Austrian Winter Pea a Good Alternative to Fallow

Chapter 8- Crops and Varieties, No. 7, Summer 1988

Roger Veseth

Austrian winter pea (AWP) harvested for seed or used as a green manure can improve the yield potential of the following cereal crop. In addition to providing nitrogen (N), the crop also helps to maintain soil organic matter content, reduces soil erosion and reduces the severity of soilborne diseases affecting cereals. In light of these advantages, AWP can provide producers with a good alternative to summer fallow and other crop rotation options in the intermediate and higher precipitation areas of the Northwest.

These are some of the conclusions of two University of Idaho researchers following two 3-year studies comparing AWP with summer fallow and spring barley as crop rotation options. Bob Mahler, soil fertility researcher, and Dick Auld, plant breeder, jointly conducted the research.

Background

The researchers point out that between 1900 and 1950, the majority of N used in crop production was supplied by legumes in the crop rotation. The use of legumes as an N source then decreased with the availability of inexpensive N fertilizers. Beginning in the 1970’s, shortages of natural gas and the resultant higher N fertilizer prices created a renewed interest in legumes as an N source, Currently, the increasing focus on low-input, sustainable farming systems also emphasizes the use of legumes in the rotation.

Earlier research by Auld and others, in the 22-to 27-inch annual precipitation areas of northern Idaho, has shown that AWP has the potential of producing a maximum biomass yield of up to 9,000 pounds/acre (lb/acre). Decomposition of AWP residue and release of nutrients are believed to proceed quite rapidly. Mahler and Auld estimate that about 80 percent of the residue N can be available to the next crop in the rotation.

Research Focus

The two separate 3-year studies were conducted between 1981 and 1985. The objectives of these field studies were to determine the effect of AWP used as either a seed crop or a green manure on soil N levels and yields of subsequent crops of winter wheat and spring barley. The research was located in a 22-inch annual precipitation zone at the UI Plant Science Research Farm near Moscow on a Palouse silt loam soil. Four cropping options preceded winter wheat followed by spring barley in each of the 3-year studies. The four rotations were: (1) green manure AWP-winter wheat-spring barley, (2) seed AWP-winter wheat-spring barley, (3) spring barley-winter wheat-spring barley and (4) summer fallow-winter wheat-spring barley.

Trial I (1981-84)

The study was initiated in fall 1981 on a field of volunteer ”Melrose” AWP. The following spring, “Advance” spring barley and fallow comparisons were added to the study. Fallow plots were roto-tilled to a depth of 2 inches every 21 days. The green manure plots were roto-tilled to a depth of 4 inches in early July, leaving approximately 25 percent of the pea residue to remain on the soil surface to help control soil erosion. For comparison with standard field equipment, the researchers point out that the green manure tillage operation was roughly comparable to a partial incorporation by disking.

After the seed AWP and barley crops were harvested, the plots were disked, cultivated, harrowed and seeded to “Stephens” winter wheat in early October 1982. As an additional experiment, N fertilizer subplots were established the following spring (April) with surface topdress additions of 20, 50, 80 and 110 lb/acre N as ammonium nitrate (34-O-O, granular), All other plant nutrients were present in sufficient amounts for optimum winter wheat yields. The winter wheat stubble was plowed after harvest, and all the plots were seeded under conventional tillage to “Andre” barley in spring 1984.

Trial II (1982-85)

Procedures for Trial II were nearly identical to Trial I except that the previous crop at the site had been a green manure crop of winter wheat instead of volunteer AWP, Also, the subplot fertilizer rates on the winter wheat were O, 30, 60 and 90 lb/acre N applied in February.

Results

Rotation Crop

Analysis of yield and N content indicate that green manure AWP had the highest potential N contribution compared to the other two crops of seed AWP and spring barley (Table 1). Release of N through mineralization from organic matter and crop residue in the summer fallow rotations in the two trials was estimated to be 40 lb/acre in 1982 and 57 lb/acre in 1983,

Nitrogen Fertilizer Equivalent

Analysis of crop biomass N content can give an estimation of the amount of N contributed for the following crop. However, the actual availability of that N is difficult to determine since it is subject to varying rates of release from the residue, potential temporary tie-up by the soil microbial population, losses by volatilization into the air and by leaching, and other factors. One method of measuring the N availability is by determining the yield response of the following crop, which is expressed as the “N fertilizer equivalent” contributed by the legume crop. The N fertilizer equivalent for these trials was defined by the researchers as the amount of N fertilizer required to produce a winter wheat yield after spring barley equal to that produced after a legume or summer fallow.

