On-seed application of peat slurry has provided the best nodulation, grain yield and nitrogen fixation in experiments comparing different methods of inoculating legumes.
The research, conducted as part of a Grains Research and Development Corporation (GRDC) investment, concluded that on-seed inoculation was “consistently superior” to in-furrow inoculation in terms of nodulation and crop productivity.
Eight field experiments were conducted over two years at four different locations in south-eastern Australia to quantify the response of lupin and faba bean to three inoculant methods: on-seed application as a peat slurry; in-furrow peat inoculant delivered as a liquid suspension at seeding; and in-furrow peat granules delivered at seeding.
Uninoculated treatments were also included in the experiments which were located at Mininera, Rutherglen and Boorhaman North in Victoria, and Culcairn in New South Wales.
Research leader Matt Denton, from The School of Agriculture, Food and Wine, The University of Adelaide, said the aim of the study was to determine whether inoculation application methodologies altered the patterns of nodulation on roots, and if so, whether this influenced nitrogen (N) fixation, crop growth or grain yield.
“The experiments were conducted to test the hypothesis that granular and liquid inoculants increase nodulation on lateral roots, with subsequent positive impacts on legume productivity,” Dr Denton said.
“But in our study, peat slurry inoculation on the seed, which increased nodulation around the root crown, had generally higher grain yields than other treatments. In contrast, in-furrow granular and liquid inoculants had more varied yield patterns.”
Dr Denton said the benefits of deeper, lateral nodulation may in fact be dependent on other factors such as soil water availability, protection of the inoculant from adverse conditions, and seasonal environmental conditions.
Symbiotic N fixation is a key biological process in legumes that supports plant growth and the production of high-protein seed and forage. The establishment of legume root nodules with a functioning N-fixing symbiosis requires that an appropriate root nodule bacteria (rhizobia) is already established in the soil, or that rhizobia are supplied at sowing by inoculation.
On-seed inoculation is the conventional form of legume inoculation and has served legume growers well for more than 100 years. This procedure ensures that inoculant rhizobia are delivered into the soil in the immediate vicinity of the emerging root. However, the procedure is somewhat time-consuming, especially with larger-seeded legumes, and it may represent a bottleneck at sowing.
In recent times some Australian growers have drilled liquid and solid granule formulations of rhizobial inoculants directly into the furrow. In-furrow inoculation has potential to mitigate against certain drawbacks of the traditional peat inoculant applied to the seed. In-furrow inoculation provides greater flexibility in sowing logistics and separates the inoculant from seed-applied fungicides and insecticides that may be hazardous to rhizobial survival.
Dr Denton said until now, gaps in knowledge existed in terms of how each of these different inoculation application methods performed in the field, under different environments and farming systems.
To assist in answering some of those questions, the GRDC invested in a study that was activated in 2008, involving researchers from The University of Adelaide, Agriculture Victoria, La Trobe University and CSIRO. Nodulation, peak biomass production, seed yield, N content and N fixation were measured for all treatments in 2008 and 2010.
“The evidence from our experiments was conclusive – in terms of most parameters of nodulation and crop productivity, on-seed inoculation was consistently superior to in-furrow inoculation,” Dr Denton said.
“However, there were few differences between the two forms of in-furrow inoculation – liquid and granules.”
Total N fixed was measured at an average of 180 kg N/ha for peat compared with 110 kg N/ha for in-furrow granules and 104 kg N/ha for liquid. Faba bean yields were the most notable in terms of difference, being an average of 2.40 t/ha when inoculated with in-furrow granules compared to 2.22 (in-furrow granules) and 2.06 (liquid).
In the study, all three methods of inoculation generally increased nodulation, N fixation and grain yield, relative to uninoculated treatments. Dr Denton said data from the experiments at Culcairn also illustrated the large economic benefits derived from using a quality inoculant, in any form, at an approximate cost of $10/hectare when soil rhizobia numbers are either low or absent.
“Under these conditions, the contribution of the additional N fixed and net N returned to soil in legume residues was equivalent to an input of $95-$115/ha of urea fertiliser, and the increase observed in faba bean grain yield of 1.94t/ha (relative to uninoculated treatments) was worth over $500/ha, based on grain prices at the time,” Dr Denton said.
GRDC Manager Soils and Nutrition – South, Stephen Loss, says legume crops and pastures deliver an incredibly cost-efficient, alternative form of N, as well as acting as weed and disease breaks for cereal crops and generating income and enterprise diversity benefits for growers.
“For many years the GRDC has recognised the importance of adequate nodulation in maximising the benefits of growing legumes and its investments in targeted research, development and extension activities have delivered considerable insights and information on inoculating legumes and nodulation,” Dr Loss said. Practical GRDC resources include the popular Inoculating Legumes guide and a new GrowNotes™ Legumes and Nitrogen Fixation Tips and Tactics fact sheet.
Meanwhile, in other related research activities, the influence of pesticides on inoculants is being investigated and an assessment of farmer inoculation practices in Australia is being undertaken.
Improving pulse options and the survival and viability of rhizobia in current farming systems is an issue identified as a high priority by the GRDC’s Southern Regional Cropping Solutions Network.