Best practices for terminating your cover crops this spring

Knowing when and how to best terminate cover crops is crucial to ensure you start the year off right, and on time.

Making decisions around when and how you terminate your cover crop is dependent on your goals for the cover crop. Although we have the added factor of chemical costs to consider, the best practice is to stick to your pre-existing plan and have a backup plan in place. It's better to be a little early than to be too late and miss your window for successful termination.

Crucial cover crop watch: Terminating grasses prior to corn. The more mature the rye, the higher the C:N ratio will be. This will lead to it competing for nitrogen which will, in turn, stunt your corn.

When do I terminate?

Soybeans

Most commonly, the cover that soybeans will be planted into is a grass cover- cereal rye or wheat. Grass covers supply large amounts of biomass, offering great ground coverage and weed suppression. Because soybeans are a legume, the nitrogen scavenging properties of cereal crops don’t pose a yield threat to soybeans at planting. 

For best practices, soybeans can be planted into cereal grasses green or terminated. If you plan to terminate before planting, be aware that large amounts of cover crop residue can hold moisture in the ground and cause issues at planting. This decision can change each year depending on the weather. In the next section we’ll discuss moisture retention from cover crops. 

Corn

In most cases, terminating two weeks prior to plant or at planting is successful. Terminating two weeks prior to planting will allow that cover crop to completely die off but not completely fall over. Planting green is possible and very effective with a previous strip-till application or with the correct planter. When planting green, make sure to terminate the cover before corn germinates. As mentioned above, being aware of the cover crop stage is crucial when making termination decisions. Cereal rye that has jointed or headed will have a greater C:N ratio and will be more apt to tie up nitrogen as soil biology starts to break down residue. Depending on your fertility program this can cause issues at germination. 

Keep in mind your planter capabilities when determining termination height of cover crops, large amounts of biomass can cause issues with proper seedbed preparation and residue build-up on the planter.

Major misconceptions that can trip you up:

"Cover crops make soil colder and wetter so kill them early."    

Not so fast. Cover crops and reduced tillage protect your soil by insulating it and buffering temperature swings. These practices ensure that during the cold snaps in spring, soil will be warmer, and during the hottest parts of the year, soil will be cooler. This means plants are less stressed and therefore use less water. Terminating these cover crops too early (three weeks or more prior to planting) can prove both myths:

1. In wet conditions, you take away a plant that was transpiring water from the soil to the atmosphere and instead leave a wet mulch on the soil surface which will push planting dates even later into the season.
2. In drier springs*, the mulch left behind after termination can protect the crop and soil from evapotranspiration.

*In cases of extreme spring drought, it may be advised to terminate earlier if there is concern for moisture competition. In most cases cover crops aid in water retention from better soil aggregation, increased infiltration, increased soil organic matter (which acts like a sponge), and soil protection.

How do I terminate?

There are several methods for cover crop termination including chemical, mechanical, and species selection (winter kill). We will focus on chemical and mechanical for now.

Chemical Termination

Chemical herbicide is the most popular tool for cover crop termination. In most years chemical is fairly affordable and easy to terminate quickly. However, this year is a different story. Rising chemical costs mean you must be targeted and intentional about your applications.

• Do not cut rates. Reducing herbicide rates can lead to poor weed/cover crop control and increase the risk of developing chemical resistance.
• Buy generic. Staying away from name-brand herbicides (such as Roundup®) can help you cut costs while still maintaining herbicide effectiveness. It is best to check with local retailers to see what generic products are in stock.
• Prioritize using glyphosate for terminating grass cover crops. Adding ammonium sulfate (AMS) at a rate of 17 lbs/100 gallons will greatly increase control of grass cover crops.
• For legume and broadleaf cover crops, using herbicides such as 2,4-D, dicamba, and Sharpen® will be better suited for control than glyphosate alone. As with all herbicide mixes, make sure to read labels for rates, tank-mix partners, and necessary adjuvants. For more information on tank-mix partners and chemical selection, click here.

As a general rule, apply systemic herbicide early in the day on still, warm, sunny days (between 9 am and 3 pm to allow for penetration). Apply before the reproductive phases of cover crops, especially grasses. Contact herbicides, like paraquat and glufosinate, can be used in cooler spring situations but make sure you get complete coverage.

Mechanical Termination

Mechanical termination is very popular in organic production and in cases where strong weed protection is desired. This type of termination is usually done with cereal rye and at boot stage (the head is swollen in the stem of the plant) when planting soybeans. It is usually done at least three weeks prior to planting.

• Use a roller crimper so the stem of the plant is truly broken and the plant dies off.
• You can also use roller crimping and herbicides in combination if it is getting later than anticipated for cash crop planning.

While mechanical termination isn't as common as chemical termination, it can still be an effective method especially for tough-to-control weeds.

Considerations:

• When planting green, scout your field early and watch for strong fungal or pest pressures.
• Remember herbicides with long residuals which are applied late in the season may impact growth of cover crops planted after harvest. This can be easy to forget and impact establishment for next fall.
• If you have a federal contract you must abide by their guidelines, find more here.
  • "For crops planted in the 2020 crop year and later, insurance will now attach at time of planting the insured crop and cover crop management practices will be reviewed under Risk Management Agency (RMA) rules for Good Farming Practice (GFP) determinations similar to other management decisions (e.g. fertilizer application, seeding rates, etc.)"