Four-year study yields insights on benefits, limits of cover crops

Welcome to Pocket Science: a glimpse at recent research from Husker scientists and engineers. For those who want to quickly learn the “What,” “So what” and “Now what” of Husker research.

What?

Cover crops — those generally grown after harvesting cash crops like corn and soybean — are planted with an eye toward supplying nutrients to soils, mitigating the erosion of those soils, and limiting the leaching of nitrate into groundwater. Though the planting of such crops has risen sharply in Nebraska, they still cover less than 4% of the state’s arable land.

That slow adoption has stemmed in part from the difficulties of growing cover crops amid the hard freezes of late autumn in the Midwest. In response, some farmers have tried swapping out a traditional technique — the post-harvest drill planting of seeds, which embeds them in soil — in favor of broadcasting seeds into still-maturing fields of corn or soybean during the late summer. The upsides of warmer days, more precipitation and lower expenses can be offset by lower germination rates of the surface-sitting seeds, leaving farmers uncertain which technique to apply. Whether certain cover crops might outperform others, and under which conditions, also remains unclear.

So what?

Nebraska Extension’s Katja Koehler-Cole and fellow Husker researchers sought to address those questions with a four-year, three-site study conducted in fields that either grew corn every year or rotated corn and soybean. The team tested not only the two planting techniques but multiple cover crops: cereal rye; a mix of rye, legumes and a brassica; a more diverse mixture of rye, oats, legumes and two brassicas; or legumes alone.

Overall, the cover crops seemed to benefit from pre-harvest broadcasting when followed by a wetter autumn, which boosted their biomass by an average of 90%. A drier fall, meanwhile, appeared to improve the productivity of conventional post-harvest plantings.

Cereal rye boasted the best production, followed by the mixes, then legumes. Both rye and the mixes took up notable amounts of nitrogen from soil, especially in fields alternating between corn and soybean, with rye showing particular promise for curbing the migration of nitrate to groundwater. The pre-harvest planting of rye did decrease corn yields slightly, by up to 4% — possibly due to that same nitrogen capture — though the mixes showed little effect on yields.

Now what?

Systematically comparing the effects of crop rotations — corn-only versus corn-soybean, for instance — should help future studies offer more specific guidance about growing various cover crops under various seasonal conditions, the researchers said. But the team’s findings do suggest that cover crop mixes, or a rye planting with an adjusted fertilizer schedule, could benefit soils and groundwater while minimizing any reductions in cash crop yields.