Weed Control on Dairy Farms

Published on Wed, 05/17/2023 - 1:44pm

Weed Control on Dairy Farms.

 By Heather Smith Thomas.

The definition of a weed is any plant growing where it is not desired—a plant that has no value in its present location, especially one that crowds out or damages the desired crop or pasture plants.

“Dr. Aaron Hager, Associate Professor of Weed Science at the University of Illinois says a weed problem on any particular farm will depend on what the primary feed crop is on that farm.  “If it’s corn grown for silage, the fundamentals of weed control in that crop are no different than the fundamentals of controlling weeds in a cornfield that will be harvested and used for grain.  The thing to remember is that when weeds grow with the crop, they only decrease crop yield—they never increase it.  They are competing for the same resources for their life cycle that the crop needs to complete its life cycle.”

The first principle is to have a plan to keep weeds under control so they do not reduce crop yield.  “If we are growing corn for silage, weeds can have the same detrimental impact on silage corn that is typically harvested quite a bit sooner than field corn for grain,” he explains.

There are many herbicide options a farmer can look at to try to manage weeds in silage corn.  “Read herbicide labels to make sure you satisfy any specified time intervals between the herbicide application and harvesting.  These are what we call pre-harvest intervals.  If we are using a product in a field that will go full season, generally we don’t have many issues with this.  However, if we are using that same product in a scenario where the corn will be harvested sooner, we need to make sure we have enough time between when the application occurs and when that corn is harvested for silage, to satisfy the label requirement, to reduce any risk for residues,” says Hager.  There might be some differences in time frame between corn grown for grain and corn grown for silage.

In pasture settings, there are some similar situations.  “We’re now talking about a perennial crop that will live for multiple years.  Often we see weeds with similar life cycles become the problematic ones in a pasture.  In corn, a lot of the most significant challenges are from annual species—weeds that complete their life cycles within the same year, just like the corn.  In a pasture, however, some of the more problematic weeds are perennials, including some undesirable grasses like downy brome, quack grass, etc. and thistles.”

They have similar growth cycles.  “One of the key factors with perennial weeds is to try to keep them from becoming established.  It may seem like a lot of work, to try to keep those first plants—such as thistles--chopped out for the entire year.  But if you don’t allow these to become established, you have saved yourself countless hours of future work and hundreds of dollars that you’d have to spend to try to control or manage an established weed patch,” he explains.

“So the number one rule is, if you see a perennial beginning to take hold or take root in an alfalfa field or grass pasture, do everything you can to curtail that weed and prevent its spread.  Many of the tough perennial weeds we deal with have underground creeping roots, and this is what allows these patches to grow rapidly.  With perennial weeds, we not only have to control everything that’s above ground, but also everything that’s below ground,” says Hager.

“A good example would be dandelions.  You can mow those plants 20 times during a summer and they continue to grow back.  We’ve cut off the top but we haven’t killed the root.  This is the same scenario with for any perennial weeds in a perennial crop system.”

Biennial weeds can also be a big problem—plants like burdock that live for two growing seasons.  The first year, it doesn’t grow tall stalks or bloom; it merely grows leaves and accumulates food reserves in its roots, like a wild carrot.  The second year of life it grows a long, deep taproot, and a tall stalk, producing flowers and burrs.  Putting forth flowers and burrs exhausts food reserves in the root and the plant dies after burrs are mature.  After the stalk comes up, it is harder to kill with herbicides because the plant is sending food up from the roots instead of down.

“Poison hemlock is another biennial that has become a big problem in pastures, and it’s very difficult to get it under control; over time you will have some plants in the first year of life and some plants in the second year of their life cycle.  We can have reasonably good success on the first year plants, but if we wait too long and now they are in their second year of growth, once those plants begin to bolt—which occurs fairly early in the spring—chemical control is very difficult,” he says.

With biennials, a person may need to keep pastures mowed multiple times during the growing season so those weeds can never mature enough to make seed.  If you diligently chop them down or mow them, they can’t keep reproducing and spreading.  If you chop or mow them every two or three weeks, there will be no seed produced, and over time the seed bank in the soil will diminish. Proper timing of herbicide applications will also help.

When weeds are stressed and not growing well, they are harder to kill; they don’t absorb as much of the herbicide.  When it is cold, or they are too dry and not growing very well, you won’t have much success.  Choose a time when the weeds are actively growing after they come up in the spring.  For a perennial or biennial that will come up again another year, you can also spray in the fall when these plants are putting nutrients down into the roots to store for winter.  If you get the herbicide to the roots, it will kill the plant and it won’t grow again the next year.

Knowing the life cycle of that plant can aid you in determining when to apply the proper herbicide—to try to kill the root as well as the upper portion of the plant.  “To control the root system on an established perennial, we need herbicide with the ability to translocate down into that rootstock.  In the spring, all the food reserves in the roots are moving upward to establish the new growth and above-ground vegetation.  The two best times to apply herbicide are right before the perennial weed flowers, and in the fall when the days are getting shorter and cooler and those plants begin to send nutrients down into the rootstock to store for reserves to get them through the winter.”

