Part 1, Section 10: Cover Crops
Cover Crops
COVER CROP SELECTION AND MANAGEMENT
Winter-annual cover crops
Winter cover crops include rye, wheat, barley, ryegrass, and hairy vetch. Crimson clover is a common leguminous winter cover crop in the southeast, but it may not survive the winter in Pennsylvania.
Winter rye is the cover crop that is most often used in Pennsylvania. Seed between early September and early October to allow for the production of enough biomass to provide good ground cover and to absorb excess nitrate from the soil. The later rye is planted, the higher the seeding rate should be to achieve adequate biomass production. Seeds have been broadcast or dribbled into standing corn, but if the soil is dry, as it often is, 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 1.10-3). The cover crops increased soybean yield the succeeding year. If rye is broadcast on the surface without incorporation, it is necessary to use a high seeding rate (2-3 bushels/A). Most uniform stands are achieved if rye is broadcast and lightly worked into the soil or drilled with a grain drill.
There are different methods of terminating the rye cover crop. It can be killed 2 or more weeks before no-till crop establishment, it can be incorporated, or it can be harvested for silage (Table 1.10-4). 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 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 most likely because the living rye has reduced water content for the young crop and there is no dead mulch available to save water after that. If rye is harvested for silage, it is therefore recommended to cut it 2 weeks prior to planting 6-8 inches above the soil surface to leave a fair amount of residue cover. Finally, the major benefits of plowing under a rye cover crop will be the improved soil structure, the protection of the soil in the winter, reduced nitrate leaching and the addition of organic matter. The same benefits result if the main crop is no-tilled into a rye cover crop, plus there will be additional soil erosion protection and water saving benefits after the cover 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 will tie up nitrogen if incorporated. Supply the following corn crop with sufficient nitrogen to avoid nitrogen deficiency. Rye can reduce corn yields because of an allelopathic effect. To avoid this problem, kill the rye before it is 2 feet high, and 2 weeks before corn planting.
Barley, wheat, and ryegrass can be used in a similar fashion as rye. They can be expected to have similar but smaller benefits for erosion control, soil quality improvement, nutrient-capture, and water conservation because they produce less biomass than cereal rye. Ryegrass needs to be monitored carefully, because it can also be a problem weed in field crops. Hairy vetch is a leguminous crop that should be seeded before the end of August in the southern part of Pennsylvania. If insufficient growth is obtained before the winter, hairy vetch may winter kill. It does 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. 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.