Farming Magazine - August, 2009

GROWING

Don't Overlook Oats

By Vern Grubinger

I’m way past sowing my oats, but not when it comes to cover cropping. In fact, that’s something I do with regularity, since I find oats to be one of the most useful tools for late-season soil spousery. There’s a lot to like about oats as a cover crop: they’re reliable, inexpensive, have a pretty long planting season and, because they winterkill in northern climates, the residue is easy to handle the following spring.

PHOTO BY VERN GRUBINGER.
Spring-seeded oats can also be used to suppress weeds and add organic matter prior to a summer or fall vegetable crop.
 
PHOTO BY VERN GRUBINGER.
A good use of oats as a winter cover is to sow them as soon as a block of crops is harvested in the fall to get the most growth and ground cover possible in the remaining growing time.

Oat overview

Oats are an annual grass. When sown for grain production, oats are planted in the spring so they can grow, flower and produce seeds before dying in the autumn. When grown for a winter cover crop, oats can be sown from end of summer through the fall. They thrive under cool, moist conditions on well-drained soil, and stands generally perform poorly when it’s hot and dry.

Multiple benefits

As a fall-planted cover crop, oats provide quick, weed-suppressing biomass, take up leftover soil nutrients and can improve the productivity of legumes when planted in a mixture. Their fibrous root system also holds soil in place, protecting against erosion. With good growing conditions and good management, including time of planting, oats can produce 1 to 2 tons of dry matter per acre from a late-summer or early fall-seeded stand, helping to maintain soil organic matter levels.

Mopping up

After the harvest of many vegetable crops in late summer, there may still be a lot of available nutrients in the soil, either from unused fertilizer, decomposing manures or compost or from the mineralization of soil organic matter. Why waste valuable excess fertility by allowing it to leach or wash away? Planting oats promptly after a vegetable crop has finished up allows the cover crop to take up excess N and small amounts of P and K, keeping those nutrients on your farm for later use when the oats break down. Studies have shown that late-summer plantings of oats can absorb as much as 77 pounds of nitrogen per acre over an eight to 10-week period.

Smothering

Quick to germinate, oats are a great smother crop that out-competes weeds while growing. Once they are done growing, they can also provide an allelopathic residue that can hinder germination of many weeds for a few weeks. To avoid concerns about suppression of a cash crop that follows oats, it’s a good idea to wait three weeks after killing the oats before planting a subsequent crop.

Nurse crop

Oats are an excellent nurse crop (companion crop) for legumes planted in a mixture with them. Adding anywhere from 35 to 75 pounds per acre of oats to the seeding mix helps slow-establishing legumes like hairy vetch, clovers or winter peas get going by holding soil in place and suppressing weeds, while also increasing biomass production of the mix versus either crop grown alone. The climbing growth habit of some legumes such as hairy vetch can take advantage of the more upright oats to develop a better canopy, with the oats serving as a natural trellis.

Establishing a good cover

For a winterkilled cover, oats are usually seeded in time to allow six to 10 weeks of cool-season growth before the first hard frost. However, even three or four weeks of growth can lead to a protective, albeit small, cover crop. The later you seed, the higher the seeding rate should be in order to assure that bare spots are minimized. Oats will winterkill at about 18 degrees Fahrenheit.

Moderately fertile soil gives the best stands of oats—if the field as been well-managed over time and the preceding vegetable crop had a high yield, then little or no fertilization may be needed. If the soils are low in fertility or if the previous crop had nutritional issues, then soil test and apply amendments as needed before sowing oats.

Broadcasting or overseeding will give the best results for the least cost, unless planting into heavy residue. To establish a thick winterkilled cover, broadcast seed at a high rate of 3 to 4 bushels per acre (or about 150 to 200 pounds per acre, in terms of 50-pound bags). Disk lightly to incorporate the seed. If drilling, seed oats at 2 to 3 bushels per acre. When seeding oats as a nurse crop with a legume, use a lower rate of 1 to 2 bushels per acre so as to avoid competition. Seeds should be planted about .5 to 1-inch deep for rapid emergence, and no deeper than 2 inches.

A light disking in the spring will break up the brittle winterkilled oat residue. That exposes enough soil for warming and timely planting. Or, no-till directly into the mulch and the residue will decompose readily early in the season.

PHOTO BY DOUG WILSON, USDA ARS.
Oat is an erect annual grass with a fibrous root system. Although it is often grown as a grain, in the Northeast it makes a good late-summer-sown cover crop which winterkills, leaving a protective dead mulch that is easily incorporated in the spring.

Spring planting

Oats can also be grown as a spring-planted cover crop or green manure, used to produce early-season organic matter and suppress weeds before a warm-season (summer-planted) cash crop. It’s relatively easy to kill the oat stand by mowing soon after the vegetative stage, when the seed are in the milk or soft dough stage. Killing the oats too early reduces the amount of biomass produced and you could see some regrowth. But, waiting too long could make tillage of the heavier growth more difficult in a conventional tillage system and could deplete soil moisture needed for the next crop if conditions are dry. Timely killing also is important because well-established or mature oat stands are high in carbon and could tie up soil nitrogen.

After you incorporate spring-planted oats, it’s a good idea to allow two to three weeks before planting a vegetable crop to avoid any allelopathic suppression of germination. The naturally occurring herbicidal compounds in oat residues that can hinder weed seed germination should break down and/or leach out of the soil after a few weeks.

Fallowing a field

A good spring cover crop mixture for resting land over the summer, before late-season vegetables or another round of cover crops for the winter, is field peas, oats and hairy vetch at a proportion of about 4:2:1 by weight. The oats provide support for the pea crop, which is followed by growth of the vetch in July. This mixture can provide excellent ground cover while also adding organic matter and nitrogen to the soil.

For more information on oats and many other cover crops, see “Managing Cover Crops Profitably,” 3rd edition, available online at www.sare.org/publications/covercrops.htm or in hardcopy for $19 plus $5.95 shipping from SARE Outreach, P.O. Box 753, Waldorf, MD 20604-0753. You can also call 301-374-9696 to order.

The author is Vegetable and Berry Specialist with University of Vermont Extension based at the Brattleboro office. He can be reached at vernon.grubinger@uvm.edu.