Contribution of cover crops in cropping systems in relation to crop yield, the nitrogen dynamic in the soil and their impacts on the soil quality

Principal Investigator

Anne Vanasse

Research Institution

Université Laval

External Funding Partners

Ministère de l’Agriculture, des Pêcheries et de l’Alimentation du Québec (MAPAQ): Programme Innov’action agroalimentaire (Cultivons l’avenir 2), Innovagrains

Ces travaux ont été réalisés grâce à une aide financière du Programme Innov’Action agroalimentaire, un programme issu de l’accord Cultivons l’avenir 2 conclu entre le ministre de l’Agriculture, des Pêcheries et de l’Alimentation, et Agriculture et Agroalimentaire Canada.

Project Start

June 2014

Project End

May 2017

Objectives

  • Quantify the effects of cover crops on cash crop yields and on nitrogen dynamic in soil through the use of meta-analysis (compile and classify data from both literature and unpublished studies) and provide information on the effects of cover crops on soil quality and on nitrogen leaching losses.

Impact

  • The identification of the best management practices of cover crops in cash crop production considering the cash crop yield and the soil quality will lead to rapid adoption of cover crop use in producer’s cropping rotation.
  • The establishment of the nitrogen contribution from cover crops will improve the production efficacy and greater economical return to producers through efficient use of nitrogen fertilizer.

Scientific Summary

A cover cropping system is a best management practice that captures and recycles excess nutrients in the soil profile while minimizing soil erosion. There has been a renewed interest in this cropping practice in the last few years. Interseeded or seeded after a main crop, cover crops improve fertility and soil quality with a considerable potential to maintain or enhance crop yields by the nitrogen contribution derived from these green manures once returned to the soil, or by the improved soil structure. The potential benefits of cover crops to crop productivity are determined by the cover crop type (legumes, non-legumes, and mixtures), the cover crop biomass production, and the timing and methods of cover crop termination.

This project aimed to document the impacts of cover crops on subsequent cash crop yields, soil nitrogen dynamics and soil quality. It also aimed to establish measurements of the N contribution (i.e. N credit) from cover crop to potentially adjust the nitrogen fertilizer application to the different crops in the fertilization plan. In this study, we performed a meta-analysis of data to combine and synthesize results from 139 field experiments (peer-reviewed or unpublished literature) to provide a comprehensive and quantitative approach of cover crop’s influence to cash crop systems. The specific objectives were to quantify (1) the effect of cover crops on cash crop yield, (2) the N contribution of cover crops to cash crop growth (compared to bare fallow), and (3) the variation of these impacts across a wide range of systems. Data were included if they met the following criteria: (1) cover crops were grown (intercropping, successive or full season systems) with a subsequent cash crop (corn, soybean, cereals); (2) a control treatment without cover crops was present; (3) the treatments were replicated; (4) the study has been conducted in a humid, temperate climate; and (5) cash crop yield, cover crop biomass and N concentrations in plant tissues were reported, which allowed us to estimate the relative contribution of cover crops to subsequent cash crop yields in terms of yield ratio (Yield with cover crops/Control Yield without cover crops).

The overall effect of cover crops on cash crop yields was significant in corn and cereal production with 16% and 22% yield increases, respectively. Conversely, the overall effect of cover crops on subsequent soybean was neutral (no yield difference with or without cover crops). Thus, the results focused on corn and cereal production that were analyzed independently.

The overall effect of cover crops on corn yield was controlled by cover crop types used, with best performance attributed to legumes (21% increase in corn yield) and mixes with legumes (16% increase in corn yield). Grass cover crops decreased corn yield by 4%, however, increasing amounts of well-distributed rains mitigated the overall negative effect of grasses on corn yield until a neutral effect in wet years (1500 mm). The negative effect of grasses also decreased as the corn fertilization increased and corn yield losses were compensated with 60-120 kg N/ha. Non-legume broadleaf cover crops had no effect on corn yield. Yield increases were reduced as corn N fertilization increased, although significant cover crop benefits to the subsequent corn crop (9% increase in corn yield) was still noticed at 120 and 60 kg applied N/ha in soils with low (<2%) and medium (2-5%) organic matter content, respectively. Above 5% organic matter content, corn yield was not impacted by cover crops. Mixes with legumes or legumes alone presented the highest amount of N accumulated in the cover crop’s aboveground biomass with 115 and 95 kg N/ha, respectively. Legumes offered the greatest corn yield benefits (between 24 to 21% greater yields with cover crops than without) in drier as well as in wet years. Conversely, mixes with legumes promoted greater corn yield in drier years (26%) but fewer benefits in wet years (11%). The effect of non-legume broadleaves on corn yield remained overall neutral and was observed regardless of rain.

Cover crop type and systems, rainfall and N content in the cover crop’s aboveground biomass were the most influencing factors of the cover crop’s benefits to subsequent cereal yield. Similar to corn production, legumes (alone or in mixture) cover crops provided benefits to cereals with 19 to 27% yield increases. Grasses had no impact on cereal yield, whereas non-legume broadleaves caused an overall 16% increase. Cover crops planted in full season or in intercropping systems respectively provided 27% and 23% yield increase, whereas cover crops in successive systems had no impact.

On the basis of the results for corn production, the use of legume cover crops that generally provide N accumulated in the cover crop’s aboveground biomassranging between 50 and 100 kg N/ha in eastern Canada would allow a decrease in corn N fertilizer rate ranging between 48 and 94 kg N/ha. However, synchrony between crop N demand and cover crop mineralization is critical in this process for which climatic conditions, soil properties and management practices act as important modulators.

The N contribution of cover crops to subsequent cereal yield has been studied less than N contribution to corn. Legumes, especially red clover planted in intercropping systems, provided a greater N contribution than grasses. At the species level, the use of pea, hairy vetch and mixture of red and white clover increased wheat N uptake more than 50% of the observations reporting positive cover crop effects on cereal crop total N uptake.

In addition to the value of cover crops as N sources, cover crops also have beneficial effects on soil quality. They can increase soil organic matter in the long term, but this effect is related to several factors, such as the number of years with cover crops, the above-ground and root biomass of the cover crops, the soil type, the type of tillage, and the climate. Cover crops increase the biological activity of the soil and seem to improve soil aggregation rapidly (<3 years). They can also increase water infiltration and reduce water erosion and sediment surface runoff. Finally, cover crops, especially grasses and Brassicas, may reduce the residual N content of the soil in the fall and reduce N leaching from fields. These benefits translate into multiple services in the agricultural ecosystem. In a context of sustainable development, cover crops deserve to be adopted on a larger scale by producers, particularly in agricultural watersheds where non-point source pollution is important. Finally, this integration of individual research has provided a comprehensive and quantitative approach of cover crop contribution to crop yields and soil protection, promoting confidence with cover crop systems.