University Park, Pa. -- Increasingly, Pennsylvania farmers are using cover crops to limit erosion from fields, control weed growth, fix nitrogen in the soil, feed livestock and produce biomass for energy. But depending on an agricultural producer's needs, all cover crops are not created equally, according to a crop and soil scientist in Penn State's College of Agricultural Sciences.
When it comes to controlling weeds, some cover crops definitely are better than others, contends Bill Curran, professor of weed science.
To help farmers determine how best to integrate cover crops into their operations, Curran and colleagues Eric Nord and Rich Smith, both postdoctoral associates, and Matt Ryan, a doctoral degree candidate in the Department of Crop and Soil Sciences, recently published a new fact sheet titled "Suppressing Weeds Using Cover Crops in Pennsylvania."
Based on decades of Penn State agricultural research, the publication will guide farmers through the process of selecting and establishing cover crops to help suppress weeds.
"The life history of a plant species affects how it may be used as a cover crop," Curran said. "Summer or winter annuals, biennials and perennials can be used for cover crops where needed. The choice of cover crop species will depend on management goals. Winter annual cover crops can generally fit into a crop rotation without requiring that land be fallowed."
Legume cover crops provide an important source of nitrogen and can replace or reduce the need for nitrogen fertilizer, noted the authors. This is of particular importance preceding nonlegume crops. Grass cover crops are particularly beneficial in erosion reduction because they have a fibrous root system and can produce many stems.
The latest in Penn State's acclaimed "Agroecology in Practice" series, the publication covers important topics, such as types and establishment of cover crops, managing cover crops to suppress weeds, control of cover crops and disadvantages of their use.
"In some cases, cover-crop mixtures may be better than individual cover crops," Curran said. "For example, oats may be used as a nurse crop for hairy vetch planted in early fall. The oats grow more quickly in the fall, providing partial soil coverage and nutrient-trapping benefits before they are winter-killed, which prevents competition with the hairy vetch in the spring."
Cover-crop planting should take into account the fertility of the soil, Curran pointed out. A soil test is a good way to begin. Pest history also should be considered, as should the history of herbicide application.
Cover crops can be established by conventional, no-till or broadcast seedings, though broadcast seeding is generally less successful. Frost seeding may be effective for the establishment of cover crops in early spring. Although less consistent, aerial seeding can allow a cover crop to be established before the cash crop is harvested.
"When selecting cover crop species, choose crops based on your objectives," Curran advised. "If weed suppression is an objective, select an aggressive species that will cover the ground quickly. If you desire a cover crop that will protect the soil through the fall and winter and suppress winter annual weeds, plant a winter cereal in late summer or early fall.
"Establishing a hardy winter cover, such as cereal rye, as early in the fall as possible will result in greater cover crop biomass over the winter and rapid growth during the spring," Curran said. "Other establishment dates may be preferable for different cover crops depending on the species and your objectives."
All cover crops help prevent erosion. Soil that is covered is less prone to erosion for at least three reasons. "First, living leaves and plant residues soften the impact of raindrops, reducing the amount of soil they dislodge," Curran said. "Second, plant stems and residues reduce the speed of water flowing over the soil surface. Third, roots hold the soil particles, preventing them from washing away."
Cover crops can be a useful tool for suppressing weeds in cash crops, but they also have many other benefits, according to the new publication. They leave organic matter in the soil, which promotes good soil structure, increases drainage and aeration, and holds nutrients. Also some, such as legumes, through their association with certain soil bacteria, are able to fix nitrogen from the atmosphere. This nitrogen is slowly released for cash crops when the cover crop residues decay.
"And some cover crops, especially grasses, can be used for livestock feed, either by grazing or mechanical harvest," Curran said.