Acid tolerant lucerne

Summary

Lucerne is a perennial pasture legume with high levels of summer production and adaptation to a broad range of agro-ecological environments in southern Australia. The summer production from this plant makes it valuable for our red meat industries, finishing livestock for market, reducing the grow-out period of weaners and increasing ovulation rates in maternal stock.

Lucerne is grown on approximately 3 M ha in south-eastern Australia, but poor tolerance to acidic soils limits its further adoption. Lucerne is ideally established on soils in the pH­­Ca range of 5.5 to 8.0, and although it is grown on more acidic soils, forage yield and persistence on these soils is often suboptimal.

Objectives

The primary aim of this research was to define the performance of new lucerne varieties and rhizobia strains selected for improved tolerance to soil acidity across a range of environments in south eastern Australia. Four sites in SA, Vic. and NSW with pH­Ca­4.1-4.3 were chosen with contrasting texture, aluminium and fertility, and this combined with a treatment to ameliorate surface pH to varying degrees with lime, was used to generate a range of environments to evaluate the performance of this germplasm.

Key findings

The results of this project have demonstrated that lucerne is more tolerant to highly acidic soils than previously described in literature. Forage production in the first calendar year after sowing ranged from 4 to 12 t/ha under rainfed conditions at three of the sites with soil pH­Ca 4.1-4.3. The long term production of lucerne in these environments is however, associated with greater risk because of decreased nodulation and lower persistence at some of the sites compared to the lime treatments.

The addition of 1.2 t/ha lime partially ameliorated the surface soil to pHCa to 4.6 and increased average forage yield from 8 to 12t/ha/yr, nitrogen fixation  by 26%, and plant persistence by 28% (excluding Boralma). The annual production of 12 t/ha had an approximate feed value of 147MJ energy and 2625 kg CP/ha of protein. This would be especially valuable to red meat producers considering that 50% of the growth occurred outside of the traditional winter-spring production period, extending the growing season into summer and autumn. The summer production occurred despite decile 1-3 springs in 2014 and 2015, illustrating the capacity of lucerne to deliver a constant feed supply, which gives confidence for feed budgeting in a variable climate.

The relatively small cost of applying lime (1 t/ha will cost up to $100/ha delivered and spread) will be recouped in the first 12 months of production, with an additional 4 t/ha of growth (valued at $720/ha using $180/t) and as much as 90 kg N/ha fixed (extrapolated from a single cut, valued at $120/ha). The addition of lime is also expected to benefit the production of lucerne and other pastures or crops grown in rotation for up to 10 years.
Poor forage production did occur at Boralma, but we believe that growth at this site was also constrained by subsoil sodicity, poor soil structure, and waterlogging. Lucerne production at this site did not respond to the high lime rate (2.8t/ha), providing evidence that problems with lucerne production were not associated with soil acidity. The poor production at this site illustrates the need to have a holistic view of the soil and landscape when considering the suitability for lucerne.

he interaction of site and lime treatments was used to create a large number of environments (4 sites x 4 lime treatments = 16 environments) that were used to investigate factors important for determining lucerne performance on acidic soils. Multivariate and principal component analysis identified pH and aluminium concentration as important contributions to variability in forage yield, nodulation and persistence (all sites except Boralma).

A further aim of this research was to compare the performance of two lucerne varieties developed using different methods for improved tolerance to soil acidity. The variety 'SARDI 7 Series 2', selected for on-farm performance in cool temperate environments with acidic soils, was compared to a new variety 'TA37', which was selected directly for traits associated with tolerance to acidic soils (solution culture screening for tolerance to low pH, aluminium toxicity and symbiotic effectiveness). In solution culture the trait-based selected line TA37 has superior root growth with increases of 32, 48 and 32% over SARDI 7 Series 2 at 0,3 and 6uM Al respectively. TA37 also nodulates with rhizobia in more acidic environments, extending nodulation by approximately 0.3 of a pH unit in solution culture. This result extended into field conditions, where TA37 had greater nodulation than SARDI 7 Series 2  between pHca of 4.1-4.4. However there were few other differences between the varieties detected in the field, each having excellent forage yield and persistence. The results support a recommendation for both TA37 and SARDI  7 Series 2 varieties to be promoted on acidic soils in south eastern Australia.  

The new strain of rhizobia produced better nodulation in solution culture than commercial strain RRI128 and improved nodulation most when combined with TA37.  The benefits of the new rhizobia strain were greatest below pHca 4.5, but negligible at pHca 5.0.  SRDI736 also survived at lower number on seed (about 35% of RRI128) which may affect its performance in alkaline soils with substantial populations of rhizobia.  All things considered, we propose a revision of the existing recommendation to replace RRI128 with SRDI736.  The revised proposal is that rhizobia strain SRDI736 is released commercially as a specialised strain for lucerne on acid soils.

Future research

The recommendation for industry is to continue to advise farmers to grow SARDI 7 Series 2 (because seed is available) in combination with the new rhizobia strain 'SRDI736', on acidic soils. Further research is underway to determine if there are environments where TA37 has an advantage over SARDI 7 Series 2, and the sowing of the two varieties as a mixture is proposed as an additional consideration.  Whilst the varieties have shown the potential to be productive, persist and fix nitrogen below pH 4.5 in the field, we recommend that soils below that pH are limed to lift overall lucerne production.  

An important output of this research will be to extend the messages to producers so that lucerne production on acidic soils can be optimised and extended into new areas. Heritage Seeds and SARDI gave a total of 16 field days and seminars to over 350 farmers, agronomists and university students at 12 locations throughout the project. Heritage Seeds also have annual field days at Howlong (850 people visits over the last 3 years) and Toowoomba where messages from this project will continue to be delivered in the future. Finally, an information package will be developed for red meat producers to summarise the results contained in this study. The delivery of this research is expected to increase the confidence of producers growing lucerne, which will have long term benefits for red meat production in Australia.