Farm Talk


March 24, 2010

The advantages of cover crops

Parson, Kansas — Cover crops offer several potential benefits for farmers but careful planning to achieve the desired goals is a must, Kansas State University Research and Extension Service crop specialists told a group of Northeast Central Kansas farmers recently at a meeting in Baldwin, Kansas.

No-till farmers are increasing the use of cover crops, and agricultural Extension services are conducting extensive crop research to help farmers profitably include cover crop plantings into their crop rotations.

Kraig Roozebaum, K-State  Extension specialist in cropping systems, listed a large number of potential benefits for seeding cover crops:

•Improve soil quality

•Increase organic matter

•Reduce soil erosion

•Reduce soil compaction

•Supply nitrogen

•Increase water infiltration

•Decrease run-off

•Suppress weeds

•Compliance with EQUIP program requirements

•Conserve soil moisture

•Reduce nitrate leeching

•Increase yields o the following crops

“Obviously, no one cover crop is going to accomplish  all of those goals, or even a majority of them,” Roozebaum said. “It's up to you to study the situation in your fields and decide what you want to accomplish because each of the specific potential benefits requires a different seeding program. Next you need to plan the specific planting time, the specific type of cover crop, and the specific harvest method and time to end the cover crop.”

Roozebaum said farmers also should consider the challenges of cover crops– expense and the extra time in the field required.

“You have to weigh carefully both the short term and long term benefits of a cover crop against the expenses and the extra time required,” he explained.   

Dave Mengel, Extension specialist in soil fertility and crop production, said that while cover crops can be grown for several purposes, they fall into three main categories–nitrogen trap crops, sources of cover and residue and sources of nitrogen.

For trap crops the use of fast-growing nitrogen-demanding crops are ideal. He suggested summer crops such as “millet or forage sorghum for planting in the summer after wheat or cereal rye, wheat, triticale or canola in the fall after summer crops.

“Most trap crops also are well suited as residue cover sources, but remember the rate of residue decomposition can be controlled to some extent by selecting a cover crop with high carbon and low nitrogen residue such as forage sorghum, millet or cereal rye and let it become fairly mature in a nitrogen deficient environment,” Mengel continued. “Adding nitrogen or terminating the crop early to lower the carbon/nitrogen ratio will speed up the decomposition of the residue.”

He said the key thing to remember is that trap crops will use the soil nitrogen taken up to support their growth. “In most cases trap crops will have a wide carbon/nitrogen ratio, so the release of the nitrogen to subsequent crops may be slow. In fact it may be the second or third crop grown that actually benefits from the trapped nitrogen. The good thing is that the trapped nitrogen is not available to move through the soil to contaminate ground water.”

Roozebaum showed extensive data and photographs of cover crop trials conducted on no-till cropping fields near Manhattan following a wheat crop harvest. The rotation pattern for the fields was wheat, cover crop, sorghum, then soybeans.

He gave a snapshot summary of some of the cover crop characteristics. Sorghum-sudan did a “good job of weed suppression and yielded 7,820 pounds of biomass per acre. The double crop soybean planting yielded 20 bushels of soybeans per acre. It's important to plant a short season soybean seed for the cover crop planting because the growing season is short. The sorghum-sudan and second crop soybeans had the same amount of nitrogen tied up in the residue.”

He said the pearl millet plots yielded good grazing. The buckwheat cover crop provided a good canopy over the field, suppressing weed growth and was especially good for a short growing season.

The cover crop yielding the most biomass with the highest nitrogen content was a millet/sunnhemp/cowpea mixture.

In all of the plots grain sorghum was planted following the cover crops.

The cost of the various cover crop seedings ranged from $10 to $90 an acre. Also to be considered if the cover crop is to be terminated with a chemical burn is the cost of glyphosate at $3 to $6 an acre.

Mengel told the group that selecting cover crops to increase nitrogen levels in the soil is probably not realistic in most cases.

“Actually most of us should be interested in weed suppression,” Mengel said. “We need to think carefully about our objectives for the cover crop. Cover crops, including legumes, routinely trap nitrogen but don't release it quickly if the carbon/nitrogen ratio in the biomass is greater than 25 to 1. Remember that organic matter mineralizes at a rate of around two percent per year primarily March through December. Soybeans for instance, with a C/N ratio of about 18:1 release nitrogen much more rapidly than corn stalks with a C/N ratio of 60:1. Corn residue leaves considerably more nitrogen than soybeans but the corn stalks break down much more slowly–it takes two to three years for the nitrogen from corn residue to enter the soil while soybean residue breaks down immediately over just a short year.”

“If you want to grow cover crops to supplement nitrogen for future cereal or forages, legumes are the first preference,” Mengel said. “Remember the carbon-nitrogen ratio still applies when determining how quickly the fixed nitrogen will be available for subsequent crops. Fine textured, low carbon-nitrogen plants, such as alfalfa, clover, soybeans or peas will decompose much more quickly releasing nitrogen much more rapidly than coarse textured plants with wide carbon to nitrogen ratios such as sorghum, millet or mature sunn hemp.”

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