Want to maximize the nutrient uptake of legumes in a cover crop? You’ll have to wait for them to reach heading and flowering. This can present a challenge to Virginia corn farmers who want to plant their corn crop by April.

Since 2018, Virginia Tech field crops agronomist Hunter Frame and his colleagues at the Tidewater Agricultural Research and Education Center in Suffolk, have evaluated a wide variety of cover crop species to determine they help with nitrogen cycling. Frame says the goal of the research is to synchronize heading dates and flowering dates of different legumes in cover crops and synchronize them to the cash crop.

Frame says the challenge is certain cover crop legume species such as hairy vetch and crimson clover head flower in mid-to-late April, so Virginia farmers can’t fully use them to their potential for nitrogen accumulation in their corn crop, which is mostly planted by that time.

Speaking at a cover crop field day at the Tidewater AREC March 22, Frame said in last year’s research the earliest small grains used as a cover crop headed and flowered around April 14. He noted that the AU Sunrise and AU Robin crimson clover reached 50% bloom on April 14, so it wasn’t quite early enough to benefit corn in the region.

The same was true for hairy vetch, which reached 50% bloom on April 22. Winter peas used as a cover crop didn’t reach 50% bloom until May. However, brassicas used in the cover crops did bloom early, reaching 50% bloom by March 24.

Frame said the goal is to develop an Extension publication to help Virginia farmers determine which cover crop species bloom early enough to provide nitrogen accumulation to benefit the corn crop.

“If you’re looking for biomass and nitrogen uptake, hairy vetch is going to be hard to beat,” Frame said.

In the Virginia Tech research, Frame said hairy vetch provided almost 280 pounds of nitrogen per acre in the biomass, which he called “absolutely phenomenal.” He added that legumes are also highly prolific in mining potash.

Frame said cover crops are beneficial in providing nutrient cycling and providing plant-available nitrogen. “We’ve measured as much behind a legume cover as 95 pounds of nitrogen in the top three feet of the profile following legume termination in the month of May. It’s breaking down very rapidly in terms of nitrogen.”

A benefit of legumes is that they can mine nitrogen, phosphorus, and potash from the subsoil. Frame said a goal of the research is to discover if those nutrients from the cover crop can be used by the cash crop following the cover crop.

Frame said legumes in a cover crop may improve potash cycling; if true, legume cover crops can help with potash deficiency while decreasing the use of nitrogen fertilizers since legumes bring nitrogen to the soil.

“If potassium is available from legume cover crops, and you can bring 300 pounds of potash to the soil surface for a cotton crop, and it’s breaking down over the season, that’s a great benefit,” Frame said. “I’m not saying we can go to zero on our potash applications because the cover crop is mining it, but if we can bring nutrients from the depth to the top and cycle it, we could lower fertilization rates moving forward using legume covers. Legumes are much better at cycling potash than small grains. The added benefit of a legume is you’re getting that nitrogen preceding corn and cotton.”

Frame said legume crops will never allow farmers to do away with applying nitrogen to corn or cotton completely, but they can be used to lower the rates of fertilizer applied to crops.

“We average anywhere from 160 to over 200 pounds of nitrogen in a legume mix based off of our biomass production. We’ve done that now for about 10 years,” Frame said.

The system works in cotton in Virginia since cotton is planted in May in the commonwealth. Frame said the goal is to make the system work in corn, which is planted there in late March to April. Frame and his colleagues are continuing to evaluate cover crop species that bloom earlier enough to provide nitrogen accumulation for corn.

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