Title: Weeds AR Wild, Ep. 06: Recommended Cover Crops and Termination Options with Jason Norsworthy (4/7/2021)


Jason Norsworthy: Welcome to the Weeds AR Wild podcast series as a part of the Arkansas Row Crops Radio. My name is Jason Norsworthy. IÕm a weed scientist with the University of Arkansas System Division of Agriculture. 

As I stare out my window today here in Fayetteville, the temperature is in the mid 70Õs. In some parts of Arkansas today, temperatures will even approach the low 80Õs. One thing is for certain and that is spring is definitely in the air. Planting season is now upon us and some of you may have planted cover crops last fall & are now beginning to think about termination, or you may have already terminated some cover crops. Those of you that didnÕt plant a cover crop may be wanting to know more about what is recommended and why. 

Today weÕre going to take a close look at cover crops. IÕd like to begin emphatically stating that rye grass should never be grown as a cover crop prior to planting an agronomic crop in Arkansas. If youÕre outside the agronomic crop production region and you would like to grow ryegrass for hay or even cattle forage, then great. Some of you might ask, why do I not like ryegrass as a cover crop? Others of you may say, IÕve heard the university weed scientists recommend rye as a cover crop before.

First, ryegrass is not synonymous for rye. When a university scientist says rye, they are referring to cereal rye, not ryegrass. 

Secondly rye grass is the most herbicide-resistant prone weed in the world today. ItÕs actually resistant to more modes of action than barnyard grass in Palmer Amaranth, two of the most problematic weeds here in Arkansas. 

Today in Arkansas we have ACCase or Group 1 resistance, ALS or Group 2 resistance & glyphosate or Group 9 resistance.

Ryegrass continues to spread northward in Arkansas at an alarming rate, and we really donÕt need folks planting it to contribute to the problems that already exist. Also some of the ryegrass that is planted contains glyphosate-resistant seed, which further contributes to our difficulty in managing this weed. 

I want to now examine how cover crops work to suppress weed. Yes, there are many additional benefits other than weed control that we get from cover crops. But IÕm going to limit my discussion today to the weed suppressive nature of cover crops and then look at how best to control them. 

One of the keys to successful weed control is making sure the soil surface is covered with a crop as much as possible throughout the growing season. After emergence of summer cash crops, whether that be soybean, cotton, corn, etc., there are generally 7 to 10 weeks needed before the crop will achieve a canopy. During this period the terminated cover crop shades the soil surface, lowering daily temperature fluctuations which in turn inhibit weed germination in subsequent emergence. Light availability can also be an important factor regulating the emergence of some weeds, especially small seeded broadleaves like pigweed that emerge on the soil surface as well as some grasses. 

IÕm a big fan of cereal rye when it comes to cover crops. However, just like soybean, cotton, or any crop, there are some varieties that perform well in my research plots & others that have been just a total bust. In regards to cereals, I also like wheat because itÕs generally cheaper than rye. In general cereals like rye & wheat have a high carbon-to-nitrogen ratio in their biomass. This leads to these cover crops being persistent on the soil surface after termination. 

Cereal rye when seeded in October should produce several tons of biomass by mid-April in Arkansas. This is based on the research that IÕve conducted over several years now. This level of biomass will result in approximately 65 to 70 percent reduction in emergence of pigweed or other small seeded annuals in both cotton or corn. In many of my plots, itÕs common to see appreciable amounts of cereal residues present at the end of the growing season.

Now letÕs turn to some legume cover crops, and letÕs think about crops like clovers, vetches, Austrian winter peat. These legumes have a lower carbon-to-nitrogen ratio, leading to rapid breakdown of these materials following termination. Here in Arkansas generally see that most legume residue has been degraded by 6 weeks after termination. For this reason, thereÕs less weed suppression obtained with legume cover crops than we see with cereal cover crops. We also need to be mindful that the legume cover crops degrade and release nitrogen. This nitrogen can be very beneficial when we think of crops like cotton and corn as well as other not leguminous crops like some of the other crops, like grain sorghum. However the nitrogen thatÕs being released can stimulate pigweed emergence. At times when we have seen my research plots is that when we terminate a legume cover crop, approximately 3 weeks, 4 weeks after termination we will see greater pigweed emergence in the legume cover crops than what we see in the absence of a cover crop. For this reason, if a legume cover crop is used, you might consider taking a look at a mixture, where you have cereal cover crops mixed with a legume cover crop. Another option that we may think about is the use of brassica cover crops. Several years ago there was a lot of buzz about the use of tillage radish. This seems to have died down some here in Arkansas over the past several years. ThatÕs partly because tillage radish really did not survive most of our winters, and it never produced the amount of biomass needed in order to cause an impact on soil physical properties as well as a weed suppression. I do know some folks today that have tried other brassicas though like turnips and mustards. WeÕve included those in many of our research trials. What weÕve seen with the mustards and the turnips is that, yes, they can persist and they definitely suppress weed emergence. They also though produce allelopathic compounds that can have a negative effect on our summer crops. IÕll talk more about that here in a moment.

