Part 1, Section 10: Cover Crops
Cover Crops
COVER CROP SELECTION AND MANAGEMENT
Cover crops can be annual, winter annual, biennial, or perennial plant species. This section focuses on the most common cover crops in Pennsylvania’s field crop rotations. These cover crops are planted after main crops are harvested, typically during the later summer/fall.
Cover crops include grasses, legumes, and other broadleaf species (Table 1.10-6). The most common grass species are rye, wheat, triticale, barley, oats, and annual ryegrass. Winter rye is the most common winter-hardy cover crop in Pennsylvania. It needs to be planted in September/October to allow for the production of enough biomass to provide good groundcover and absorb excess nitrate from the soil. Seeds have been broadcast or flown onto standing corn, but if the soil is dry, as it often is in late summer, stands tend to be poor and competition from corn is too great unless the corn is harvested for silage. Broadcasting rye with a fertilizer spreader into standing soybeans at time of first leaf drop has been successful in southeast and central Pennsylvania (Table 1.10-4). The cover crops increased soybean yield the succeeding year. Most uniform stands are achieved if rye is broadcast and lightly worked into the soil or drilled with a grain drill, however.
There are different methods of terminating the rye cover crop. It can be killed 2 or more weeks before no-till crop establishment, incorporated, or harvested for silage (Table 1.10-5). If water stress during the growing season occurs, a dead rye mulch will provide water-saving benefits for the following crop. If the rye is killed shortly before crop establishment and rains are insufficient, there is a risk that the main crop will suffer water shortage early on because the rye has taken up most of the soil moisture. Once rains resume, the dead mulch will save water. If the rye is taken off as silage immediately before crop establishment, negative effects on the main crop are likely because the living rye has reduced water content for the young crop and there is no dead mulch available to save water. The major benefits of a rye cover crop will be improved soil structure, protection of the soil in the winter, reduced nitrate leaching, and the addition of organic matter. If the main crop is no-tilled into a rye cover crop, there will also be additional soil erosion protection and water-saving benefits after the crop is killed. Nitrogen taken up by the rye will become available very slowly if the rye is killed after it heads out. Timing of nitrate release will depend on whether the crop is incorporated or not. If rye has headed out it will have a high C:N ratio and may tie up nitrogen. It is therefore recommended to supply the following corn crop with sufficient nitrogen to avoid nitrogen deficiency. Although rye can reduce corn yields because of an allelopathic effect, it seems this effect can be largely overcome by supplying enough nitrogen to the corn.
Wheat, triticale, and barley can be used in a similar fashion as rye. These cover crops are less winter hardy than rye (in the order triticale, then wheat, and then barley). When grown to maturity, rye matures first, then barley, then triticale, and finally wheat. Rye grows extremely fast in the spring and can be challenging to manage because it grow to be 6 feet tall. Triticale is similar to rye. Common wheat and barley varieties are shorter than rye and triticale and are therefore easier to manage. However, wheat and barley accumulate smaller amounts of biomass than rye and triticale and can be expected to have similar but smaller benefits for erosion control, soil quality improvement, nutrient capture, and water conservation. All these cover crops need to go through a vernalization period before becoming reproductive and will therefore stay vegetative in the fall. Spring oats do not have to go through a period of vernalization before becoming reproductive. The result is that, if planted early enough, oats will begin to produce a stem and sometimes a head before it winterkills. The result is that oats, when established in July/August, can produce more biomass in the fall than winter cereals. The oat residue can be quite resistant to decomposition because of its high C:N ratio and in this scenario can leave proper mulch cover for spring no-till planting. However, no living roots will be present after a killing frost (December/January), and the soil improvement achieved with oats and nutrient scavenging are therefore smaller than with the winter cereals. Annual ryegrass is another grass species that can survive the winter in Pennsylvania if established on a timely basis (up to mid-Sept in southern Pennsylvania). It can produce high-quality forage in the fall. It should not go into the winter with more than 6 inches of growth, or it may suffocate itself. Annual ryegrass needs to be monitored closely in the spring to avoid it going to seed (can become a weed in following crops). It is not very sensitive to being killed by glyphosate (in contrast to the cereals).
Legume cover crops include hairy vetch, crimson clover, red clover, and field peas. Hairy vetch is a leguminous crop that should be seeded before the second week of September in the south of Pennsylvania, the end of August in the central part, and by mid-August in northern parts of the state. If insufficient growth is obtained bfore the winter, hairy vetch may winterkill. It des not grow very much in the fall, but takes off rapidly in the spring. The more growth can be obtained, the more nitrogen is fixed. In a review of studies, hairy vetch contributed the equivalent of 5 to 150 pounds of nitrogen per acre to following corn. A vigorous hairy vetch stand can meet most of the nitrogen demands of a corn crop. Hairy vetch and rye make an excellent cover crop mix. Both need to be killed by tillage or herbicides in the spring, although some researchers and farmers are now experimenting with rolling hairy vetch and winter cereals at time of flowering to kill them without herbicides or tillage. Hairy vetch may not be killed by glyphosate alone (2,4-D and banvel are more effective). Crimson clover is another legume that has survived the winter consistently in trials in southeastern Pennsylvania. It needs to be established early, much like hairy vetch. In the few studies where it was determined crimson clover contributed up to 80 pounds of nitrogen per acre to following corn. Crimson clover has tough stems that can be challenging to plant through with a no-till planter. Red clover is a good choice if it can be established before the summer heat. It can be broadcast in February/March in a growing winter cereal such as wheat or barley and will then take off after the harvest of these crops. Field peas are not winter hardy in most of Pennsylvania but may have a place after small grain harvest. The large seed makes it necessary to use a high seeding rate.
Buckwheat is a cover crop that is known for its fast growth; it is an excellent smother crop that can suppress weeds. The two brassica species that have received most acclaim as cover crops are rape and forage radish. Rape can survive the winter in southern Pennsylvania if established before mid-September, whereas forage radish winterkills. They both establish quickly in the fall and enjoy high-fertility conditions (such as in a field with manure history). The radish has a big taproot that is used for “biodrilling”—it creates large holes in the soil that can be access points for water and air. The forage radish suppresses winter weeds and, because it decomposes quickly, can leave the soil bare in the spring. It is therefore recommended to mix it with a small grain such as oats (winterkills), wheat, or rye.