Fitting the wheat and soybean succession

Early sowing of soybeans, in October or even in September, in addition to pushing for the exclusion of the cultivation of these areas in winter for grain production, can reduce the yield of soybean grains and the economic return from agricultural activity. This suggests opportunities to improve the fit between winter and summer crops, with the potential to increase food production.

The commercial supply of soybean cultivars of an indeterminate type and early cycle made it possible to anticipate the sowing of this crop in several regions of southern Brazil. Until the mid-2000s, sowing took place preferably in the month of November in succession to the wheat harvest, and in recent years, there has been a migration to October in several situations. Lacking scientific confirmation, this change, in the view of some rural producers and technicians, would serve to increase the productive potential of soybean cultivation due to the lower risks of water deficiency and/or Asian rust pressure in critical periods of the crop, as has been verified in the months of January and February.

A complicating factor in this practice of anticipating soybean sowing is the interference in the crop succession system within rural properties, in several situations negatively affecting or even making winter cultivation unfeasible. In southern Brazil, the cultivation of agricultural areas in winter to produce grains corresponds to around 20% of the area cultivated for grains in summer (see article “Sustainable diversification” – March 2016, nº 202, Revista Cultivar Grandes Culturas), despite the available area and infrastructure. If winter crops are harming the main economic crop, which is soybeans, there would be another negative factor attributed to winter crops, especially wheat, the main grain crop in the cold season.

In the search for technical indicators to evaluate and identify cropping strategies for winter wheat and soybean crops in succession, Embrapa, in partnership with teaching and/or research institutions in southern Brazil, conducted field work during several harvests. agricultural, therefore, representing different meteorological, soil, management and productive potential conditions. With Setrem, in Três de Maio, Rio Grande do Sul, we sought to represent the moderately hot, humid and low-altitude region for winter crops and with considerable water restrictions for soybeans. At Embrapa Trigo, in Passo Fundo, Rio Grande do Sul, the region represented the cold, humid and medium-altitude region in winter and with better water supply in summer compared to the western region of Rio Grande do Sul. Already in partnership with Fapa/Agrária in Guarapuava, Paraná, which conducted representative trials in a cold, humid and higher altitude region in winter and with water availability in summer suitable for high grain yield. In these locations, 24 to 28 strategies for combining winter and summer crops were evaluated. In winter the following were cultivated:

To simulate the practice of producers, soybean cultivars were used combining determinate and indeterminate growth types with relative maturity groups (GMR) ranging from 4.9 to 6.3 (NA 4990 RG, NS 4823 RR, A 4725 RG, BMX Energia RR, BMX Ativa RR, BMX Apolo RR, TEC 5833 IPRO, FUNDACEP 62 RR, AFS 110 RR, BRS Tordilha RR, A 6411 RG, FPS Urano RR, TMG 7262 RR and NA 5909 RG). These soybean cultivars come from several breeders and represent groups of cultivars offered commercially. Likewise, winter crop cultivars come from different breeders. For both situations (winter and summer), the cultivars and crop management practices were defined together with technicians from each region. In this way, they represent current regional practices.

The results obtained indicated that the higher soybean grain yield expected with earlier sowing is not widespread in southern Brazil. On the contrary, it would only be occurring in specific conditions where the winter crop harvest occurs from mid-November and continues until the beginning of December, which is why soybeans are sown late. Thus, in the Northwest and Middle Plateau regions of Rio Grande do Sul, an average reduction of 18% and 19,4% in grain yield was observed when soybean sowing was brought forward (Figure 1). 

On the other hand, there are differences in grain yield between soybean cultivars when sown earlier than in the traditional season (Figure 2). This differential behavior of cultivars is an opportunity for producers and technicians to choose those that are most responsive in terms of grain yield at each sowing time. For example, in the Northwest of Rio Grande do Sul, if the decision is to adopt a cultivar with a specific growth type and anticipate soybean sowing and, in this way, give up grain production in winter cultivation, there is a risk to the soybean grain yield. be 50% lower than possible with other available cultivars. Still in this region, in general, the best soybean grain yields were obtained with cultivars of indeterminate type. In the Middle Plateau of Rio Grande do Sul, the greatest variations in soybean grain yield with early sowing occurred with cultivars of a determined type and intermediate cycle, although considerable losses also occurred with early sowing of cultivars of an indeterminate type and cycle precocious. On the other hand, in the Central-South region of Paraná, the soybean cultivars evaluated showed a grain yield gain of 5% to 30% in early sowing.

Because soybeans are the species most cultivated in summer, although generally in monoculture, and in winter the area cultivated for grains is much smaller, the results obtained clearly indicate opportunities to adjust the coexistence of the two crops, wheat and soybeans, on agricultural properties. of southern Brazil, including making it possible to increase the area cultivated in winter to produce grains. In regions where early soybean sowing does not bring benefits - it even reduces grain yield compared to November sowing, therefore, the opposite of what is reported by some technicians and rural producers - the best way is to maintain the winter crop to produce grains (adjust the sowing time and the choice of cultivars so as not to extend the cycle too much) and soybean sowing at the traditional time, after harvesting the winter crop. This makes it possible to obtain a higher grain yield in the summer and guarantee or increase the cultivation area in the winter, improving the contribution of agriculture to the production of food for humans and animals, in addition to being more agronomically coherent. In the highest and coldest regions, a dilemma arises, as anticipating soybean sowing increases its yield. However, winter cereals have high grain yields, in addition to economic returns, and cannot be ignored in grain production systems. In such a situation, the high grain yields obtained from winter crops could compensate for much of the lower grain yield from soybeans in succession. The analysis of grain production and economics, looking at the system and not at an isolated crop, is fundamental to realizing this opportunity. Another opportunity for this situation is to adopt soybean cultivars with better performance when sown after the winter crop, that is, with less loss of grain yield potential when sown at the end of November or during December. 

The studies conducted by Embrapa and partners reinforce current technical indications for wheat and soybean crops, as well as widespread agronomic knowledge. However, in each region, opportunities were perceived to better fit cultivars from this succession of crops that have been successful for a long time in southern Brazil. Decision-making to adopt the wheat and soybean succession has become more specific, technical and not generalist, as is sometimes suggested. The interaction of cultivars x sowing times, whether in crop succession or within each crop, influences plant growth and development and must be observed in management practices that aim to enhance grain yield in a sustainable manner. Finally, it should be noted that excluding wheat from the grain production system, in the winter period, in the expectation of having a better yield from soybean cultivation, means burdening soybean cultivation with all the maintenance and depreciation costs of the crop. structure present on the properties (machines, implements and silos, for example), in addition to lower efficiency in the use of the available structure, since less grain will be produced per unit area.

Mércio Luiz Strieder, João Leonardo Fernandes Pires, Embrapa Trigo; Marcos Caraffa, SETREM

Article published in issue 204 of Cultivar Grandes Culturas.