An inoculant Rhizobium strain for improved establishment and growth of hexaploid Caucasian clover (Trifolium ambiguum)

Strains of Rhizobium leguminosarum bv. trifolii present in New Zealand soils are ineffective on Caucasian clover (Trifolium ambiguum) and inoculation with an effective strain of rhizobia is essential for nodulation. However, problems have been encountered with nodulation of oversown Caucasian clover and this study was conducted to determine whether nodulation could be improved by the use of an improved strain of rhizobia. Five strains of rhizobia were compared with a re-isolate (ICC 105) of the recommended New Zealand commercial inoculant strain (ICMP4073b) for their effect on the establishment of oversown hexaploid Caucasian clover in Canterbury and Otago, South Island, New Zealand. When strains were incorporated in peat inoculant and used to inoculate seed in a commercial pelleting process, one strain (ICC 148) increased the percentage of seedlings nodulated over that obtained with ICC105, from 23% to 49%. In a further evaluation of these two strains, seed was inoculated and lime-coated in the laboratory and stored for either 1 or 20 days before oversowing on 7 tussock grassland sites. Inoculation with ICC 148 resulted in a 1.3-fold increase in the percentage of seedlings nodulated over that obtained with ICC 105. In addition, dry matter (DM) weights of individual seedlings were increased with strain ICC 148. The dry weights of individual Caucasian clover plants after 6 and 13 months increased 1.5 and 3.0-fold, respectively, when inoculated with strain ICC 148. Inter-tussock Caucasian clover DM after 17 months was 1870 kg ha −1 for plants inoculated with ICC 105 and 3480 kg ha −1 for plants inoculated with ICC 148. Foliage N contents in spring were increased from 2.6% to 3.1%, when seed was inoculated with ICC 148. These results strongly support a change in the Rhizobium strain recommended in New Zealand for hexaploid Caucasian clover to strain ICC 148.