No-till Winter Wheat After Green Manure Legumes

Chapter 2 —Systems and Equipment, No. 15, Summer 1990

Roger Veseth

There is increased interest in use of legume crops as green manures in the Northwest. Fixation of nitrogen (N) for reducing fertilizer costs and the addition of crop residue for increasing soil organic matter content and improving soil structure are well recognized benefits. Another common and important benefit is increased yield of cereals following green manure legume crops compared to other crops. In addition to more available soil N, this yield increase after a green manure legume is often due to a “rotation effect. ” This can be the result of reduced incidence of soilborne cereal diseases, more carry-over soil water, improved soil physical condition and other benefits. However, in order to fully reap the benefits of a green manure crop, producers also need to control soil erosion after the legume is tilled, particularly overwinter in the following cereal crop.

Soil erosion can be severe where winter cereals follow low-residue crops, such as dry peas or lentils, or summer fallow when intensive tillage systems are used to prepare a finely-tilled, low-residue seedbed. High erosion potential is also possible if the residue from the green manure legume crop is buried with intensive tillage. After years of green manure crops in the crop rotation, soil can become more resistant to erosion because of increased organic matter content and improved soil structure. To realize this benefit, however, soil erosion must first be controlled.

Fortunately, recent research has shown that conservation tillage systems can be used with green manure legume crops to provide protection from soil erosion during the following winter without sacrificing the benefits of the green manure legume.

Background Research

The first in-depth research study to evaluate the potential for using reduced tillage and no-till systems following green manure legume crops in the Northwest was conducted near Pullman, WA, from 1984 through 1986. The study was coordinated by Dave Bezdicek, Washington State University soil microbiologist and STEEP researcher, in cooperation with other researchers from WSU and the USDA-Agricultural Research Service (ARS). STEEP (Solutions To Environmental and Economic Problems) is a comprehensive research program on conservation farming in Idaho, Oregon and Washington. Each year since 1975, STEEP has involved about 100 land-grant university and ARS scientists in the three states.

A brief summary of the results from this first study is provided below in order to set the stage for discussion of a recent research effort. More details on the 1984-86 study are available in PNW Conservation Tillage Handbook Series publication No. 14 in Chapter 2. The Handbook can be ordered through County Extension offices in the three states.

The original study consisted of three tillage treatments with several different green manure legume crops. The tillage treatments included conventional tillage (moldboard plowing and shallow disking), reduced tillage (shallow disking twice) and chemical-kill/no-till. Bezdicek found that winter wheat yields under reduced tillage were generally equal to or higher than yields under conventional tillage. However, yields under the chemical kill/no-till treatments in their study were significantly lower than the other two treatments.

The reasons for the yield reduction with chemical kill/no-till were not clear. There were too many unanswered questions to draw any firm conclusions about the potential for this practice. Bezdicek conducted detailed laboratory and field studies in 1987 and 1988 to determine if reduced N availability from the legume residue was a factor in the lower yields. Austrian winter pea plants were produced with a’ ‘tagged” form of N fertilizer (N-15) in the greenhouse. Pea plants were chemically-killed or clipped. The pea residue was then taken to field plots and either incorporated into the soil or left on the surface before planting winter wheat.

Bezdicek monitored the movement of N-15 in the soil and uptake by the winter wheat to determine the availability of N from the legume crop under different tillage practices. The research results showed that soil availability and winter wheat recovery of N from legume crop residue left on the soil surface was only slightly lower than from soil-incorporated legume residue. Although N losses were slightly higher when the peas were chemically-killed and left on the surface, Bezdicek concluded that the difference in N recovery alone could not have accounted for the large reductions in yield with the chemical-kill/no-till treatments. Additions of N beyond 60 pounds per acre did not appreciably increase wheat yields in these plots.

Other possible factors which needed further study included winter wheat injury from the herbicide used to kill the legumes, and increased soilborne disease with chemical-kill/no-till seeding compared to conventional or reduced tillage. The herbicide used to kill the green manure legume crop in the initial study is also no longer registered for crop use in the area. So there has been a need to evaluate the effect of other herbicides to kill green manure legume crops prior to no-till or reduced tillage seeding of winter wheat.

1988-89 Research Project

To further explore the potential for no-till winter wheat following green manure legumes, a new field research project was conducted in 1988 and 1989. The study was a cooperative effort between Bezdicek and Alex Ogg, a plant physiologist with the USDA-Agricultural Research Service at WSU.

Study Details

The research site, located just north of Pullman, was on a deep Palouse silt loam soil on a 5 percent slope with a north aspect. This is in a 20-inch annual precipitation zone. The site had been cropped with spring barley for the previous 2 years. Winter wheat was planted after two legumes. Early spring-seeded Austrian winter pea (AWP) was used as a green manure (dry soil conditions had prevented planting the previous fall). Spring dry pea, harvested for seed, was included in the trial for comparison. This is the predominant rotation crop before winter wheat in the area.

A variety of treatment combinations were used with AWP (Table 1). The AWP was killed with either a nonselective herbicide or by clipping with a sickle mower. To facilitate seeding, residue was then flail-chopped or removed from the plots. Tillage treatments included no-till and reduced tillage. The spring pea treatment and one AWP treatment were shallow-disked. All of the other AWP treatments were no-till. One chemical kill/no-till treatment also included soil fumigation before planting wheat to help determine whether soilborne diseases or other biological factors were associated with chemical-kill and/or no-till seedings. All of the treatments were seeded to Hill 81 soft-white winter wheat with the USDA-IV cross-slot, no-till drill.

