April Rains Affect May Forages; Alfalfa Turns Yellow in Wet Fields

Alfalfa fields fading from green to yellow may mean too much water. Farmers see the change after record-setting rains.

Waterlogged soil kills nitrogen-fixing bacteria on roots, says Craig Roberts. The University of Missouri Extension specialist alerted area specialists of the problems. There are fixes.

A large part of southern Missouri had super rains. Pat Guinan, MU Extension climatologist, said April 2017 will be one of two wettest Aprils on record. You must go back to 1994 to find a wetter April.

This year, dozens of counties had from 10 to 20 inches of rain.

On the weekly MU teleconference, the agronomists share ideas across the state.

"We learned about waterlogged forages in the Great Flood of 1993," Roberts said in a teleconference.

Flooded crops in river and creek bottoms aren't the only problem, Roberts said. Hilltop soils become water logged also.

Excess water cuts oxygen in the soil. It also kills soil microbes, hurting plant growth.

An early unexpected sign may be alfalfa turning yellow. That's a sign of nitrogen deficiency.

Most people think, but alfalfa makes its own nitrogen. Why does the legume need nitrogen?

Too much water drowns the rhizobium microbes on alfalfa roots. They are the bacteria taking oxygen from the air to feed the plant.

In the previous big flood, Roberts came up with a way to re-inoculate the fields.

Before planting legumes, farmers coat seeds with fresh inoculum. That carries the microbes into the soil.

Roberts' cure is to treat orchardgrass seed with the inoculum. Then drill grass into the thinning stand of alfalfa. A normal practice is to drill grass seed into the thinning alfalfa stands. This boosts hay yields.

By treating the grass seed, inoculum re-enters the soil. Bacteria broadcast on top of the soil won't survive, Roberts warns.

With fresh inoculum, the root nodules regrow. The alfalfa will flourish.

Fresh alfalfa seed cannot be seeded into an old stand. As a defense, existing alfalfa kills the new seedlings.

As a first step, Roberts says, "Check the roots on yellowing alfalfa." Likely there will be a lack of robust nodules.

The inoculation of orchardgrass seed does two things, he said. It adds grass to the hay crop. But the inoculum preserves the alfalfa.

It's a wild idea, but it should work, Roberts said.

State specialists warned against working wet fields before they dry. Wet tracks add soil compaction. That also limits oxygen in the soil.

Farmers with flooding problems can seek help from regional specialists through their county MU Extension Center.