Aug 16, 2020 9:55 AM

YOUNKER: Using cover crops for weed suppression

Posted Aug 16, 2020 9:55 AM

It has been a very challenging summer when it comes to controlling weeds in fallow and wheat stubble fields. While timely rains have been great for fall crops, they have made staying ahead of the weeds very difficult. About the time you get ready to do an herbicide application it rains again keeping the sprayer out of the field for a few more days.

For producers using tillage to control weeds its even been worse. In many parts of the state there has been very few days where field conditions have been dry enough to do any sort of tillage operation. If a rain occurred shortly after the field was worked the weeds just re-rooted and kept growing.

Combine that with a growing list of weeds that are now resistant to several different types of herbicides it makes you wonder if we’ll ever get a handle on effective weed control in these fallow cropping systems.

One weed control strategy that many times gets overlooked, and certainly underutilized, is planting cover crops during the fallow periods of the rotation.

Cover crops provide weed management benefits in several different ways. Many small seeded weed species, like kochia, palmer amaranth and other pigweed species need sunlight for seedlings to emerge. Living cover crops, or the residue that is left after it is terminated, reduces the sunlight reaching the soil surface thus reducing the number of weeds that come up. Cover crops also compete with weeds for moisture, nutrients and sunlight keeping the weeds in a weak, easy to kill condition, or preventing them from coming up at all. Some cover crop species, like rye and some brassicas, control weeds through a process called allelopathy. This is where decaying residue release chemicals that inhibit the germination of weed seeds.

Here are a few strategies to consider when utilizing cover crops for weed suppression in dryland cropping systems.

* Know what your problem weeds are and get the cover crops well established before those weeds emerge. For western Kansas most of the time those weeds are going to be kochia and palmer amaranth. Kochia comes up early in the spring, so to get ahead of this weed a cover crop needs to be planted as soon as possible after the fall crops are harvested or very early in the spring. If time allows a fall seeding is the preferred method, but once it gets to about October 25th, I would delay the planting to early spring in mid to late February.

* Select species that grow aggressively and shade the ground quickly and provide a mat biomass on the soil surface once the cover crop is terminated. For a late fall or early spring planting cereal grains like rye, triticale, barley, oats and wheat work well. For a summer mix that is planted after wheat harvest, forage sorghums, sorghum sudans and millets are good species to use. Brassicas like turnips and radishes can also provide a fair amount of biomass before freezing out.

* Take advantage of the allelopathy effect some species, like rye provide. This process works like using a preemergent herbicide. Once the cover crop is terminated, chemicals are released from the decomposing plant residue that inhibit weeds seeds from germinating. This works well in controlling palmer amaranth when terminating a cereal grain cover crop right around the time it is heading in early to mid-May.

* Terminate the cover crop early if there is a persistent dry period and the long-term weather forecast is not favorable for adequate moisture to replenish the soil profile. The key to making cover crops work in these dryland fallow systems is for the cover crop to utilize the moisture we would lose anyway through evaporation, runoff and transpiration through weeds, but still leave adequate soil moisture for the next cash crop.

* Always look at the economic side of things and compare your current weed control cost and what it will cost to incorporate cover crops into your system. Herbicide applications can get pricey and many times don’t work well. Multiple tillage operations can also add up to a substantial amount of money.

Cover crops can help with weed suppression, but they are not a “silver bullet.” They are just another tool in the toolbox, that along with other weed control measures, can help in controlling weeds more effectively.

Dale Younker is a Soil Health Specialist at the USDA Natural Resources Conservation Services office in Jetmore.