This summary covers a four-year study looking at the benefits of including Brassica green manures in the potato rotation in Maine. Some members of the Brassicaceae have been reported to suppress soil pathogens when incorporated as green manures. The predominant rotation among potato growers in Maine is to plant potatoes followed by oats or barley, and then go back to potatoes. Our initial goal was to grow a Brassica green manure in the same season as a cash crop of barley. However, from results of the first season of the study, this appears to be unworkable in northern Maine. Where the winter rapeseed green manure became well established, it interfered with barley harvest when the grain lodged into the underseeded green manure. Where the green manure was established after barley harvest, the remaining growing season was too short to obtain significant growth from the green manure. Accordingly, the grower cooperators choose to redirect the study and work on evaluation of a Brassica green manure as a sole crop completely replacing the barley crop in northern Maine. Also, it was decided to work on establishing a Brassica green manure after barley further south, in central Maine, where the growing season is longer, and barley is often harvested early as silage. The grower-cooperators also chose to conduct the experiment with large plots (60 feet wide) that better fit their farming equipment. This limited the number of treatments that could be evaluated. It was decided to work with ‘Caliente’ green manure mustard, as this variety has been shown to benefit potatoes in Washington. The growers felt with limited treatments they preferred to work with a variety that had been proven elsewhere.
In 2003 and 2004 replicated on-farm trials were established to compare green manure mustard (variety ‘Caliente 119’) versus barley as a control. Plots were 60’ wide and more than 200’ in length. In 2003 there were three sites, in Presque Isle, Crouseville, and Exeter, Maine. In 2004 there were four sites (Presque Isle, Crouseville, Exeter, and Easton, Maine). In 2004, one of the sites (Exeter) did not have a barley control – the grower used annual ryegrass as a check. Potatoes were grown the following year. Plots within a field were uniformly managed by the grower during the potato part of the rotation. Tuber yield was determined at each site. At the Presque Isle site, incidence of white mold on stems, and of Rhizoctonia and powdery scab on tubers were monitored.
The following conclusions can be drawn from this work:
1. Significant reduction in Rhizoctonia on tubers was consistently observed where mustard was grown as a green manure.
2. There was a small, but statistically significant, increase in white mold observed in potatoes following green manure mustard.
3. No effect on powdery scab or common scab from mustard green manure was observed.
4. There was a consistent trend for greater yield after mustard green manure compared to barley. Across seven replicated trials, the average yield increase was 8 %.
The goal of this work was to evaluate the short-term effects of a mustard green manure on potato yield and disease incidence for potatoes grown in a rainfed cropping system. Diseases of interest were black scurf (Rhizoctonia solani), powdery scab (Spongospora subteranea), and white mold (Sclerotinia sclerotiorum). On-farm trials were established in Maine in 2003 and 2004. Mustard (Sinapis alba and Brassica juncea blend) was sown at a seed rate of 10 lbs per acre, mowed at late flowering, and then disked into the soil. In the following season, tuber yield was measured at each site and compared to that of potatoes grown in check plots. Incidence of white mold on leaves and stems, and Rhizoctonia incidence on tubers, were evaluated at one site over two years. On average across all the sites, total tuber yield was 8 percent greater following mustard green manure versus barley. Mustard green manure was associated with decreased incidence of Rhizoctonia on tubers, and increased incidence of white mold on leaves and stems, in the following potato crop. No effect was observed on incidence of powdery scab.
1) The feasibility of intercropping and the optimum time of planting will be well-defined by the third year of the project.
2) The farmers working with the project will be planting a rapeseed (or other Brassica) green manure on a production scale (either intercropped or sole cropped – depending on results) on fields outside the study area by the third year of the project.
3) We will have large demonstration/observation strip plots (1 acre in size) incorporating a rapeseed green manure into the potato cropping system on other farms by the third year of the project.
Mustard green manure plots were established in on-farm trials over two seasons (2003 and 2004). In the 2003 there were three sites (Presque Isle, Crouseville, and Exeter, Maine). In 2004 there were four sites (Presque Isle, Crouseville, Exeter, and Easton, Maine). In 2004, one of the sites (Exeter) did not have a barley control – the grower used annual ryegrass as a check. The trials were set up in a randomized complete block design at each site with four replications, except for the Easton site which had only two replications, and the Exeter site in 2004 which had three replications. ‘Caliente 119’ (High Performance Seeds, Moses Lake, Washington) was chosen as a mustard cultivar because it has a history of commercial use as a biofumigant for potatoes grown in the Pacific Northwest. At five of the six sites, mustard and barley plots were established in mid-May to early June. At the remaining site, mustard was planted in early August after harvest of small-grain silage in late July. In all cases, barley (sown at the same time as the mustard) was included as a control treatment. Mustard was sown at a rate of 10 lb per acre at each site. Plots were 60 feet wide and varied in length based on the shape of the field. Mustard biomass samples (three 0.3 m2 subsamples per plot) were taken within one week of mowing. Mustard plots were mowed with a rotary mower at late bloom and disked into the ground.
Each field was uniformly managed during the potato cycle of the rotation. Potato yield samples were hand-dug from each plot after vine killing. Three subsamples (area of 4.5 m2) per plot were taken at the Presque Isle site, and two subsamples per plot were taken at the other farms. Tubers were graded and weighed. Data from subsamples within a given plot were averaged to give one value for each plot. To analyze yield data across sites and years, values were averaged for each treatment in a given trial, and then each farm and season was treated as one replication in a randomized complete block design. In this way, differences in management between farms would be accounted for as block effects and not be confounded with treatment effects. The yield data across sites was then analyzed as a randomized complete block design using the PROC GLM routine in SAS statistical software (SAS institute, Cary, NC).
