Determine how to optimize soybean in rotation with other crops, such as canola, wheat, and corn, as well as its potential negative impacts. Quantify the impact of the frequency of soybeans in the rotation on soil health parameters.
Crop rotations in Manitoba have shifted significantly in the past 10 years with the rapid expansion in soybean acres. This has raised important questions about the opportunities and implications on how to optimize soybean in rotation with other crops, such as canola, wheat, and corn that are commonly grown in Manitoba. This eight-year rotation study conducted at two contrasting locations in Manitoba (Carman and Kelburn) evaluated the impact of the frequency of soybeans in rotation on soybean production and soil health in central Manitoba. This report summarizes the second cycle of the 4-year rotation (2018-2021). The rotation treatments are as follows 1) Continuous soybean (S-S-S-S) 2) Canola-Soybean-Canola-Soybean (Ca-S-Ca-S) 3) Corn- Soybean -Corn- Soybean (C-S-C-S) 4) Wheat- Canola- Corn- Soybean (W-Ca-C-S). This project provided a comprehensive assessment of the frequency of soybean in Manitoba crop rotations on the performance of a soybean test crop after eight years of contrasting rotation treatments. It also evaluated the impacts of these eight-year crop rotation treatments on soil health indicators in Manitoba. As expected, the continuous soybean treatment resulted in reduced soybean growth as measured by biomass at both study locations and reduced yields at one of two study locations. Growing soybean in rotation with other crops increased soybean test crop yield at Carman the carman site, while drought conditions at Kelburn that resulted in significantly lower yields likely masked differences among rotation treatments with the exception of the Ca-S-Ca-S treatment that had higher soybean yields than all other treatments. Differences in soybean association with symbiotic organisms, such as brady rhizobium for nitrogen fixation and arbuscular mycorrhizal fungi for phosphorus uptake were influenced by rotation treatments earlier in the growing season but in general did not persist later in the soybean test crop growing season. Soil health indicators for labile pools of carbon and biological cycling of carbon, nitrogen, and phosphorus that were used in this study identified differences in rotation treatments after eight years. Results from both the soil enzyme and active carbon measurements identified the value of including high residue crops (corn or wheat) when designing rotations with high frequencies of soybean in Manitoba. Although influenced by the dry conditions that dominated during this second four-year rotation cycle, results from this study point to the value of growing soybean in rotation with other crops using a number of different performance measures. Study results also support the long term value of growing soybeans in rotation with other high carbon crops as a path to sustain soil health and to maximize the benefits from symbiotic relationships between soybean and beneficial organisms such as brady rhizobium and arbuscular mycorrhizal fungi.