By Trenton Roberts, Research Assistant Professor, Crops, Soil and Environmental Sciences
If you are considering implementing winter cover crops into your crop rotation this fall the single biggest piece of advice that I can give you is to PLAN, PLAN, PLAN. The potential benefits of cover crops can be easily lost if you do not take the time to develop an effective cover crop management plan that involves cover crop variety selection, planting date, seeding rate and cover crop termination method and timing. There are a multitude of benefits that can be realized from the implementation of winter cover crops into production systems in Arkansas. The impact/potential benefits can vary based on location and cash crop, but include erosion control, nutrient retention, weed suppression, crusting prevention and increased water retention. In my opinion, one of the greatest benefits that can be realized from the use of cover crops is the reduction in sediment loss to our primary and secondary waterways in the state. Sedimentation and turbidity are two of the primary impairments seen in many waterways across the state and is a direct result of erosion from cropland. In order to help you develop an effective winter cover crop management plan, please ask yourself the following questions:
1) What Cash or Summer Crop will be planted in the spring?
For cereal crops such as corn, rice and grain sorghum consider planting legumes or blends with legumes as the primary component. Winter legumes well suited to Arkansas include: Austrian Winter Fieldpea, Hairy Vetch, Common Vetch, Red Clover and Berseem Clover. Legume cover crops can offer many benefits such as nutrient retention, erosion control and also have the added benefit of fixing N (similar to soybean) which can be available to the cash crop, thus reducing the need for N fertilizer. Research in Arkansas has shown N credits from Austrian Winter Fieldpea to be as high as 150 units of N per acre! When planting winter legumes, remember that inoculation of seed with the proper inoculant is required to ensure adequate N fixing capabilities. Small seeded winter legumes (vetches and clovers) should be blended with cereals to maximize establishment and provide additional ground cover.
For crops such as soybean and cotton consider planting winter cereals or blends with cool-season mustards and cereals. Winter cereals provide large root systems that trap and retain nutrients, and help stabilize soil to prevent erosion. Winter cereals also provide large amounts of biomass with relatively low input costs. These high levels of biomass are great for erosion control, weed suppression, crusting prevention and increasing organic matter. The best adapted winter cereals for Arkansas are Cereal Rye, Winter Wheat (can be bin-run), Winter Oats and Triticale. The seed cost on these can be relatively low and they can provide adequate amounts of biomass with little to no added input costs. Try and avoid planting soybean following winter cover crops that are predominately winter legumes as there can be increased pest pressures and residual N from winter legumes can limit or inhibit nodulation and N fixation by soybean.
2) What am I trying to achieve by planting a cover crop?
Knowing why you want to plant a cover crop is of vital importance and helps you to better pick the species that will best help you meet those needs, it will also play a big role in helping you plan when and how you terminate as well as when and how you will plant your following cash crop. Don’t just plan to plant a cover crop because it is the hot topic, have a reason and that will make your experience so much better.
If your primary interest is erosion control, weed suppression, preventing crusting or building organic matter then you want to consider cover crops that will produce large amounts of above ground and below ground biomass. Winter cereals and some legumes like Austrian Winter Pea and Hairy Vetch can produce large amounts of biomass which is ideal for all of the aforementioned desired outcomes. However, you must be prepared to deal with issues at termination, such as adequate herbicide rates/coverage to effectively terminate the crop, and the high amount of residue that will be on the soil surface at planting. All the added benefits of high biomass cover crops can be lost if they have to be incorporated or plowed up prior to planting. Most cash crops in Arkansas can be planted into these high residue situations with simple additions/adjustments to your current planting equipment, but you must plan ahead and be prepared.
