Effect of cowpea and pea inocula on cool-season grasses

Rhizobium or Bradyrhizobium bacteria, which form nitrogen fixing nodules on the plant roots, also produce plant growth hormones that can stimulate plant growth, or other compounds that may inhibit pathogenic bacteria. Non-legume crops (corn and rice), inoculated with these bacteria have shown greater plant growth than un-inoculated plants due to stimulatory effects of plant growth regulators released by the rhizobial bacteria. Inoculated forage legumes are often planted with forage grasses. The possible effects of rhizobacteria inoculum on forage grasses have not been evaluated. Here commercial inocula and non-sterile soil to simulate field conditions were used to evaluate the possible effects of Rhizobium and Bradyrhizobium on the early growth of three cool-season forage grasses (Italian ryegrass (Lolium multiflorum Lam.), tall fescue (Festuca arundinacea Schreb.) and tall wheatgrass [Elytrigia elongata Nevski)]. The grasses were inoculated with cowpea (Bradyrhizobium sp., a warm-season legume inoculum) or pea (Rhizobium sp., a cool-season legume inoculum) inocula. The grass seedlings were grown under controlled environmental conditions and harvested at either 4 or 6 weeks after emergence. Both inoculum reduced the plant dry weight, but the effect was greater with cool- than with warm-season inoculum. The pea inoculum reduced root and shoot dry weight by 29% and 20%, respectively, while the cowpea inoculum reduced root and shoot dry weight by 11% and 2%, respectively. Although only early seedling growth was evaluated, such a delay or inhibition in seedling growth would reduce the competitiveness of the grasses and result in reduced yields.