During 2009 and 2010, on-farm testing was conducted in southwestern Oklahoma to evaluate the use of summer cover crop in a continuous winter wheat cropping system. Guar, cowpea, soybean, and mungbean were seeded in late June following wheat harvest. Cover crops were terminated after 60 days of growth. The use of cover crops did not did not decrease wheat grain yield in the following wheat crop. Data generated from the test locations indicated cowpea to be the best choice of a summer cover crop. Inclusion of a cover crop appears to help increase diversity into a monoculture wheat cropping system.
Producers in western Oklahoma have identified two main areas that they feel need to be addressed to increase the sustainability of their cropping systems: 1) lack of crop diversity and 2) reduction of N fertilizer/increase in nitrogen use efficiency.
Lack of crop diversity: Continuous monocultures are the predominant cropping systems in this region. This lack of diversity increases pest pressures and increases needs for chemical inputs, especially nitrogen fertilizer and herbicides. Dependence on inorganic inputs threatens long-term sustainability. Growers in this region also rely heavily on conventional tillage practices which do not encourage the build-up of organic matter. Ongoing economic pressure from increasing fuel and nitrogen prices provides an ideal opportunity to educate producers about the use of cover cropping systems in order to move toward long-term sustainability. A recent survey in Garfield County Oklahoma (western OK) indicated that 79% of producers would consider the use of legumes as cover crops for nitrogen savings, but lack of knowledge and information was prohibitive (Cardwell, 2007).
In areas of western Oklahoma where precipitation (< 900 mm yr-1) is the main limiting factor in dryland cropping systems the use of cover crops has generally been viewed as unacceptable due to limited precipitation. The current general consensus of many producers in the western part of Oklahoma is that no suitable summer crops exist for their climate and no suitable alternative exists to replace wheat forage for cattle, so they are reluctant to grow anything except winter wheat. Quality of winter wheat has continued to decline in this area because of increased weed and insect populations as a result of minimal crop rotation. Another aspect of limited rotation is that no-till systems have not become popular in this region because of yield reduction under no-till with continuous winter wheat. In order for Oklahoma producers to successfully implement no-till in their cropping systems they must be willing to rotate crops. One potential is through the use of cover crops, especially during the summer months when temperatures are high and rainfall is highly variable. Even if producers continue to plant winter wheat each fall, using a summer cover crop would introduce some diversity into their systems.
Reduction of N fertilizer: Nitrogen contribution from leguminous cover crops is well documented throughout the US. Use of tradition-based fertilization practices results in approximately 65% of applied nitrogen fertilizer being lost to volatilization and leaching (Johnson and Raun, 2003). We have the technology to better predict crop N needs through use of optical sensor instruments. We propose sensor-based nitrogen recommendations as a mechanism to change the farmer behavior of tradition-based fertilization practices and move towards a more sustainable system.
The sensor-based system, accurately accounts for nitrogen mineralized by biological processes and, therefore, provides nitrogen recommendations that more accurately reflect crop needs. In contrast to soil sampling and yield goals, sensor-based nitrogen recommendations allow the producer to accurately gauge the amount of N that has been mineralized through natural biological processes (http://www.soiltesting.okstate.edu/SBNRC/SBNRC.php).
Cardwell, Bart. 2007. Conservation Till 101 Attendee Survey. Enid, OK. 8 February 2007.
Johnson, G.V. and W.R. Raun. 2003. Nitrogen response index as a guide to fertilizer management. J. Plant Nutr. 26: 249-262.
The objectives of this on-farm study will be to 1) determine the effect of cover crops on grain yield of the subsequent wheat crop and 2) determine nitrogen contribution using the Green SeekerTM Optical Sensor.
Four legume cover crops (cowpea, mung bean, guar, and soybean) were planted in a continuous wheat cropping system following wheat harvest in June of 2008 and 2009. Cover crops were planted in strips across each field in a randomized complete block design following wheat harvest. Strips were replicated a minimum of three times. The control treatment was the producer’s typical fallow practice during the wheat crops. Strips were 30 ft in width (depending on producer’s equipment) and a minimum of 100 ft in length. In 2008-2009 we had one location, the other two locations were to dry to establish after wheat harvest. In 2009-2010, 3 locations were established. Cover crops were planted following wheat harvest (June 1-30) and chemically terminated after approximately 60 days of growth. Then winter wheat was seeded in the fall. Wheat grain yields were collected from each treatment at each location. Wheat was harvested with the producer’s combine and grain yield was measured and recorded with a weigh wagon.
