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Nodulation of Crimson Clover by Introduced Rhizobia in Mississippi Soils¹

ABSTRACT

Crimson clover (Trifolium incarnatum L.) has long been recognized as an important annual legume in the winter grazing programs of the South. Considerable effort has been devoted to the development of cultivars which are resistant to disease, reseed reliably, and produce more fall and winter forage. Little information on the bacterial microsymbiont has been reported, and questions exist which concern Rhizobium strain-host cultivar interactions, persistence of inoculum strains, and nitrogen fixation rates under field conditions. For this reason, five strains of R. trifolii were evaluated in competition with indigenous populations in nodulating crimson clover at two field sites in Mississippi. Proportions of nodules due to the introduced strains were measured by acquired and intrinsic antibiotic resistance. First season data indicated significant differences among the amended strains both within and between field locations. Three of the strains (162K10, RP113-14, and RP111-51) were recovered at a high frequency (>85% of the nodules sampled) in one site. At the second location, almost one-half of the nodules assayed were produced by the indigenous rhizobia when competing with 162K10 and RP113-14. In contrast, the proportion of nodules due to the other two strains, a local isolate (207) and an Australian culture (TA1), varied between 16 and 55% at both locations. Forage yield and percent crude protein levels reflected a positive association with the most competitive Rhizobium strains. These results indicate that selections for highly effective and adaptive Rhizobium strains can increase both crimson clover production and symbiotic nitrogen fixation.

¹ Contribution from the Dep. of Agronomy, Mississippi State Univ., and the Mississippi Agric. and Forestry Exp. Stn. Approved by the Director as Technical Paper no. 4,988.

² Research Associate and Soil Microbiologist, respectively. P. O. Box 5248. Department of Agronomy, Mississippi State University, Mississippi State, MS 39762.

Received for publication October 8, 1981. Accepted for publication January 19, 1982.