Nitrogen-fixing Covercrops for California Rice Production (2/4)

G.S. Pettygrove and J.F. Williams
CULTURAL PRACTICES
Establishing a covercrop following rice can be difficult. The combination of wet, heavy soils and uncertain weather can lead to plugged discs and half-finished fields. But most years, an adequate stand can be established that will produce a significant amount of N and organic matter for the soil. There are several ways to grow vetch or other covercrop species, giving the grower some flexibility to deal with soil and weather difficulties.
Cover Crop Rotations
Rice growers have used covercrops in their rice rotations in several ways:
1. In continuous rice: Plant the covercrop in the fall after rice harvest and following straw incorporation or burning. Incorporate the covercrop in the early spring as part of seedbed preparation. Covercropping in a continuous rice rotation is not a common practice because of the limited time and wet soil conditions, but it can work in some years and on lighter soils.
2. In a two-year rice-fallow rotation: Plant the covercrop immediately after rice harvest and following straw incorporation or burning, then let it go to seed in the spring. This will produce a volunteer crop the following fall.
3. In a two-year rice-fallow rotation: Plant the covercrop one year after rice harvest. Seeding the covercrop the year after fallowing allows time for straw incorporation and decomposition and results in a better seedbed for the covercrop.
4. Broadcast the seed in the water before harvest just as the field is beginning to drain. Water sowing is the traditional method of planting a green manure crop in rice and sometimes can produce much better establishment and growth than later planting dates. However, it is not compatible with fall burning or incorporation of rice straw.
Selecting Covercrop Species
Covercrop species suited for rice production in California are described in Covercrops for California Agriculture (Miller et al., 1989). The best covercrop species for rice are purple vetch, woollypod vetch, and fava bean (bell bean). The vetches will grow during the coldest months of the year. They are relatively tolerant of wet conditions in the early winter and will produce well if there is a drying period in March and April.
Purple Vetch. Purple vetch, Vicia benghalensis, is the most commonly used species for covercropping in rice. It is a viny species and is markedly hairy or pubescent overall. It can survive 20°F temperatures. Purple vetch produces slightly later in the spring than woollypod vetch so is probably a better choice for heavy rice soils.
Woollypod (Lana) Vetch. Woollypod vetch, Vicia dasycarpa, may be a good choice for lighter soils or where the covercrop must be incorporated earlier in the spring. The Lana cultivar is the most recently introduced variety that is successful in California. It resembles hairy vetch (V. villosa) and is more prostrate than purple vetch. It flowers about three weeks earlier than purple vetch and is somewhat more winter hardy. Lana vetch is not as tolerant of saturated soil conditions as purple vetch or bell beans.
Bell bean. Bell bean, Vicia faba, is a small-seeded fava bean. Its growth habit differs from purple and woollypod vetch. It has coarse, erect, succulent stems, a large taproot, and larger leaflets and seeds than the other vetches grown in California. It is similar in adaptation to purple and Lana vetch but is more sensitive to low temperature. It is more tolerant of flooding than Lana vetch. Under good conditions, it can outproduce purple vetch but has not been used much by rice growers. (see Case Study 2).
Seedbed Preparation
Before a covercrop is planted, rice harvest residue should be chopped, disced, or rolled so that it is in close contact with the soil. Straw on the surface can be tolerated if it is uniformly distributed, but the covercrop will not become established where thick straw rows are left on the surface.
If weather permits, discing the straw residue and then broadcasting the covercrop seed is effective. Fall discing incorporates the straw, making less work in the spring and allowing for better contact of the seed with the soil. Usually one or two passes with a stubble disc is required. A smooth, soft soil surface is neither required nor desirable. Plowing often does not produce a good seedbed for covercropping, because it will bury the rice harvest residues in a layer. Decomposition of the residue in this layer under saturated, wet winter conditions can lead to production of phytotoxic compounds.
Winter drainage is essential for a healthy covercrop. Saturated soil conditions in the winter will result in a poor stand or will drown plants that have already become established. If possible, rice checks should be left open and borrow ditches and pits left intact. Drains should be put in where needed.
Planting the Covercrop
Broadcast Seeding. Seed should be broadcast by air or ground rig then covered using a disc or rice roller. The roller does a good job if the clods will crumble and cover the seed and will leave the soil smoother for spring field work. In many cases, an adequate stand of purple or Lana vetch (but not bell beans) can be obtained with no post-seeding tillage. Purple or Lana vetch can be planted where the soil surface is covered with rice straw. Seed is round and will bounce and roll down onto the soil surface. Stubble will protect seedlings and provide a scaffolding for seedlings to climb.