Your climate and geographic region will determine when this occurs, and also will determine which kinds of problem plants you’ll have.  “What we face in a grass pasture in northwestern Illinois may be completely different from the weed species we see in the very southern part of the state,” says Hager.  The weeds in the west may be quite different from the common ones in the east or midwest.
“I saw an article recently by Dr. Bradley in Missouri about weed species he is now seeing more frequently in pastures.  He listed a couple of species that I don’t think I could even recognize.  That’s not to say we don’t have them here in Illinois, but they are not as common.  Also, we don’t have as much pasture area in Illinois as we did when I started working here 30 years ago.  Our livestock industry is much smaller now, especially the dairy side, compared to a state like Wisconsin, for instance.”

Sometimes seeds come into a region or a particular farm—or from one area to another on the same farm, by various routes.  “There are many different ways that seeds can move.  For instance, if I just spent five hours chopping out an established stand of poison hemlock from one of my pasture areas, and then decide to go scout my other pasture that’s 15 miles away, I may be the source of seed movement.”  There may be some seeds on the tools, clothing or vehicle.

Animals are another common way seeds are spread.  Seed-heads and burrs may stick to fur, hair, feathers or wool.  Many plants evolved mechanisms for animal transport.  Other plant species are well adapted for wind transport, such a dandelion seeds.  Other seeds are spread when animals eat the plant and the seeds end up in the feces, surviving the digestive process and taking root where the animal defecates.

“Burr cucumber can be very problematic in corn.  Let’s say you harvest the corn and use it for silage or as grain.  If the temperature in the silage or stored corn doesn’t get high enough to kill it, that seed may remain viable.  Then when it is fed to livestock, it could possibly pass through their digestive tracts.  These seeds may germinate on the ground and start to grow in the feedlot, or if the manure is applied to a field, those seeds can move to the field, resulting in new patches,” he explains.

Follow directions when using herbicides: Don’t risk residues where you don’t want them - Albert Adjesiwor, Extension Weed Specialist  at the University of Idaho says that there’s not much difference whether it’s a dairy farm or cattle ranch when people want to kill weeds, but if they want to use the manure from pens to spread over crop or pasture land, don’t spread any manure that is likely to leave a residue that might contain herbicides in the manure—herbicides such as aminopyralid (Milestone), picloram (Tordone), etc. and affect the crop or pasture.  “Always keep this in mind if you will be taking the manure somewhere else,” he says.

If there are cattle in a pasture, field or pen near where you are spraying, make sure you do it on a day that’s not windy or the herbicide might blow off target and into where the animals are, or where they might be eating later.  “There are some regulations about grazing in areas where you apply herbicide.  You want to make sure the herbicide doesn’t get onto what the animals might be eating.  If there is any residue that might show up in the milk, you might be in trouble,” he explains.

There are some rules on how soon you can graze a pasture after it has been sprayed.  “You may have to wait at least 30 days before you can graze or feed the animals in that area, and with some products you have to wait longer.  Always read labels to see what the grazing restrictions might be,” says Adjesiwor.

“When spraying, you need to know whether the product is prohibited for use on something that the animals might be grazing, or near something they might be grazing or eating.  It also depends on the kind of weed you are targeting.  If it’s a thistle or bindweed and you spray it at the wrong time in its life cycle it is not going to die.  It must be the right timing, and the right kind of herbicide to get to the roots or those weeds,” he explains.

When trying to spray walkways or gaps between pavements  people use products that are often called pre-emergence herbicides (bare ground herbicides) or soil sterilants, to keep any weeds from coming up.  “Keep in mind that those products remain in the soil longer; you wouldn’t want to use them near manure that will be spread on fields, or where this herbicide might leach into a field or pasture,” he says.

It pays to read labels, and if you have questions you can contact a weed specialist.  “If someone calls me, wanting to apply a certain herbicide, they might ask if it works well for a certain weed.  The first thing I do is pull up the label and then I can tell that person that yes, you can do xyz but cannot do abc with this product.  It’s hard to give any recommendations without knowing the specific situation and what weed they are dealing with.”  There are some general principles, but if there are questions, it helps to know who to contact for answers, when making certain decisions.

“If you decide to use a certain herbicide it will be good for you to know the restrictions, the grazing tolerance, etc.  If it is prohibited at all for certain uses, you don’t want to take any chances on having any amount that would show up in the milk, for instance.”

Get Advice When Needed
Every farm has its own list of problem weeds.  There are many resources that dairy farmers can utilize and consult for advice on dealing with these.  “Every state has weed control guides and these list herbicide efficacy for certain species,” says Hagar.  “Many of these weed guides also have sections discussing weed control in forages.  If multiflora rose is the problem in your pastures, you could look at a weed control guide from a university to see which herbicide would be most effective.  You will generally find ratings of various products in terms of how effective they are.  If the scale used is one to ten, typically the higher the number, the better the efficacy.  If it’s a choice between a product rated as a 6 for multiflora rose versus one that’s rated as nine plus, you’d want to pick the one rated at nine plus.”

Your state weed control specialist or Extension office can also answer questions.  “We can try to help, and if we can’t help we can contact a source that might be able to help.  Even if we don’t have the answers right away, we may know where to look for answers,” says Hager.