Table 1. Influence of residue management practices for Austrian winter pea green manure crops on yield of the following winter wheat crop under reduced tillage and no-till, 1988-89, Pullman, WA (Bezdicek — WSU, OGG – USDA-ARS).

Since the moldboard plow-conventional tillage and disk-reduced tillage treatments in the 1984-86 study did not result in significantly different winter wheat yields following green manure legume crops, only the disk-reduced tillage and no-till treatments were included in the 1988-89 study. Both of these conservation tillage systems after green manure legume crops provided adequate soil erosion protection.

The addition of 96 pounds N per acre of fertilizer was also evaluated in each treatment to determine if soil N availability was a factor in winter wheat response to the treatments.

Results

The researchers reported an average yield increase of 18 bushels per acre with the addition of 96 pounds N per acre of fertilizer on all treatments (except the soil fumigated plots). There were no N fertilizer-treatment interactions, hence an average of yields from the fertilized and non-fertilized treatments will be discussed here.Aside from the soil fumigation treatment (No. 2),winter wheat yields in most all of the other treatments were not significantly different (Table 1). Winter wheat yields on nearly all of the green manure AWP treatments were equal to or greater than the yield of wheat seeded after disking of the spring pea residue (Treatment No. 4), which was the second lowest-yielding treatment in the study. Yield in this treatment was 16 bushels per acre lower than the comparable AWP treatment (No. 1) seeded after disking the residue. One lower-yielding exception in the AWP treatments was Treatment No. 7, where the AWP residue was removed after clipping before no-till seeding the winter wheat. This treatment resulted in the lowest winter wheat yield in the study, 20 bushels per acre lower than the similar no-till treatment (No. 6) where residue was not removed. This type of yield loss with residue removal would be expected since the N in the residue was not available to the winter wheat.

Winter wheat yields following green manure AWP (except where residue removed) were similar or higher than following spring pea harvested for seed. Producers need to closely evaluate the economic costs and returns of the two crops in their winter wheat production systems. Additions of more legume residue for increased soil organic matter, improved soil structure, and other longer-term benefits of green manure legume crops also need to be considered in the comparison.

The chemical-kill/no-till treatment with soil fumigation (Treatment No. 2) gave the highest yield, 144 bushels per acre (Table 1). This can be compared with the similar non-fumigated treatment (No. 3) yield of 104 bushels per acre. This kind of dramatic wheat yield response to soil fumigation is not uncommon in wheat research projects in the area. The use of soil fumigation is not economically feasible as a production practice for wheat, but is used as a research tool to help determine the attainable yield with available water, and the impact of soilborne pests on wheat yield potential.

The researchers attributed the yield response partially to control of Pythium fungi (which causes seedling blight and root rot) and other soilborne pathogens, and about 26 pounds N per acre more available soil N. Pythiwn populations on winter wheat roots, determined in December, were 90 percent lower in the fumigated treatment than in the similar non-fumigated treatment (No. 3).

In non-fumigated treatments, spring populations of soil microbes (many of which are involved in residue decomposition and nutrient cycling) in the winter wheat year were significantly higher in treatments where the AWP residue was disked (No. 1) compared to being left undisturbed in the similar no-till treatment (No. 6). However, winter wheat yields were not significantly different between the two treatments. Populations of soil microbes were also higher following AWP compared to spring pea when both residues were disked, as would be expected because of the greater amount of residue from AWP.

Management Implications

Only a few of the treatments in the 1988-89 study were meant to be practical management options for producers (Treatments 1, 3 and 6) where AWP was killed by clipping or herbicides, and winter wheat was planted under reduced tillage or no-till. Most of the other treatments, such as those involving soil fumigation or residue removal, were included only from a scientific standpoint to help isolate production limitations. In addition, the need for flail-chopping in production fields would depend on the amount of residue, subsequent tillage practices, residue-handling capacity of the drill used in planting winter wheat, and other factors. Shallow disking or other reduced tillage operations, instead of clipping, could also be used to kill the green manure legume crop.

The results show that reduced tillage and no-till can be viable conservation tillage options for planting winter wheat after green manure crops such as AWP. There were no significant differences in winter wheat yields after green manure AWP whether it was no-till seeded into the clipped or chemically-killed residue, or seeded under reduced tillage where the peas were killed by clipping. For comparison, winter wheat yields in the 1984-86 study were also not significantly different between the disk-reduced tillage system and the moldboard plow-conventional tillage system following AWP and other green manure legume crops.

Producers need to explore conservation tillage options with green manure legume crops. This combination offers the potential for reduced N fertilizer costs and high winter wheat yield with the increased soil N and “rotation effect.” It also offers soil erosion protection, increased soil organic matter content, improved soil structure and other-soil quality benefits.

Use of Trade Names

Research results are given for information only and are not to be construed as a recommendation for an unregistered use of a pesticide. Always read and follow label instructions carefully. To simplify the information, trade names have been used. Neither endorsement of named products is intended nor criticism implied of similar products not mentioned.