Robert Mullen, Jim Hoorman, Rafiq Islam
Ohio State University
Dr. Robert Mullen:
Conservation benefits of cover crops, especially reducing soil erosion, have been clearly demonstrated; however, the agronomic benefits are not as clear and vary with crops and timing. Although a cover crop following corn should be beneficial, corn is typically harvested in late October when cool temperatures make it difficult to establish a cover crop. A Canadian study, which evaluated four cover crops following corn and using both mold-board chisel plow and no-till methods, documented a yield increase of approximately 40 bu/acre for all cover crops and both methods when no additional nitrogen fertilizer was applied. However, when nitrogen fertilizer was applied at a rate of 134 lbs/acre there was no increase. Comparison of fall versus spring kill on the no-till plots showed no additional accumulation of nitrogen during the winter months when nitrogen was not added, but a slight increase for the red clover and oilseed radish when nitrogen was added.
Research conducted by Dr. Don Eckert, OSU, in Wooster and Northwest Ohio from 1983-85 showed mixed results for cover crops increasing yields. A 2005 literature review of 82 cover crop research studies concluded that it is possible to supplement nitrogen with a cover crop (leguminous species) but the amount of increase is difficult to determine. In Ohio, it would appear that cover crops have the greatest potential to increase yield if they are legumes planted after wheat in a wheat-soy-corn silage rotation; however, the costs of seeds and planting should be considered.
Jim Hoorman:
Soil organic matter (SOM) and the ratio of carbon to nitrogen (C:N), which impact nutrient availability and stability, are affected by tillage and cover crop practices. Conventionally tilled fields have 1-3% SOM as the bacteria that dominate have 20-30% efficiency in recycling carbon. In comparison, long-term, no-till fields have 2-4% SOM and 40-50% carbon use efficiency primarily due to the existence of fungi that form a mycorrhizal network with plant roots, increasing uptake of nutrients and providing glomalin, a sticky substance essential for soil structure.
The availability of nitrogen in the soil is affected by the microbial decomposition of organic materials and the C:N ratio. Organic materials with a C:N ratio greater than 20 decompose slowly and require additional nitrogen which may be removed from the soil or added via manure or inorganic fertilizer. Grass cover crops killed after May 1 have a higher C:N ratio, tending to tie up nitrogen in the short-term as microorganisms remove it from the soil during decomposition. Grass covers killed around April 1 have a lower C:N ratio, decomposing and releasing nitrogen to the next growing crop more quickly. Cover crops can also decrease soil erosion by 90% and protect streams and lakes by decreasing sediment transport by 75%, reducing pathogen loads by 60% and reducing nutrient and pesticide loads by 50%. The two most cost effective practices for reducing soil erosion are long-term no-till farming ($3 per ton of sediment saved) and using cover crops ($2 per ton of sediment saved).
Dr. Rafiq Islam:
Ongoing research at Piketon, the Ohio Agricultural Research and Development Center/OSU in southwestern Ohio, is evaluating the effect of 32 different cover crops for increasing biomass contribution of N for crop production, recyling of other nutrients (e.g. P and K), control weeds, and improve soil quality. Of those tested, cowpeas and winterpeas provided the highest biomass N contribution and sudan-sorghum grass, spelt, annual ryegrass, cowpeas, and winter peas had greater biomass. Cover crops such as cowpeas and winterpeas were planted following wheat in a corn-soy-wheat rotation. Analysis of corn yields from plots using cowpeas and winterpeas, no cover crops with added nitrogen and with only added nitrogen indicated that cover crops cannot completely replace nitrogen fertilizers. However, incorporation of suitable leguminous cover crops in no-till system can reduce the nitrogen fertilizer application by 50 to 80%, and greatly improve the soil quality. Challenges of using cover crops include difficulty in establishing the cover crops, competition of the cover crop for nitrogen, and immobilization of soil nitrogen. Finding the right combination of cover crop and production crop is also an issue that needs to be addressed.
Discussion:
Columbus participant questioned the cost of rye grass and whether the amount used would result in a heavy enough crops. Mullen replied that the amount was from the Ohio Agronomy Guide and the cost would be $19/acre.
Participant in Columbus noted that if a cover crop is actively growing in a no-till system, it is relatively easy to kill; however, conventional till or transitional fields may have poorer crops which are difficult to kill. Hoorman noted that it is important to identify different ways to inter-seed a cover crop so it can be established early and quickly. Annual rye has the benefit of growing throughout the winter.
Columbus participant asked when cowpeas are planted and harvested. Islam noted they are drought tolerant, so can be easily planted in mid to late July immediately after harvesting wheat. They are killed by the cold in winter so no herbicide is needed. Winterpeas are cold tolerant and can be planted in October after harvesting soybeans. Corn can be planted within winterpeas residues in end of May or early June.
Mullen asked how the nitrogen fertilizer equivalent of the cover crop was determined. Islam responded that the total N for the soil was measured before the cover crop was planted and after it was killed, then the difference calculated. Because no fertilizer was added, it was assumed any additional N was from the cowpeas and winterpeas biological N fixation Mullen noted that there is a high variability of total N in soil. Because the cover crop adds a very small percentage of N, there is a high margin of error.
Warren Dick (OSU) noted the corn-soy-wheat rotation used. If there is increasing pressure to grow more corn for ethanol production, is there any method for planting the cover crop after corn other than flying it in? Islam noted that next year he will be testing winterpeas after corn with no additional N.
Mullen noted that much of the corn being planted is Bt corn which stays green longer and may be an issue because its breakdown will be delayed. (?)
Harold Keener (OSU) asked if the herbicides used to kill the cover crop could have a residual effect. Islam responded that it could be a potential problem.
A Columbus participant asked for clarification on the 5% acetic acid (commercial vinegar) used to kill the cover crops. Islam is more effective on broad leaf plants than on grasses.
Notes by Mary Wicks
The OCAMM (Ohio Composting and Manure Management) Program is funded by OARDC/OSU.