Benefits of Inoculating Legume Crops with Rhizobia in the Northern Great Plains

Inoculation of forage and grain legumes with rhizobia is an important process to maximize biological N2 fixation capacity in these crops. Inoculation has the potential of increasing dry matter yield, N yield, and residual N levels. However, yield responses to inoculation are not universal in the region. In fields that have previously grown the same grain legume crop (i.e. contain an endemic rhizobial population in the soil), positive yield responses occur from one-third to one-half of the time. Yield responses to inoculation are dependent upon many factors, but legume species and soil N levels prior to seeding are two important factors. However, given the modest cost of inoculation compared to the potential agronomic benefits, producers are well advised to seriously consider inoculation of their legume crops in all circumstances. Introduction Biological N2 fixation by legume-rhizobia symbioses is vitally important as an N input in agroecosystems in the northern Great Plains of North America. Inoculation of grain and forage seed at planting is generally recommended to maximize the potential for nodulation and N2 fixation in these crops. However, producers in this region commonly raise questions on legume inoculation. What benefits can we expect from inoculating grain and forage legumes with rhizobia? Do we always need to inoculate legume crops, even if a field had previously grown the same inoculated crop? Can we depend upon inoculation to meet all the N requirements of the legume crop? The objective of this article is to provide background information on the inoculation of legume crops and to review the benefits of inoculation of legume crops with rhizobia in the northern Great Plains (i.e. the northern United States from Montana to Minnesota and the southern regions of the three Canadian prairie provinces, Alberta, Saskatchewan and Manitoba). The primary focus of the review is to address the question of whether producers always need or should inoculate their legume crops. Comparisons of inoculant formulation (peat-based, liquid, or granular; e.g. 17) and its placement (seed treatment or seed row placement; e.g. 23) are not addressed. The review focuses on responses of grain legumes, although inoculation of forage legumes is also briefly addressed. Benefits of Rhizobial Inoculation Bacterial species within the genera Allorhizobium, Azorhizobium, Bradyrhzobium, Mesorhizobium, Rhizobium, and Sinorhizobium (commonly referred to as rhizobia) can infect, nodulate and symbiotically fix N2 in legumes. However, legumes express ‘host specificity’, meaning that only certain species or subspecies (i.e. biovars) of rhizobia will infect certain species of legume (Table 1). For example, pea is infected by Rhizobum leguminosarum biovar viciae, while common bean is infected by Rhizobium leguminosarum biovar phaseoli. Sometimes different legume crops (e.g. faba bean, lentil and pea) can be infected by the same species or biovar of Rhizobium, however, even within these crops often certain strains of rhizobia are more effective in one crop compared to another (8). Commercial rhizobial inoculants have been available for legume crops since the 1890's (12). Currently, a wide variety of commercial legume inoculants are available for use in the northern Great Plains (Table 2). Producers inoculate their legume crops to increase yields and decrease the input costs of N fertilizer. Yield responses to inoculation, even in fields with soil rhizobial populations sufficient to infect the particular legume host, are common (but not universal) in field trials in the northern Great Plains (Table