Soybeans in succession to black oats and wheat can have 54% higher productivity

Sowing black oats and wheat in southern Brazil, during autumn/winter, can increase soybean productivity by 54%, compared to areas left fallow. This is one of the findings of the article Performance of soybeans in succession to black oats and wheat, published in June 2020 in the Brazilian Agricultural Research (PAB) magazine, produced by researchers from Embrapa and four universities.

The experiment was conducted in the autumn/winter of 2017 and 2018 and seven treatments were applied: fallow (1); black oat or wheat straw, without roots (2 and 3); plots with black oat or wheat roots, without straw (4 and 5); and plots with straw and black oat or wheat roots (6 and 7). According to Embrapa researcher Alvadi Balbinot, soybean performance was estimated based on the variables: plant density; leaf area index; chlorophyll content, estimated by the SPAD index; accumulated dry matter, grain productivity and yield components. 

“We confirmed that the effects of black oat and wheat roots proved to be as important as those of the straw cover left on the soil in explaining the productivity increases of 1.467 kg/ha, that is, 54% higher than in areas that they remained fallow in autumn/winter”, highlights Balbinot. “The agronomic performance of soybeans is better in the combined presence of roots and black oat or wheat straw, when compared to areas left fallow”, he emphasizes.

Direct planting and crop diversification

For the authors of the article, the direct planting system in straw presents economic and environmental benefits when compared to conventional tillage, especially when it is possible to maintain plant diversity and biomass production. Even so, fallow agricultural areas between two soybean harvests, from March to September, are still common in Brazil. “We understand that cultivation systems with low plant diversity and consequent low biomass addition are the main cause of soil degradation under direct planting”, explains the researcher.

According to the authors, black oat has a great dry matter production capacity, resulting in adequate soil coverage under direct planting, high nutrient cycling and weed suppression. Furthermore, the authors say that the species can be easily dried for planting subsequent crops. 

Roots can improve the physical quality of the soil, favoring water infiltration and retention and also the release of nutrients for subsequent crops. On the other hand, straw reduces the rate of water evaporation in the soil, soil warming peaks, weed infestation and the occurrence of soil erosion.

As for removing black oats or wheat for silage or hay production, it may reduce the benefits of these crops on subsequent soybean yield. “Therefore, the producer must consider this information in the decision-making process”, assesses the scientist.

Relationship of straw and roots with lack of water

In the 2017/2018 harvest, water availability was adequate throughout the soybean development cycle. In 2018/2019, there was a water deficit during flowering and grain filling. In this harvest, black oat and wheat crops benefited soybeans, in relation to fallow. “Therefore, it is possible to infer that the positive effects of straw and roots on soybean productivity are mainly associated with the reduction in water stress observed during the grain filling period”, he highlights. “From this point of view, the positive effects of black oat or wheat roots on soybean productivity occur largely through the improvement of soil structure”, he assesses. 

He further explains that root growth from previous harvests contributed to fracturing compacted soil layers and creating a complex network of continuous and stable biopores. “It is well known that biopores produced by previous crops play an important role in increasing water infiltration into the soil, hydraulic conductivity and gas diffusion, providing greater availability of water and oxygen to the roots of subsequent crops”, highlights the researcher. Furthermore, the improved soil structure facilitates soybean rooting and water absorption from deeper layers. 

Balbinot also points out that straw also favors water storage in the soil, as it reduces losses through evaporation and runoff. “Therefore, straw on the soil surface likely increased water availability for soybean plants, mitigating the negative impacts of the 2018/2019 water deficit periods on legume productivity.” 

Another advantage of straw retention, according to him, is the reduction in soil temperature, providing a better environment for the growth and functioning of the soybean root, which increases the efficiency of water use by the plant and grain yield.

Effect on Biological Nitrogen Fixation (BNF) 

Another factor probably related to the positive effects of black oats or wheat on soybean performance is the increase in FBN. This is because, according to the authors, both water deficit and high soil temperatures impair the efficiency of bacteria used in FBN in tropical conditions. “As roots and straw contribute to improving the physical quality of the soil, it is expected that environmental stresses will be lower in these soils, consequently increasing soybean productivity”, he concludes.