Results were disseminated by presentations made at grower conferences in Maine in 2005 and 2006, at the USDA-SARE Sustainable Agriculture Conference in Burlington, Vermont in 2004, in an article in Spudlines Newsletter in 2006, and in a paper submitted to Crop Management (an on-line journal) which has been accepted with minor revision for publication.
1) Trials to evaluate intercropping a Brassica green manure with barley were implemented and evaluated with growers in the 2002 season.
2) Based on the outcome of these trials, growers decided that it would be good to focus the trial work in 2003 and 2004 on evaluating the benefit of sole-cropped Brassica green manures in northern Maine. They are particularly interested in evaluating the ‘Caliente’ oriental mustard used by potato growers in the Columbia Basin of
3) Replicated field trials to evaluate a Brassica green manure as compared to the standard barley rotation were implemented in three on-farm trials in the 2003 season, and on four on-farm trials, and seven on-farm observation blocks in the 2004 season. Several farmers planted some area to mustard green manure (at their own expense) in the 2005 season. Three of four farmers who had on-farm trials purchased seed to plant mustard green manure on other parts of their farms.
4) In both the 2004 and 2005 seasons, the mustard green manure helped to control Rhizoctonia and increased total yield of ‘Russet Burbank’ and ‘Shepody’. Across seven on-farm trials there was an average yield increase of 8% following mustard versus barley. This yield benefit was statistically significant. We did not see control of powdery scab in our trials. White mold incidence was greater where mustard or canola were grown the previous season. Thus the mustard green manure provided a yield benefit, and it helped control Rhizoctonia. On the other hand, white mold incidence was worse following the mustard green manure. Therefore, where white mold is an issue, farmers may want to use a different green manure in their efforts to improve soil quality.
5) The Maine Potato Board, a grower controlled commodity group, contributed financial support
for analysis of leaf and soil samples taken in the 2005 season. Farmers have been interested in the outcome of the work.
6) In 2005 and 2006 a local seed company brought in mustard (Brassica juncea) seed for sale to
growers as a green manure. In 2005 there were approximately 110 acres planted. In 2006
there were approximately 165 acres planted to mustard green manure by farmers interested in
improving their potato crop.
“Evaluation of a Mustard Green Manure in Maine” printed in the April 2006 issue of Spudlines newsletter. This newsletter goes to Maine potato farmers and business people.
Sexton, P.J., A. Plant, S.B. Johnson, and J. Jemison. 2006. Effect of a Green Manure Mustard on Potato Yield and Disease Incidence in a Rainfed Environment. Crop Management (in review).
Additional Project Outcomes
Impacts of Results/Outcomes
On-farm trials were conducted over two rotation cycles at multiple sites. The mustard green manure was associated with eight percent greater in tuber yield versus the check plots where barley was grown as the previous crop. The mustard green manure was associated with decreased incidence of Rhizoctonia on tubers, and with a slight increase in incidence of white mold on stems and leaves. There was no effect observed on powdery scab. We estimate that the increase in potato yield resulted in an average net return of $40 per acre for growers using the mustard green manure. There should be a note of caution however as it seems advisable to avoid use of mustard on potato fields where there is a history of white mold problems, and on varieties that are highly susceptible to white mold. It also seems advisable to avoid short rotations with other crops that are susceptible to white mold, as well as to avoid short rotations with other members of the Brassica family.
A survey which, amongst other things, inquired about use of green manures was sent out to 200 potato growers from the “Spudlines” mailing list. Thirty farmers responded. Thirty three percent of the respondents indicated they had some knowledge of mustard green manures. Seventeen percent indicated that mustard green manures enhanced yield of potatoes. Benefits of mustard green manure noted by growers included: addition of organic matter to soil, biofumigation, mellower soil, and control of Rhizoctonia. Problems associated with the mustard green manure noted by growers were: volunteer mustard becoming a weed, no income during the non-potato part of the rotation, and white mold. The significance of the survey results lies in the fact that at the beginning of these trials, mustard green manures were almost unknown in northern Maine. By the end of these trials a significant portion of the farming population is now aware of mustard green manures and knows something about the technologies strengths and weaknesses.
There is enough demand for mustard seed in the last two years that private companies have begun bringing in seed and offering it for sale. The end result of this is that mustard green manures have enough of a start in northern Maine that the practice will continue to develop on its own from this point. This project has helped to get this practice off the ground and it has also identified white mold as a disease that has to be considered and managed where mustard green manures will be used (at least in the Northeastern USA).
Working with the farmers on the project and trying to include all the costs involved, we estimate a variable cost of $115 per acre to grow the mustard green manure. Our results indicate an average payable yield increase of about 8 % over use of barley as a rotation crop. Using a conservative value on potato prices ($5.70 per cwt) and a yield response of 27 cwt per acre we estimate a net return of about $40 an acre for growing the mustard green manure.
Almost no one was using mustard green manures at the beginning of this work. Currently, there are about 150 acres of mustard green manure being used in northern Maine. As growers work with the practice it will spread further and further. This technology contributes to the diversity of the cropping system by giving growers another alternative to small grains. It is associated with decreased incidence of Rhizoctonia on tubers, which may contribute to decreased pesticide use in the future.
Areas needing additional study
Some work should be done to compare the value of different varieties of mustards as green manures. Also, it would be beneficial to develop regional seed production as it is expensive to transport seed from points further west.