LetÕs turn now and talk about cover crop termination. I like to terminate my cover crops generally around 14 days prior to planting the summer crop. When terminating a cereal cover crop, such as again, wheat or rye, I like to use glyphosate products or Roundup because itÕs highly effective. But generally I am going to mix that Roundup with an auxin herbicide like 2,4-D or dicamba if IÕm terminating prior to April the 15th. The reason that IÕm going to do that is in many fields IÕm still going to have some broadleaf weeds present that IÕm not able to control with glyphosate alone. I also like to mix auxins with my glyphosate because glyphosate alone is not going to terminate a legume cover crop. Legumes are very tolerant to glyphosate. Another mixture that works extremely well in my research has been the use of Gramoxone plus Paraquat as a PSII herbicide to help terminate a wide assortment of cover crops. If I have corn present or IÕm going to plant corn behind the cover crop, I like to use something again like Paraquat plus Atrazine. If soybean will be planted, consider Tricor or another metribuzin-containing product and mixing that with Paraquat. If IÕm going to plant cotton behind the cover crop, consider using Cotoran, Diuron, Caparol and mixing that with Paraquat. For brassica cover crops like mustards or turnips I prefer Paraquat plus a photosystem 2 inhibitor over that of the use of glyphosate and an auxin. But I want you to know that I do not recommend brassica cover crops alone or even blends if cotton is going to be planted following the cover crop. The reason why is as I mentioned earlier, the brassica cover crops have a tendency to produce allele chemicals or natural toxins, natural herbicides and what IÕve observed in some of my research is at times weÕll see lower cotton yields when we plant cotton behind a brassica cover crop than what we see in the absence of a cover crop. 

Another question that I commonly get from folks is how large should a cover crop be when we make a decision to terminate. And what I tell you is donÕt be afraid to let the cover crop get some size to it. Remember that the more cover crop biomass that you have in a field, the greater the weed suppression will be in the subsequent crop. I do recommend that cover crops be terminated before we get viable seed production from the cover crop, otherwise thereÕs a potential that youÕre going to have to contend with progeny or with weeds from that cover crop in the subsequent cash crop. Also donÕt be afraid of the residue. IÕve never had any problems planting in the standing cover crop using a standard planter with a good colter, double disk openers and a well set press wheel. IÕm not a fan of using row cleaners when planting into a cover crop. What I have seen is that when row cleaners are actually used, they have a tendency to grab the cover crop and it wants to ball up rather quickly which has caused major issues from a planting standpoint. 

Another question that I get as it relates to cover crops is: Should I remove the pre-emergence residual herbicides from a weed control program? The answer to that is absolutely not. Yes, some of the residual herbicides are going to be tied up with that biomass or with that residue, but thereÕs also going to be bare soil thatÕs going to be present in that field especially in the row where the crop was planted. And weÕve got to make sure that we get herbicide to that bare soil or else weÕre going to have some emergence. From an economic standpoint really the value of a cover crop will be recouped through improved soil moisture conservation, also utilization by the crop, that soil moisture, as well as less need for bedding and tillage operations, as well as at times we do see improved crop yields when we have a cover crop in front of our summer cash crop. From a weed control standpoint, the cover crop is helping to ensure that your weed control programs remain effective by bringing diversity into that system. 

As I close today, I hope you consider using a cover crop in the future if youÔre not currently using one. As weeds continue to develop resistance to herbicides, selection for this resistance must be reduced and cover crops are a sound strategy that substantially diminishes weed emergence. 

I hope you found todayÕs podcast to be informative and useful. On next weekÕs podcast, Dr. Butts will discuss soybean residual herbicides and our current recommendations. Again thanks for joining us for this episode of the Weeds AR Wild podcast series on the Arkansas Row Crops Radio.

End notes: Arkansas Row Crops Radio is a production of the University of Arkansas System Division of Agriculture. For more information please contact your local county extension agent or visit uaex.edu.