Table 1. Seed yield, seed nitrogen (N) content, straw yield and straw N content of Austrian winter pea (AWP) grown for either green manure or seed and spring barley at Moscow, ID In 1981-82 (Trial I) and 1982-83 (Trial II) (Mahler and Auld, Ul).

In Trial I, the green manure AWP, seed AWP and summer fallow rotation treatments provided N fertilizer equivalents of 84,76 and 65 lb/acre, respectively. The N fertilizer equivalents for the same treatments in Trial II were 84, 57 and 57 lb/acre.

Soil Nitrogen

Initial soil samples taken in the first fall of each 3-year study indicated that 27 lb/acre available N was in the top 3 feet in Trial I and 62 lb/acre in Trial II. The researchers point out that N was the only plant nutrient limiting wheat yield potential. All other plant nutrients were present at adequate levels for optimum yields with available water.

Inorganic soil N samples were collected from the surface 3 feet during May in the winter wheat and spring barley crops in each trial (Table 2). During the winter wheat year, inorganic soil N content was significantly higher after the green manure AWP and seed AWP than after fallow and spring barley in both trials. Contributions of soil N from these first rotation crops into the second crop year was uncertain, although N content tended to be highest after the AWP for green manure.

Table 2. Effect of crop rotation on inorganic soil nitrogen content measured in-crop in May for 2 years after the initial rotational crop in 1982 (Trial I) and 1983 (Trial II) at Moscow, ID (Mahler and Auld, UI).

Fertilizer Response

In Trial I, the researchers found that winter wheat yields were significantly increased with all N fertilizer application rates and after all four previous crops. The wheat yield responses to N were similar after green manure AWP, seed AWP and fallow. Wheat yields after barley were comparatively lower at all N application rates.

Winter wheat yield responses to N fertilizer in Trial II were similar to Trial I with one exception. The application of more than 30 lb/acre N did not improve winter wheat yield following the green manure AWP. This lack of yield response to higher application rates of N fertilizer additions may be attributed to the large amount of N added to the soil in the green manure AWP dry matter (Table 1). Much of the crop N was probably mineralized by soil microbes, providing nearly adequate available N for maximum wheat yields with the available water.

Cereal Yields

Yields of winter wheat following green manure AWP, seed AWP and fallow were not significantly different when averaged over all N fertilizer rates (Table 3). However, yields for all three of these treatments were significantly higher than the yield following barley. The researchers attribute part of the difference in yield to the lower level of inorganic soil N (Table 2) following barley and to the increased potential for soilborne diseases common to both wheat and barley.

The researchers point out that even though the fallow treatment resulted in wheat yields similar to those following the two AWP treatments, the yield gain after fallow was at the expense of soil organic matter decomposition without any crop residue contribution. In other words, the inclusion of AWP provides a more sustainable, long term crop rotation than fallow.

Table 3. Effect of crop rotation on yield of Stephens winter wheat and Andre spring barley in 1982-84 (Trial I) and 1983-85 (Trial II) averaged over N fertilizer rates at Moscow, ID (Mahlar and Auld, UI).

Conclusion

Mahler and Auld conclude that AWP can bean important addition to the crop rotation. It provides N for the following cereal crop, contributes to the soil organic matter, reduces soil erosion and reduces the incidence of soilborne diseases affecting cereals, Their research shows that AWP, as a green manure or harvested for seed, contributed an estimated N fertilizer equivalent of 57 to 84 lb/acre for the following crop, This N fertilizer equivalent was greater than from summer fallow.

The AW produced yields of the following winter wheat crop that were similar to yields after fallow, plus contributed crop residue to help maintain the soil organic matter content for sustained soil productivity. Inorganic soil N content and winter wheat yields following spring barley were lower than after the three other rotation options.

Since winter wheat yields were similar between green manure AWP and seed AWP, the researchers point out that the seed AWP-winter wheat-spring barley rotation would probably be the most economical. This is because income is received each year in the crop rotation. They feel that the interest in AWP in rotation will increase as N fertilizer costs increase in the future. Other important features of AWP in crop rotations are improved cereal yield because of decreased severity of diseases and long term maintenance soil productivity.