Extension of the current 12-month expiry of rhizobial inoculants in Australia to 18 months would have commercial benefits for the manufacturers and resellers. The dilemma, however, is that numbers of rhizobia in the inoculants decline over time and individual cells may lose efficacy. The research undertaken in this study shows the effect of lupin and chickpea inoculant age (i.e. 0, 6, 12, 15 and 18 months old) on numbers of rhizobia, rhizobial cell characteristics and efficacy. For the latter, assessments included colony size on plates, survival on inoculated beads, and infectiveness and effectiveness in field experiments at 3 sites.

Assessment of commercially produced inoculants at the Australian Legume Inoculants Research Unit (ALIRU) laboratory indicated that, on average, chickpea and lupin inoculants had counts of about log₁₀ 9.6 when fresh, delivering >log₁₀ 6 rhizobia/seed. At 12 months, the average counts had fallen to log₁₀ 9.4, delivering slightly less than log₁₀ 6 rhizobia/seed. By 18 months, average counts were log₁₀ 9.3, delivering log₁₀ 5.9 rhizobia/seed. The lines of best fit indicated decline rates of 0.0005 log₁₀ units/day. We found no evidence that the rhizobia in the older inoculants (i.e. >12 months old) had lost any ability to grow on nutrient agar, survive on inoculated beads, and nodulate and fix nitrogen with the host plant. While the chickpea and lupin inoculants produced currently in Australia are as efficacious after 18 months of storage at 4°C as they are when fresh, efficacy of other inoculant types may fall below acceptable levels at <12 months.