Poor performance of preinoculated pasture seed

Take home message

• The rhizobia strain in root nodules can now be identified by DNA test.
• A survey of 20 irrigated pastures in the NSW Murray Valley indicated an 80% failure rate of preinoculation.
• The main problem appears to be storing seed too long after preinoculating.
• 'Old’ seed needs an additional inoculation treatment.

Background

Annual pastures, primarily subterranean clover, were common on irrigated mixed farming properties in the NSW Murray Valley throughout the second half of the 20th century. These pastures however did not survive the millennium drought and were basically absent from the region by 2009. With the return of reasonable water allocations in 2010, some landholders began to re-establish annual pastures into their system. The majority of these new pastures were originally sown with preinoculated seed.

The rhizobia strain present in all Group C inoculants since 2005 is code named WSM1325. This replaced the former Group C inoculant rhizobia strain (code name WSM409) due to its superior performance. All annual clover pastures sown since 2005 should therefore be nodulated with the current Group C strain (WSM1325).

A survey was conducted in spring 2017 of 20 irrigated annual pastures in the Berrigan, Finley, Deniliquin and Bunnaloo areas. This was a small part of a wider survey conducted on dryland pastures in the eastern half of the Murray Local Land Services region and in the Riverina Local Land Services region.

The survey included extensive background information on the age of the pasture, inoculation method used, herbicide usage, insecticide usage and fertiliser practices. At each site a soil test was taken and the pasture was sampled to determine nodulation health and the rhizobia strain(s) present in the root nodules. This paper will only report on the findings of the rhizobia strain testing.

Method

At each site, four pasture sods were dug up and the soil washed from the roots. The nodulation was assessed against a standard chart and an average ‘score’ was determined. The roots were cut, padded dry and sent to the MALID ID laboratory in Perth to determine the rhizobia strain present.

Pastures established since 2005, which were originally sown with preinoculated seed, were analysed to determine the success of this inoculation technique. The test could determine if the rhizobia present was the current Group C strain or one of the old Group C strains or another strain altogether.

Results and discussion

The results of the paddocks sown with preinoculated seed are shown in Table 1.

Table 1. Rhizobia strain present in the root nodules of the sampled plants. Samples taken from paddocks sown with preinoculated seed.

The results showed that that preinoculated seed failed in over 80% of cases in our surveyed paddocks (Table 1) which was an extremely disappointing result. It should be noted however that only 20 pastures were sampled and a larger sample would be needed to more accurately indicate the success rate of the technique.

This result, while disappointing, was not unexpected. The GRDC report ‘Inoculating Legumes: A Practical Guide’, published in 2012, reports on surveys of inoculant quality (rhizobial numbers) of preinoculated seed sampled at the point of sale. This report indicated that only 32% of preinoculated subterranean clover seed had adequate viable rhizobia at point of sale (i.e. a 68% failure rate). It also reported that no seed had adequate rhizobia numbers 50 days after preinoculation.

If seed purchased is then stored on-farm for some weeks before sowing, the number of viable rhizobia would further decline. Therefore, our results of an 82% failure rate in the field are not inconsistent with the survey that showed a 68% failure rate at the point of sale.

It appears that the main problem is that landholders are not ensuring that they sow the seed within six weeks of the date of preinoculation.

Irrigated pastures present an additional challenge to successful inoculation because the seed is commonly sown into dry soil and the seed may sit there for several days before it is irrigated. This means that seed inoculated on-farm at point of sowing with peat based inoculum will also be ineffective as this rhizobia will only survive in adequate numbers for a day when in dry conditions.

Irrigators attempting to inoculate any bare or out-of-date preinoculated seed need to consider how to do so effectively. Sowing peat inoculated seed will only be effective if it is sown into moist soil. If sowing into dry soil which will later be irrigated, a more suitable inoculation method, such as a clay based granule, needs to be used.

Conclusion

The use of preinoculated seed for establishing subterranean clover pastures has become widespread. In fact, most of the newly released varieties are not available as bare seed. While it is a very convenient method to use, it has not proven itself to be a very successful method.

The success of the technique relies on the use of freshly preinoculated seed. Growers are advised not to rely on preinoculation if the seed is sown more than six weeks after the inoculation date (which is usually written on the bag or the seed label). Older seed should have an additional inoculum source, such as being sown with a clay based granule.

Acknowledgement

The survey reported on in this paper was funded by the Australian Government through the National Landcare Programme. The author would like to thank them for their support.

Contact details

John Fowler
Murray Local Land Services
449 Charlotte St, Deniliquin 2710
(03) 5881 9933
0427 079 138
john.fowler@lls.nsw.gov.au