Evaluating rhizobial inoculants for Saskatchewan fenugreek production

Objective: Acquire up to 30 pure strains of Sinorhizobium meliloti and commercial Rhizobium inoculants registered globally for use on fenugreek, sweet clover, and alfalfa. 

We acquired 6 commercial inoculant products from Argentina, Canada, Lituania, South Africa, and the United States, and 15 pure Sinorhizobium meliloti (S. meliloti) strains isolated from Australia, Canada, South Africa, Spain, and the United States. 

Objective: Identify commercial inoculants and pure strains that are able to initiate nodulation. 

Four of the commercial inoculants and 12 of the pure strains were able to initiate nodulation in at least 1 of the 2 fenugreek varieties we tested, CanaFen and Fenucold. 

Objective: Assess up to 10 strains from the commercial inoculants and pure strains for the ability to conduct biological N fixation. 

We tested 3 commercial inoculants and 4 pure strains that were best able to initiate nodulation for their ability to fix nitrogen in CanaFen and Fenucold. The top performing strain overall was USDA1150 and the top performing commercial inoculant was RIZOLIQ TOP-Alfalfa from Rizobacter. 

What are the results of this project in more detail?

• We tested 6 commercial inoculants and 15 pure Sinorhizobium meliloti strains (the species that is known to initiate nodulation on fenugreek) for their ability to initiate nodulation in the Canadian fenugreek varieties CanaFen and Fenucold. There was considerable variation in number, colour, size, and distribution of the nodules initiated by the different S. meliloti strains. Two of the commercial inoculants registered for alfalfa and sweet clover and 3 of the pure strains failed to initiate nodulation in either fenugreek variety. Interestingly, the pure strains we tested that were isolated from fenugreek (USDA1177, USDA1178 and USDA6693) were all poorly compatible with either variety of fenugreek examined here, further highlighting the specificity of the interaction. A successful interaction between rhizobial strains and host legumes requires recognition of distinct molecules excreted from the host by the rhizobial strain and concomitant recognition of molecules from the strains by the legume host . The variability in nodulation that we found reinforces the concept of specificity on either side of the host-microbe symbiosis. It is important to note for researchers, manufacturers, and farmers alike that making inoculant choices based on broad compatibility groups at the genus and species level may lead to poor nodulation and, in some cases, no nodulation at all.  
• We tested 3 commercial inoculants and 4 pure rhizobial strains for their ability to conduct biological nitrogen fixation in CanaFen and Fenucold. We repeated the entire nitrogen fixation greenhouse experiment twice, once in the winter/spring and once in the summer. Like in the nodulation assessments, there were major differences in nitrogen fixation across both the rhizobial strains and fenugreek varieties. Differences between the same inoculant-variety combination across the duplicate experiments showed the influence that the environment plays in biological nitrogen fixation symbioses. Because we used the same growing medium for the entire project, we controlled for differences in soil structure, composition, fertility, and microbiology, which could no doubt further confound interpretation and selection of a superior rhizobial strain or strains for inoculation of fenugreek in Saskatchewan.  
• There are no rhizobial inoculants specifically registered for use in fenugreek. Although fenugreek can initiate nodulation with compatible strains of Sinorhizobium meliloti, the ability to both initiate nodulation and fix nitrogen are governed at the S. meliloti strain and fenugreek variety level. This specificity means that not every strain of S. meliloti can initiate nodulation on every variety of fenugreek. Where functional nodulation occurs, fenugreek can derive up to 58% of its required N from biological N fixation under the right conditions. The best performing rhizobial strain overall that we tested was USDA1150, an S. meliloti strain isolated in Australia in 1959, which contributed to high levels of N fixation in both the seed-type fenugreek variety CanaFen and the forage-type variety Fenucold.  
• Our recommendation for inoculant use on fenugreek in Saskatchewan is the commercial product RIZOLIQ TOP-Alfalfa produced by Rizobacter.  RIZOLIQ TOP-Alfalfa is registered for use on alfalfa in Canada (CFIA registration # 2019056S).
• It is important to note that failure to perform in this fenugreek study is no indication of a strain or product’s ability to perform as an alfalfa or true clover inoculant.

What benefit will these outcomes bring to farmers in SK?

Because fenugreek can fix up to 58% of its required nitrogen through association with compatible S. meliloti strains, identification of a high-performing commercial inoculant that is currently registered in Canada allows for fenugreek growers to reduce their potential fertilizer costs. Our study also highlights the specificity between individual strains of S. meliloti and varieties of fenugreek, which, together with our recommendation for using RIZOLIQ TOP-Alfalfa, will prevent money being wasted on products that may not fix nitrogen in the fenugreek varieties grown in Saskatchewan.