Regular manure applications combined with forage-based rotations are the envy of crop producers when soil organic matter (SOM) levels and soil resilience are considered. Cover crops planted with an application of manure after wheat harvest may be the next best option.

Manure application during the growing season is preferred for several reasons:

  • Nutrients from manure can be utilized by growing crops.
  • Risk of compaction is reduced and the addition of cover crops can further reduce existing compaction and help increase water infiltration.
  • Risk of phosphorus loss from manure is lower during the growing season. Phosphorus loss from soils during the non-growing season represents about 80% of the annual loss.
  • Soil micro-organisms respond to the nutrients from manure, especially when applied with growing crops, and often result in increased biomass production. A variety of cover crop species will add diversity while soil micro-organisms will enhance root systems.

There are a variety of methods for seeding cover crops before or after manure application. Conventional methods have manure applied followed with a tillage pass to incorporate manure and the broadcast cover crops. In recent years, slurry seeding has become popular and is being done by mixing cover crop seed with manure in the tanker; adding a seeder to the tanker (pictured below) that can place cover crop seed into the injection strip and/or other innovative equipment designs. One pass application, improved seed placement and reduced compaction risk are all benefits. The interaction of manure nutrients with cover crop growth and the benefits to the soil microbial populations and increased biomass production can be significant."

Cover crops applied in the same pass as the manure application saves a trip over the field and can improve placement.
Figure 1. Cover crops applied in the same pass as the manure application saves a trip over the field and can improve placement. This design can also seed cover crops into standing corn with a side-dress manure application.

A side-by-side study done in 2015 after wheat harvest compared several different cover crop species mixes with and without manure (in this case digestate, which is similar in composition to liquid hog manure). A multi-species mix with 10 different cover crop species was compared to a mix with 3 species. The results are shown in the image below and in Table 2, Approximate Biomass Yield of Three Cover Crop Mixes with and without Organic Amendment. The areas that had the digestate applied were easy to distinguish. Biomass yield (average dry matter yield measured from three 1m2 sections per treatment) shows the impact from the organic amendment and approximate difference from the various species.

Synergistic effect of manure on cover crop growth. Nutrients from manure increase microbial activity that increases cover crop top and root biomass.
Figure 2. Synergistic effect of manure on cover crop growth. Nutrients from manure increase microbial activity that increases cover crop top and root biomass.

In each treatment, the cover crop with added organic amendment had a higher biomass yield, however the multi-species mix with 10 species did not yield as much biomass relative to the 3-species mix or oats alone. Observations at harvest would suggest that where the digestate was applied, there was more growth of nitro radish. The radish have a lot of top growth and large root, but dominates over the other species so that there are very few other cover crop species growing near the radish. Multi-species mixes seems to give a bigger benefit where an organic amendment is not applied, while species that include mainly cereals (oats, barley, cereal rye, etc.) and a smaller amount of radish and/or legume will give higher biomass yield where manure is applied.

Table 1. Approximate biomass yield of three cover crop mixes with and without organic amendment
Cover cropApproximate yield with manure (ton/ac)footnote 1Approximate yield without manure (ton/ac)footnote 1Approximate yield — increase from manure (ton/ac)footnote 1
Oats3.62.833%
Multi-Species Mix: Planted at 40 lbs/ac  
33% Oats, 4% Nitro radish, 2% Brassica, 2% Sorghum Sudangrass, 1% Phacelia, 2% Sunflowers, 4% Sun hemp, 5% Turnips, 25% Crimson Clover, 23% Austrian Peas		2.101.7517%
3 Species Mix: Planted at 30 lbs/ac  
14% Nitro Radish, 16% Crimson Clover, 70% Oats		2.851.8336%

The above data represents one site -one year 3,500 gal/ac digestate was applied in mid-August. The nutrient composition of digestate is similar to hog manure.