Can serradella increase nitrogen in pastures?

Serradella nodules (left) and subclover nodules (right).

There is growing interest in using serradella in high rainfall pastures as an alternative to subclover due to its potential to fix more nitrogen and better withstand dry season conditions.

Frustrated by the lack of nitrogen-fixing nodules on his subclover roots, Busselton producer Len Hamersley is trialling two varieties of the alternate legume this year.

Len recently hosted an event by Western Beef Association Inc that gave 39 producers a chance to learn more about the factors affecting clover nodulation and whether serradella could be a suitable replacement.

South West NRM Sustainable Agriculture Program Manager Peter Clifton spoke at the event about the issues with sub clover and the potential for serradella.

Peter said nodules on legume roots indicated presence of rhizobia, the nitrogen fixing bacteria that form a symbiotic relations with the plant. If the nodules were pink, it generally meant they were fixing nitrogen.

The more pink nodules there were on the roots, and the bigger they were, the more nitrogen that was fixed.

“Well-nodulated legumes can produce about 20 to 30 kilograms of nitrogen per tonne of legume dry matter. But as Len has found, subclovers are often poorly nodulated,” Peter said.

“We became aware of an issue in about 2016 when Manjimup agronomist Paul Omodei suggested that only 1 in 10 paddocks were nodulating adequately,” Peter said.

Looking for answers

So in 2019, South West NRM and Western Beef Association surveyed nodulation of sub clover roots across 22 farms with permanent pasture in the South West.

The survey found that subclover nodulation was inadequate at 80% of sites, meaning plants typically had less than 20 small pink nodules and less than 3 large nodules.

“The majority of sites didn’t have any large nodules, and in half the sites, nodules were scarce at best,” Peter said. And it wasn’t an uncommon situation.

More comprehensive research in NSW found 93% of surveyed sites had inadequate nodulation.

Why clover might not be nodulating

While Len couldn’t explain why his subclover wasn’t nodulating, Peter suggested some possibilities that were raised in the 2019 project:

Root disease – 70% of sampled plants had severe root rot from pathogens such as Phytophthora, Pythium and Rhizoctonia.
A deficiency of Molybdenum – 40% of sites surveyed had low Molybdenum which is essential for nitrogen fixation by rhizobia. (Note: high molybdenum can impact animal health, so tissue testing and seeking advice is critical).
Low soil pH – As soil pH drops below 5 (measured in CaCl₂), conditions in soil can become toxic for rhizobia and numbers decline. In the South West survey, soil pH averaged 4.75 in the topsoil and 4.4 in sandy subsoils. These results may also explain low molybdenum. Liming may be an option to increase soil pH and gradually increase the availability of molybdenum.
Less effective rhizobia strains – Legumes are typically inoculated with the most effective strain of rhizobia at seeding. But these strains can exchange genetic material with background strains and become less effective over time. Re-inoculation needs to occur about every 5 years, but surveys suggest this is only done on 50% of sites and typically with seed coating formulations. Seed coating varies in quality with the number of rhizobia on the seed sometimes inadequate at the point of purchase.
Nutrient availability – in NSW, nodulation was positively associated with phosphorus and sulfur, although low phosphorus and sulfur was much less common in sites surveyed by South West NRM (5-20%) compared to NSW sites (40-73%).

Benefits of serradella

In comparison, serradella had several benefits:

Inoculated with a strain of rhizobia that is less sensitive to low pH soils compared to the clover inoculant (tolerance to pH of 4 rather than 5).
Less susceptible to root disease.
Sinks its roots to a greater soil depth quicker, it is less susceptible to false-breaks, and deeper roots improve resilience in dry seasons.
More efficient in the uptake of phosphorus compared to subclover.

Serradella varieties

Serradella is an annual pasture legume suited to mediterranean climate zones in Australia and is suitable for hay and silage production. Several varieties of French serradella and yellow serradella have been developed for Southern Australia, typically for low to medium rainfall regions and acidic soils.

While Len is trialling new varieties of French serradella (Fran₂o) and yellow serradella (SerraMax), DPIRD Senior Research Officer Clinton Revell said Margurita French serradella could be another option to consider for the high rainfall region because it is later maturing compared to the other varieties.

Clinton said serradella nodules usually looked better than subclover (assuming correctly inoculated).

“Subclover nodulation can be problematic on acidic soils and serradella would be expected to have an advantage there,” he said.

One concern voiced with serradella was whether it could compete with ryegrass in a mixed, high rainfall sward.

“Ryegrass is very competitive, so can get the jump on legume seedlings,” Clinton said.

“You need legume plant density to be competitive. Serradellas tend to be a bit more upright than subclover so a bit more vulnerable to early grazing, but can withstand moderate grazing pressure at 4-6 weeks as a way to keep the ryegrass in check.

“French serradellas have the advantage of lower cost seed (can use it in the pod form). Serradellas are vulnerable to budworm in spring and this can reduce seed production. Emerging seedlings can also be damaged by red-legged earth mite.”