To investigate soybean responses to and interactions between granular inoculant rates and contrasting nitrogen (N) fertilization practices.
To investigate soybean responses to and interactions between granular inoculant rates and contrasting nitrogen (N) fertilization practices; to improve upon N management recommendations for a growing number of new soybean producers in Saskatchewan.
Field trials were initiated in 2015 and conducted over three seasons at three Saskatchewan locations (Indian Head, Outlook, and Melfort) to improve provincial soybean nitrogen (N) management recommendations. The observed N removal and estimated uptake clearly illustrated the need for strong nodulation and N fixation to support high soybean yields. While it generally increased vegetative growth, meaningful benefits to starter N were inconsistent and usually only observed in the absence of granular inoculant. While, with poor inoculation, starter N was occasionally beneficial, there was also evidence of negative impacts on nodulation, therefore, this practice is not recommended. At all sites with no previous soybean history, there were strong yield responses to dual inoculation. At the only site with a history of soybeans there was no benefit to dual inoculation. Overall, these results agree with other research and support the recommendation of dual inoculation in fields with limited soybean history. While not recommended as a general practice, in-crop N applications have a fit for mitigating yield loss when poor nodulation is confirmed. While low seed protein concentrations have occasionally been flagged as a concern for Saskatchewan soybeans, these results indicate that optimal management for yield and protein are similar. Nitrogen fertilizer was never beneficial and occasionally reduced protein when inoculation was adequate. Sometimes, seed N concentrations continued to increase at granular inoculant rates beyond those required to maximize yield; however, the responses were usually similar.