If your primary interest is nutrient retention/nutrient cycling then consider tillage radish or deep-rooted winter cereals. Following the harvest of corn, grain sorghum, cotton and soybean there can be a significant amount of residual nutrients remaining in the soil profile. These nutrients can be lost via erosion and leaching, which is often times money that you spent for commercial fertilizer. Tillage radish have been promoted for compaction alleviation, but in the majority of our poorly-drained soils, their taproot tends to grow up rather than down. The primary benefit of growing tillage radish in Arkansas are that they are nutrient sponges. Those large taproots or tubers are really just giant sponges that soak up a lot of the residual nutrients in the soil and hold on to them until the next crop needs them. Winter cereals and their deep root systems can also be great at retaining nutrients, but there is one primary difference- when those nutrients are available to the following crop. Winter cereals are great at absorbing and retaining nutrients, but they are somewhat slow to breakdown and release those nutrients to the following crop, especially when they are not incorporated back into the soil. Tillage radishes, however, decay very rapidly and the nutrients held within the tubers or taproots are quickly available to the following crop- almost like a preplant fertilizer application.
If your primary interest is N fixation for N credits to offset N fertilizer needs then legumes or legume based blends are your species of choice. Winter legumes like Austrian Fieldpea and Hairy Vetch can produce large amounts of biomass (in the spring) and can provide significant N credits to the following crop. Winter legumes such as clovers can fix N credits, but can be costly and hard to establish with a much lower rate of return on those N credits. For cash crops like rice, corn and grain sorghum a winter cover crop like Austrian Fieldpea can provide a minimum of 30-50 units of N with minimal spring growth and as much as 150+ units of N if allowed to grow into April. There have been some problems with establishment and foraging waterfowl on single-seed legumes which can be overcome with a blend of cereals and legumes. If you are really interested in N credits consider mixing Austrian Fieldpea or Hairy Vetch with a spring oat. The spring oat will serve as a nurse crop to aid in the stand establishment of the winter legume that will die with the first hard frost and not impede legume growth the following spring.
3) How will I terminate the cover crop?
This is one of the most important questions to ask yourself and should be considered before you plant. Every cover crop species and blend requires a little bit of different chemistry to be effectively terminated/burned down. Glyphosate is not the silver bullet for burning down cover crops. In the following crop specific blog articles we will go into more depth on what herbicides are effective for each cover crop. There is the old adage that if all else fails you can just incorporate the cover crop, but in most cases (where you are not interested in N credits) this is a waste of time and money as the biomass that you were needing to fulfill your goals is no longer on the surface to provide those benefits like weed suppression and erosion control that you had been hoping for.
4) When will I terminate the cover crop?
Cover crop termination will be related to the desired outcomes that you are trying to achieve as well as the species that is planted. For cover crops that will be incorporated for N credits termination is not required, but strongly suggested. For these legume and legume based blends where N credits are the main focus, try and apply the burndown with adequate time prior to incorporation to let the residue “melt” to make tillage easier. The hard part regarding legumes and N credits is the timing of the burndown to optimize N credits and cash crop planting date. The longer you let the legume grow (until flowering) the more potential N credits you can gain. However, most winter legumes like Austrian Fieldpea and Hairy Vetch do not really start producing biomass and fixing N until early March. In a normal year a mid-March termination date of Austrian Fieldpea will result in roughly 30-50 units of N credit and a Mid to late April termination date will yield 150+ units of N credit. For cereal or cereal based blends where the biomass will not be incorporated, the termination date can be more easily determined. The main concern with cereals or unincorporated winter cover crops is ensuring that there is no “green bridge” of living or succulent plant growth which can harbor unwanted pests during cash crop emergence and establishment. Typically a termination date two weeks prior to planting is sufficient to ensure that there is no “green bridge” and reduce the potential pest pressures that might occur from a living cover crop during cash crop emergence and establishment.
Over the next few weeks we will be covering cover crop specific information including planting dates, seeding rates and herbicide burndowns for the primary cover crops for suggested use in Arkansas. There are many amazing short and long-term benefits that can be achieved by introducing cover crops into our Arkansas production systems, but often times poor planning can lead to disasters. In order to maximize the benefits of cover crops on your farm and optimize the input costs, take the time to ask yourself these four simple questions and make a plan before you ever put a seed in the ground. Please feel free to contact us with any questions that you might have concerning cover crop use in Arkansas.