By DALE YOUNKER
Jetmore NRCS Office
You don’t have to travel very far down a country road to see that a lot of famers are struggling to control herbicide resistant weeds. A good number wheat stubble and fallow fields have been overrun by Palmer Amaranth. This pigweed species also known as “Careless Weed” because it can care less what you do to try to kill it, it will still come back, and most of the time with vengeance. Many herbicides are no longer effective in controlling it and that list seems to grow longer every year.
In wet years tillage seems to be just about as ineffective. If there is any part a of a plant’s root still attached to any soil with any moisture it comes right back. And even if you get them killed, there are millions and millions of seeds just waiting to germinate and grow with just the slightest amount of moisture.
It is obvious that weed management strategies in dryland cropping systems are becoming less effective as time goes by and something different needs to happen to get a handle on these difficult to control weeds. Using cover crops, during the fallow periods, could be at least part of the answer.
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, easier 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 things to think about when considering cover crops for weed suppression.
· Know what your problem weeds are and get the cover crops well established before those weeds come up. Most of the time in western Kansas, 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 in late summer or early fall on fallow fields. If planting behind a fall crop and there is time and moisture is available to get the cover crop established, a fall seeding could be used. But if it gets too late, or moisture is limited, delay the planting to early spring in mid to late February.
· Select species that grow aggressively, shade the ground quickly and provide a mat of residue 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 if planted early enough in late summer.
· 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. A word of caution using rye. Termination in the spring needs to be very timely to timely prior to viable seed development to prevent it from being a weed in following wheat crops. If that is a concern, I would use other fall cereal grains like, triticale, wheat, or barley.
· 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 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 are not a “silver bullet” when it comes to weed control and may not fit into everybody’s cropping system. They are just another tool in the toolbox, that can help in controlling weeds more effectively.
For more information about this or other soil health practices you can contact me at [email protected] or any local NRCS office.
Dale Younker is a soil health specialist with the National Resources Conservation Service in Jetmore.