Direct Drilling. When soil conditions are dry enough, direct drilling of the covercrop seed through the stubble will produce good seed-soil contact. Usually, a conventional drill is not heavy enough to cut through rice straw, and a heavier, no-till drill is needed. Rice harvest with a stripper header or use of a good straw spreader will reduce the thickness of straw windrows, making the seedbed more suitable for direct drilling.
Water Sowing. For water sowing before rice harvest, seed should be broadcast from two days before to two days after water is drained from the field. This method works best in late-planted fields that are drained in mid- to late-September. Vetch will grow well if rains come shortly after harvest. In fields that are drained in August or early September, there is the risk that the vetch will die from drought before the first fall rains. At harvest, the combine cutter bar should be set high enough to leave 6 to 12 inches of the covercrop plants intact. Straw windrows left by the harvester can smother or kill young plants, so it is important to spread the straw. Water sowing is not appropriate for bell beans.
Seeding Rate
It is important to minimize the seeding rate, as seed is the main expense of covercropping. Recommended ranges of seeding rates in lb/acre are:
Purple vetch40 - 60
Woollypod vetch40 - 65
Bell beans125 -175
The lower end of the range is recommended for broadcast seeding before November 15 for seedbeds in good condition. Later planting and poorer seedbed condition justify higher seeding rates. Seeding rates as low as 25 lb/acre for purple vetch have been used where seedbed conditions are ideal. This might be adequate where one or more of the following factors are present: longer rotations, re-leveling of fields after harvest and before planting of covercrop, straw removed by burning or baling, thorough rice straw incorporation, or drill seeding.
Fertilization
Legumes generally respond well to phosphorus and sulfate fertilizers and, on soils with pH below 5.5, to lime additions. P deficiency is commonly seen in crops rotated with rice such as safflower and wheat. This is due to the rapid reversion of soluble phosphate to insoluble, highly unavailable forms of P following drainage of rice fields. P deficiency in covercrop legumes can be diagnosed by visual symptoms and plant tissue analysis. P-deficient legumes grow slowly, develop dark or dull green foliage, and in some cases may develop a reddish tinge. Tissue P concentrations of less than 0.15 - 0.20% (total P) may indicate P deficiency. Soil analysis is not reliable for diagnosing P deficiency in a rice rotation.
The cost of phosphate fertilizer could well make covercropping uneconomical. Growers should consider several alternatives. If P fertilizer is already normally applied during the rice seedbed preparation, consider applying it in the fall in order to provide benefit to the covercrop. If the covercrop is drill planted, phosphate fertilizer can be applied through the drill, and a much lower rate (20 to 40 lb P2O5/acre) will be required than with broadcast application. On acid soils, phosphate fertilizer plus lime will produce a better response than fertilizer alone. Waste sugarbeet lime is an ideal liming material for this situation, because it contains some plant-available P.
Inoculation with Nitrogen-fixing Bacteria
If a field has not been covercropped with a legume for a long time, the covercrop seeds should be treated with a commercial inoculant prepared by a reputable laboratory. The inoculant specific to the crop being planted must be used. Do not purchase inoculant long before you plan to use it. Do store it under refrigeration. If the same legume is grown on the land repeatedly and at short intervals for several years, the bacterial population will build up in the soil such that further seed inoculation may not be needed. For more information, see UC Bulletin 1842 (Phillips and Williams, 1987).
Incorporation of Covercrops
Covercrops are usually incorporated by discing as a normal part of rice seedbed preparation. In some years when unusually good growth occurs, it may be hard to disc the material down due to large biomass and tough stems. If such a problem occurs, the covercrop can be flail chopped, then immediately incorporated. Waiting even a couple of days after chopping may make it more difficult to incorporate the material.
Estimating the Nitrogen Contribution of Covercrops
The total amount of N in the covercrop at the time of incorporation can be estimated from the fresh weight of the above-ground plant material.
  1. Cut and weigh the fresh covercrop from a 16 square foot (4 ft x 4 ft) area.
  2. Multiply the fresh weight in pounds by a factor to estimate the pounds of nitrogen per acre contained in the covercrop. Factors are: Vetch - 16; bell beans - 10; berseem clover - 13.
  3. Repeat this sampling 5 to 10 times over the field, depending on its uniformity. Average the results. Samples should be free of dew.
For example, if you harvest 5 pounds fresh weight of vetch from a 4 ft x 4 ft area, you know there are approximately 80 lb/acre of covercrop N (5 x 16=80). The multiplication factors apply to a wide range of growth stages. As the covercrop approaches maturity, N content lessens but dry matter percentage increases. Thus the factors remain stable with advancing maturity.
This procedure estimates N in the above-ground covercrop biomass. This is not the same as fertilizer equivalent (or replacement) value. Some field studies have shown that one pound of covercrop N replaces more than one pound of fertilizer N. See the Case Studies section below.

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Modified: 4 Dec 1997 Comments to jayoung@ucdavis.edu