Seven Months of Bare Ground

Drive across Iowa in mid-November and you will pass field after field of nothing. The corn came out in October. What is left is stubble, and under the stubble, soil. It looks orderly. It looks like a job well done.

It is also the start of the longest stretch of the year when the world’s most productive farmland holds itself together with almost nothing living in it. From harvest in October until a new corn canopy closes the following summer, roughly seven months pass with little or no growing cover. Rain falls on bare soil. Snow melts across it. Spring storms arrive before the new crop has any leaves to speak of.

The consequences are measurable. The Iowa Daily Erosion Project puts average hillslope soil loss at about 5.8 tons per acre per year. Soil reforms at roughly half a ton annually. We are drawing the account down about ten times faster than it refills, and Iowa has lost an average of 6.8 inches of topsoil a year since 1850. The same water carries nitrogen left over from fertilization. Iowa alone supplies roughly 29 percent of the nitrate reaching the Mississippi-Atchafalaya Basin, feeding a summer dead zone in the Gulf (water so short of oxygen that the aquatic organisms cannot survive in it).

The fix seems obvious, and it is not a new idea. Plant something to hold the ground over the winter. Cover crops do exactly this, and they work. The problem is that they must be planted again every year, in a narrow window after harvest when the weather and the equipment rarely cooperate, then terminated every spring before the corn goes in. Each cycle costs money, time and seed, and adoption across the Corn Belt has stayed low for reasons that have little to do with belief in soil conservation.

So researchers asked a different question.

What if the cover were perennial?

Planted once, left in place, alive underneath the corn year after year.

This is the perennial groundcover system, and it changes the economics entirely: the establishment cost is paid once rather than every autumn. In field trials, perennial groundcover has kept more than 85 percent of the soil surface covered through the year, while building soil carbon and nitrogen and reducing erosion.

A groundcover still growing in May and June is not a groundcover. It is a weed. It takes water and nitrogen that the corn needs, and the corn can sense it. Light reflected off the nearby leaves triggers a shade avoidance response, and the plant invests in height rather than grain. The plant grows tall and green, reaching for the sun, then has less energy to invest in producing its fruit – the corn. Kentucky bluegrass, a cool season grass, matches bare-ground corn yields only when chemically suppressed before the corn emerges. That works, but it adds a cost and a spray pass.

What we want is a grass that suppresses itself.

Some cool-season perennial grasses do exactly this. They undergo summer dormancy: leaf growth stops and the leaves and stems above ground turn brown and die back as days lengthen and temperatures rise. The grass is not dying in summer, and it is not wilting for lack of water. Summer dormancy is an internal developmental program triggered by long days and high temperatures, and it happens whether or not the soil is moist. The grass shuts down on its own and greens up again in autumn.

For a groundcover, this is close to ideal. The plant is active in the cool spring and autumn shoulders, when bare soil is most exposed, and goes quiet in midsummer, precisely when the corn is claiming every resource available. No herbicide required.

My research as a FFAR Fellow at Iowa State University focuses on one candidate species: Sandberg bluegrass, Poa secunda. It is a native perennial bunchgrass of western and central North America, among the first grasses to green up in spring, setting seed and browning by early summer with pronounced summer dormancy under long days and heat. In recent corn trials, a summer-dormant groundcover did not reduce corn yield.

Moving a western grass into Iowa raises questions nobody has answered yet, and those questions are the basis of my research. Can it survive an Iowa winter, which is colder and wetter than anything in its native range? The species is enormously variable in its genetics and reproduction, so which plants should we start from? And what happens inside the plant when it goes dormant, at the level of which genes switch on and off, so that we can breed for the trait rather than hope for it?

I am grateful to the FFAR Fellows Program and to my sponsors, the Raymond F. Baker Centre for Plant Breeding at Iowa State University and Corteva Agriscience. The program has pushed me to explain this work to people outside my field, which has proven harder and more useful than I expected.