How to be successful with cover cropping - best practices

How to be successful with cover cropping - best practices

In the warmer regions of our continent, the combines have already started working in the fields and the barley harvest has begun. However, a large part of Europe is still waiting for the harvest to start. During this last calm period, it is worth rethinking the follow-up actions after the harvest, specifically the long-term optimization of soil productivity, which means building organic matter in the soil. More specifically, about introducing cover crops. The goal of this short article is to answer four key questions regarding the practical application of cover crops this year and in the coming years: What? How? Where? When?

What to sow as a cover crop?

It is definitely worth choosing species mixtures. In this way, the impact of unfavorable conditions is limited, as is the case with a single-component cover crop. One of the elements determining success is the selection of components with different root system structures: fibrous root system and taproot. In addition, proper mixtures are characterized by better canopy architecture (low-, medium- and high-growing) reducing the risk of weed infestation of the cover crop and ensuring effective biomass building. The proof here can be the field tests of eAgronom presenting dry matter yields (t/ha).

Sowing date 08.08.2022

The above results should be treated as an example. Finally, it is important to select components adapted to the characteristics of a given farm. Of course, the components of the mixture cannot be plants that normally appear as main plants in the rotation, and it is better to avoid those that are sensitive to identical agrophages.

The choice in the category: ready-made mixture or self-prepared depends on many economic factors, but it is worth paying attention to the significant share of phacelia or vetch in the mixture. According to German studies, these two species allow the soil to retain much more water than leaving the soil without cover. This is especially noticeable in years with high evapotranspiration. Phacelia reduces water losses from the soil by 19.2%, and vetch by 25.5%.

(Source: Gernot Bodner, praxis-agrar.de)

If the farm prepares its own mixture, it is worth using a simple formula for determining the mass proportions of the components.

Mass of a single component to create a mixture =  Standard sowing rate for the species (kgs/ha) / Number of species in the mixture

This formula should be considered as a starting point, because often some components of mixtures, e.g. buckwheat, are very competitive compared to other components. The mixture should not be dominated by one component.

How to sow? Available techniques for sowing cover crops

Regarding the previous chapter, if the mentioned mixture is composed of seeds of very different sizes and thousand-seed weight, then unfortunately there is no possibility of proper wide broadcast sowing with discs. This actually applies to most mixtures. The problem will be uneven distribution of the seed material. Even when using regular cereal drill or broadcast sowing with an applicator, there is a certain risk that small seeds that fall to the bottom of the tank will more often end up on half of the field. Therefore, it is important to properly mix the components of the mixture and refill the tank more often with smaller batches of seeds, if possible - this applies more to the row drill. Cover crop seeds applicators often have smaller tanks. 

Therefore, the row grain drill and applicator remain to be considered. The second method will certainly be cheaper and faster, but it will not ensure proper emergence in the absence of optimal soil moisture and the accumulation of a large amount of straw on the soil surface. One of the problems may be phenolic compounds, such as benzoic, salicylic and syringic acids. They have a phytotoxic effect, which means that they can inhibit the growth and development of plants, especially young seedlings. A higher sowing rate by 20-30% is recommended.

Cover crop emergence sown with an applicator mounted on a mulch harrow

Combi drilled cover crop emergence

Where to plan cover crops?

The location of cover crop in the crop rotation is not complicated: after early harvest crops and before the planned spring crops of the next calendar year.

However, a certain difficulty is the risk of herbicide residues used in previous main crops (or even earlier!) before the cover crop. Active ingredients may remain in the soil or on crop residues. Unfortunately, the database here is very limited, but the risk is unfortunately greater in the case of reduced tillage. Too high or too low soil pH prolongs the decomposition period of the active ingredient. Legumes in the cover crop mixture will be particularly sensitive. Examples of several popular active ingredients and the period of their presence in the soil are given below:

It is worth remembering that fields with too low pH will constitute a very difficult growth environment not only for the main crop, but also for cover crops, and here, additionally, taking into account the need to use current soil resources, the cover crop will have particularly difficult vegetation due to the reduced root system.

The influence of pH on the structure of barley roots (indicator plant) - Grzebisz 2008

Of course, it is possible to choose cover crops that are more tolerant of lower soil pH, such as rye or lupine. In the case of these plants, water use for vegetation will be higher. Cover crops will never be a remedy for low soil pH or other serious soil problems, they will rather be an indicator of these problems.

When to sow cover crops?

The sowing date is an absolutely crucial element that determines the success of introducing cover crops. The rule is simple: the earlier the better, but one should not forget about the risk of cover crops flowering and seed formation in the case of using a very early sowing date. If there is such a risk, it is better to consider winter components in the mixture, e.g. the aforementioned hairy vetch or other winter species: winter peas, winter beans, winter oats. Plants that will not flower before winter under normal conditions even in the case of very early sowing. However, they will require more intensive actions in spring.

The early sowing date of cover crops is a cost-free factor increasing efficiency, because the cost of sowing will be identical regardless of whether it is done at the beginning of August or the beginning of September.

The influence of sowing date on biomass of cover crop - tillage radish (Author: Merili Toom)

The influence of sowing date on biomass of cover crop - hairy vetch (Author: Merili Toom)

However, if prolonged harvest or other factors cause a delay in sowing cover crops, a theoretical solution (a bit controversial) is to initially support the cover crop with a minimum dose of nitrogen. Nitrogen will be intended to improve the C/N ratio in the topsoil so that the cover crop can properly fulfill its function. Nitrogen should not be considered only as a cost here, because it will be available to the successor plant and a better-grown cover crop will retain more nitrate nitrogen, the so-called residual nitrogen remaining in deeper parts of the soil after harvest, which would undergo further leaching during the winter and become inaccessible to subsequent crops.

Effect of nitrogen addition (+25 kgs/ha) on mustard and phacelia biomass (mass doubled)

Summary

Effective introduction of cover crops is a real art, but the game is worth the candle. Of course, correct termination procedures such as mulching or rolling the cover crop are equally important.

Ultimately, it is primarily about introducing more organic matter into the soil and ensuring its productivity for many years. Cover crops also generate additional funds in the eAgronom Carbon Program and within the framework of area subsidies from the European Union's community funds.