How big should you let cover crop get before terminating it to plant corn or soybeans? What if you don't want to terminate it? You may want to keep it and let it grow and harvest the cover crop for forage. Or harvest it for seed production, so you can plant your "saved seed" for your next cover crop instead of buying cover crop seed.

Those are a couple of cover crop questions some farmers are asking this spring. Brian Lang, Iowa State University Extension field agronomist in northeast Iowa provides the following answers and information to help you make these decisions.

YOUR DECISION: "There is no set calendar date requirement for when to kill a cover crop," notes ISU agronomist Brian Lang. "It mostly comes down to when the producer is ready to plant in the spring."

Regarding timing of termination of a cover crop, the following link is to an article explaining the rules for terminating winter hardy cover crops. There is no set calendar date requirement for when to kill a cover crop.  It all comes down to when the producer is ready to plant in the spring.

Thinking About A Cover Crop? Start With Developing A Plan
Taking time to design your cover crop plan will increase the successful establishment of the crop and potentially allow for improved staggering of fall harvest.

Early termination is more important for rye ahead of corn planting
"If you have a winter rye cover crop ahead of corn production, we like to kill the rye at less than 1,000 pounds per acre of dry matter production, which is about 5 to 8 inches tall depending on stand density," says Lang. "The purpose for killing it this soon is to minimize any allelopathic affects, lessen potential residue cover, possible nitrogen tie-up with residue, and improve soil warm-up before planting corn."

What if you are planting soybeans? An early termination is not important for rye ahead of soybeans, he explains.  Rye cover grown closer to soybean planting time offers significant weed control and rarely adversely affects soybean yield potential.  ISU Extension weed management specialist Bob Hartzler wrote two recent articles about this topic. You can read "Cereal Rye, Allelopathy and Corn" here and "Terminating Cover Crops" here.

Keeping a cover crop for forage use, or for seed production
Some farmers may want to keep a cover crop growing instead of killing it. Instead of planting corn or beans in that field, they want to harvest the cover crop for forage for cattle. Or they want to let the cover crop grow and harvest the seed and use that seed instead of buying seed to plant their next cover crop.

Cover crops that were drilled last fall are usually fine for either option -- forage or for seed, says Lang. Aerial seeded stands may not have achieved a dense enough stand.  Having enough of a stand to create cover is not the same as establishing a full stand for forage or seed production. "There are no detailed guidelines for which to assess a rye stand for anticipated yield of forage or seed," he adds, "but if we extrapolate from well researched winter wheat production guidelines, we could consider the following general guidelines."

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For winter wheat (and spring oats, barley and wheat), a "full stand" has about 90 seed heads per square foot. That is about 30 plants per square foot assuming each plant produces two tillers. Tillering is enhanced by four factors: 1) good soil moisture, 2) good light interception into the canopy, 3) available nitrogen, and 4) cool weather conditions. 

Calculate how much seed or forage your cover crop will produce
Thirty pure live seeds (PLS) per square foot equals 1.3 million seeds per acre which is about 73 pounds of rye seed per acre which is about 1.3 bushel per acre. This assumes everything that was planted grew. Calculating PLS takes into account the seed tag information of percent germination and purity. 

"We often seed a 'full stand' of oats, barley and wheat at 90 to 100 pounds per acre for grain production and 90 to 110 pounds per acre for forage production, drilled for efficient stand establishment," says Lang. "In winter wheat production, University of Wisconsin assessments in spring that find less than 70 tillers per square foot suggests applying a nitrogen fertilizer application at green up to enhance more tillering. For rye production, spring nitrogen recommendations would be in the range of 20 to 60 pounds per acre depending on cropping history (i.e. lower end following soybeans and higher end following corn). If manure was applied in fall, it should supply enough nitrogen for rye forage or seed production."

For tips on rye grain production see the following publication. It was published in 1990, but little has changed except for some of the varieties. Current variety trial information is available. And variety descriptions available online.

On another topic, only a little bit of alfalfa winterkill
Another question some people are asking: How bad was winterkill of alfalfa this year? Agronomists and farmers in Iowa who grow alfalfa report there is only a little bit of it. "In traveling across northeast Iowa last week I found some winterkill of alfalfa, but only in older stands," says Lang. "The newer seedings looked great. Much of the winterkill in older stands is very patchy and in areas small enough not to bother with. Some areas might be large enough to justify interseeding some ryegrass to fill-in some of these spots for the last year of the stand."

At this time, it is extremely difficult to make a roadside view assessment, he says. You need to walk the field. Also misleading from a roadside view is that some of the orchardgrass was set back by frost, but it is recovering. If you want to try to assess your own stand at this time, Lang recommends you watch this video first. It is from Dan Undersander, University of Wisconsin forage specialist, and it nicely illustrates assessing a stand this early in the season. Go to Alfalfa Stand Assessment at www.youtube.com/watch?v=hoJUU1rmCko.

"Once the alfalfa gets about 6 to 8 inches of growth, we can assess stands by the stem-count method," says Lang. "It is a much more accurate assessment of plant health and yield potential. It is a fairly quick and easy method, and is explained in the following publication.