potential, they are sacrificing spray coverage and weed control. Again, our recommendation is to use flat fan nozzles if all you are after is the best weed control.

Boom height is another consideration with weed control, because if your boom is too high, you will have more drift, less coverage, and less performance. Finally, if the canopy from your crop or your weeds is too thick, you will not get as much spray coverage, and you are likely to see your weed control suffer.

Well, it was quite a list we covered today with regards to herbicide performance. However, it is important to keep all these things in mind if you are trying to get the best weed control on your farm.

Again, if you spray a very susceptible and fairly small weed when it is actively growing, you will get tremendous weed control if you get good spray coverage.

However, if your target weed is not very susceptible, it's big, the weather is adverse, and your spray coverage is poor, don't expect to kill that weed.

Everyone always wants to cut herbicide rates to save money. Well, if all the factors we talked about today are in your favor when you spray, maybe you can cut rates to some degree.

However, if all or even some of the factors are against you, you may actually have to increase your rates to get the performance you desire. We encourage you to visit with your agronomist or ag chemical supplier for more details on the herbicides you intend to use and the weeds you intend to kill.