SOUTHERN PASTURE & FORAGE CROP IMPROVEMENT CONFERENCE Lexington

Nothing is more important for effective nodulation of legumes that the availability of sufficient numbers of an infective strain of rhizobia in the root zone of seedlings. In some cases, the soil will contain sufficient numbers of infective rhizobia but modern approaches to the use of genetically superior strains of rhizobia practically demands the use of some means of inoculation, thus raising the potential for effective N2-fixing nodules being formed. The means by which inoculation is achieved varies both for legume species and by country. In Australia’s recent past, peat cultures containing at least 10 rhizobial bacteria per g have been recommended for direct application to seed applied at a total rate of 10-10 cells per ha were standard practice. This approach, coupled with a reliable means of certifying the quality (as viable cells of a specific nodulating strain) of the commercial cultures for a shelf life of at least 12 months in cool conditions proved very effective in Australia for the past 50 years since means of quality control were developed. Worldwide and in Australia other means of delivery of inoculants have been sometimes favoured. For example, liquid cultures in sealed flexible plastic containers with proprietary additives and a stated shelf life of one year have been favoured for field crops like soybeans. In other cases, the use of lyophilised (freeze-dried) cultures has been applied. Both of these techniques have the advantage of not requiring a good quality inert carrier but may raise questions regarding an effective certification procedure. Ideally, independent methods of quality assurance of inoculants will be available and in Australia the Australian Legume Inoculants Research Unit provides such a service. In the absence of such quality controls it is a recommended practice that users of commercial inoculants should conduct their own demonstration field trials by comparing the nodulation performance of at least one seeding run of un-inoculated forage legume with the inoculated plants at large. Introduction Inoculation of legumes with specific strains of rhizobia has now been practiced in the USA and the Americas, Europe and Australia for around 100 years. In Asia, inoculation was promoted more recently, by agencies such as NifTal based in Hawaii funded by USAID, the international agricultural research institutes (CGIARs) such as ICRISAT and ICARDA and the Australian Centre for International Agricultural Research (ACIAR). Early Australian experience was summarised by Brockwell (1977). The following review material is abstracted mainly from Deaker et al. (2004). Australian practices Legume seed has commonly inoculated in Australia in the past half-century with peat cultures of rhizobia. Their commercial production began in 1953 using finely milled peat as the bacterial carrier. Following widespread nodulation failures, the quality of inoculants was improved by the