Cover crops were chosen to maximize nitrogen contribution and produce large amounts of biomass. The use of the Green SeekerTM Optical Sensor allowed us to determine the nitrogen contribution from the cover crops by determining the yield potential of the various N rates in the Ramped Calibration Strip. Potential grain yield was predicted using in-season spectral measurements, determined by using a handheld GreenSeeker sensor, collected between January and March in each year. The Ramped Calibration Strip Applicator applies increasing levels of N in a strip across a fixed distance (100 ft) (Figure 1). The levels applied range from 0 at the start of the strip to 120 lb N per acre. This fertilizer was applied pre-plant.
One Ramped Calibration Strip was placed in each cover crop strip to determine N contribution from that given cover crop. Wheat in that strip will be top-dressed with the amount of N that is indicated by the Calibration Strip.
In Feb. 2009, the one cover crop plot established in the summer of 2008 (El Reno, OK) was top-dressed based on NDVI (Table 1). The use of the Green SeekerTM Optical Sensor allowed us to determine the yield potential of each cover crop plot and apply N accordingly. Wheat grown after cowpea required more N (numerically) compared to the other cover crops and fallow. This was due to the higher yield potential indicated by the NDVI values. All of the wheat planted after cover crops, with the exception of mungbean, looked visually better compared to the fallow treatment (data not shown). The reason for the mungbean being similar to the fallow could have been from a poor stand of mungbean. Wheat grain yields were significantly higher in the plots that were planted to cowpea when compared to the fallow plots that had no cover crops growing on them during the summer months (Table 2).
In the fall of 2009, wheat was once again sewed after the termination of the cover crops. Three sites were established but only two sites were harvested for grain. No top-dress N fertilizer was applied at any of the locations since no N was called for at the time of NDVI measurements. This was in large part due to the environmental conditions that were experienced. We had a wet and warm fall that probably resulted in increased mineralization of organic N. Wheat grain yield at El Reno following cowpea once again was numerically higher than the fallow treatment (Table 2). Our yield potential was limited at this location because of delayed wheat planting due to wet soil conditions. At Gould in 2010, wheat grain yield was higher in the fallow treatment compared to the soybean cover crop treatment (Table 2). No other differences were observed.
Nitrogen contribution from the legume cover crops was indirectly measured with the Green SeekerTM, however, we did not try to estimate exact N contribution of cover crops. It is not known how much of the N from the cover crops was available to the following wheat crop. Presumably, at least a small amount would be available. We may have not truly seen the N contribution potential in the short study time.
Based on observations from these three sites, the inclusion of a cover crop such as cowpea can be beneficial to a monoculture winter wheat system. Cowpea appears to be the best choice for a legume cover crop from the crops that were evaluated. However, a longer rotation may be needed to break some of the pest pressures created form years of continuous wheat. Utilizing technology, such as the Green SeekerTM, allows producers to correctly measure the N needs of the wheat crop. This technology takes a lot of the guessing work out of N contribution from legume cover crops.
Educational & Outreach Activities
Four field days were held at the locations from 2009 to 2010. These field days were organized to highlight the On-farm SARE research project that we have with the producer in each area of southwestern Oklahoma. A total of 100 producers attended and were able to view cover crop plots first hand. To this date, data generated from these locations has been used in four presentations that had an audience of 325 producers. One newsletter article has been published in the Plant and Soil Sciences Extension newsletter that has a subscription of > than 600 producers, consultants, and educators. Two local newspaper articles were written on field days. In addition, one fact sheet is in publication that used data from the on-farm study.
Over 425 producers have seen the use of cover crops first hand and/or in meeting presentations. The knowledge of cover crops has increased in SW Oklahoma due to on-farm studies conducted from this project. One newsletter and fact sheet has or will be distributed as a result of the project.
At the moment it is difficult to gauge farmer adoption. Through verbal communication of producers at meetings and fields days we have definitely increased awareness of utilizing cover crops.
Areas needing additional study
Continued research in water use and nitrogen contribution of legume cover crops needs to be conducted. Some producers are still skeptical of the use of cover crops because of the perception that it decreases yields of their cash crops.