Leaders in Organic

PROFILE

SALINAS, Calif. – Cover cropping in organic systems was a new area of study for Dr. Eric Brennan when he launched his research at the Agricultural Research Service’s (ARS) research station in Salinas in 2001.

As the first ARS researcher nationwide dedicated solely to organic systems, Brennan’s first step in setting his research agenda was to meet with farmers to hear first-hand what farm practices were in use and what challenges farmers faced. A universal interest, says Brennan, was cover cropping.

“My reason for focusing on cover crops was to pick something that would be broadly applicable; would apply to small-scale growers; large-scale growers; diversified growers; growers that had a less diverse rotation,” says Brennan.

Over his eight years of inquiry, Brennan has made numerous discoveries about the value cover cropping brings to organic production of high-value vegetable crops like spinach, lettuce, and broccoli.

The Salinas Valley features a mild climate, fertile soils, and dry summer conditions. Cover crops, says Brennan, play a huge role in improving and maintaining soil quality in these tillage-intensive conditions. They also affect nutrient leaching, soil biology and weed management -- areas of concern for many farmers.

“Good soil management cuts across the system. It doesn’t matter whether you are a conventional grower or an organic grower. The soil needs organic matter, and the most sustainable source of organic matter in this area is cover crops,” says Brennan.

Eric Brennan/USDA-ARS
One of Eric Brennan's many cover crop trials, this one located in Hollister, California in 2004.

Seeding Rate is Key
One of Brennan’s most significant findings involves cover crop seeding rates. He discovered early on that typical seeding rates were based on recommendations for cereal grain production of crops like rye.  Rye is planted as a cover crop to reduce leaching, add organic matter, and to suppress weeds. Doubling or even tripling the seeding rate has a nominal effect on crop cost, but can, in some fields, produce a several-fold increase in weed suppression.

“That’s important because organic growers are trying to minimize weed seed production during the cover cropping period because…the seeds that are produced while cover cropping can germinate during the subsequent vegetables and increase your weed management costs,” says Brennan.

Cover Crop Frequency Makes a Difference
While cover cropping provides a multitude of benefits, the practice complicates early spring plantings and takes land out of cash crop production, thus some farmers choose to cover crop infrequently. Brennan says research from his long term (seven year) trial on a loamy sand soil showed that  annual cover cropping is optimal and that annual compost applications cannot substitute for a lack of cover crop.

“In our long term trial, if we cover crop every year, we have several times higher lettuce yields than if we cover crop every fourth winter. So, in this sandy soil at our research farm, frequent cover cropping is essential. If you can’t cover crop frequently in this soil, you will probably have to use much higher fertilizer levels,” says Brennan.

Seed Mixes Provide Challenges
Cover crop seed mixes provide research challenges, says Brennan, and there is still a lot of research to be done in this area. He says more focus is needed to develop cover crop mixes that will suppress weeds, reduce nitrate leaching, and reduce the need for supplemental fertilizers in later vegetable crops.

“When you grow one crop it’s complicated, but when you grow several different species in a mixed planting, they can interact with each other. We’re just starting to get some good information on what some optimum mixes may be,” says Brennan.

Eric Brennan/USDA-ARS
Eric Brennan standing in one of his fava bean cover crops in one of his early cover cropping research trials at the USDA-ARS research station.

Looking Ahead
In addition to long-term studies in high-till systems, Brennan’s research focus will begin to evaluate no-till methods of managing cover crops over the next five years. No-till involves leaving cover crop residue on the soil surface rather than incorporating it into the soil with spaders or discs. Brennan believes a proven no-till or reduced-till regime may help farmers save money, do a better job of soil management and reduce dust, a key issue in California.

“Like any management system, when you try something new, it may take a few years to tweak it before it will actually perform at its best,” says Brennan.

Results of Brennan’s cover crop research were published in the January 2009 issue of Agronomy Journal.