Legume decline traced to crop chemicals

Image of Carl Moltoni

Carl Moltoni is hoping a switch to a serradella-based pasture rotation will lift productivity and solve a herbicide problem.

PHOTO: Evan Collis

A dramatic decline in subterranean clover on a Dandaragan, Western Australia, property has stimulated a shift in rotations, herbicide use and soil management for the better, according to owner Carl Moltoni

Carl initially thought the subclover decline was due to the combined impact of a run of dry seasons and his non-wetting soils on clover establishment, but a call to the Department of Agriculture and Food, WA (DAFWA), revealed the pasture problem was actually chemical in nature.


Grower: Carl Moltoni
Location: Dandaragan, Western Australia
Farm size: 4200 hectares (40 per cent cropping and 60 per cent pasture)
Annual rainfall: 450 millimetres
Soil types: loamy sand, sand over gravel and sand
Soil pH (CaCl2): 5.5

“The first thing DAFWA asked me was whether I had been using sulfonylurea herbicides in my cropping program,” Carl says.

Sulfonylurea chemicals (SU) seriously affect root growth and nodulation in legume pastures and crops such as lupins. To minimise their impact, sufficient rainfall is needed between SU use and legume plantings.

“The chemical companies advise that about 400 millimetres of rain is needed to wash the sulfonylurea through the soil profile, but our experience shows that more than that might be needed,” Carl says.

“It could be that the sulfonylureas are hitting a hardpan, but more work is needed to determine what is going on.”

Carl started using sulfonylurea when he bought a neighbouring property with trifluralin-resistant ryegrass in 2009: “I had to switch to the sulfonylurea Logran® to manage weeds on that property and because this required a new seeding system of press wheels and knife points I began using Logran® across both properties to streamline the seeding process.”

Within just two years of sulfonylurea use, Carl saw a dramatic decline in his subclover pastures.

Analysis of surviving pasture plants by DAFWA’s pasture agronomist Dr Angelo Loi and rhizobiologist Dr Ron Yates showed the subclover pastures were displaying classic signs of sulfonylurea toxicity – yellow, deformed and stunted plant tops and dark coloured, poor-condition root systems lacking in nodules and root hairs.

Previous DAFWA research has shown that even residual concentrations as low as one thousandth the rate of recommended sulfonylurea applications can prune legume root growth, so ensuring sulfonylureas are fully degraded before a legume rotation is essential (Figure 1).

Dr Loi advised Carl to stop using sulfonylurea and suggested he shift into a serradella-based pasture rotation to lift legume percentage in his pastures using pasture seed harvested from on-farm nursery paddocks.

“I wanted to be able to achieve tighter pasture rotations on my better paddocks so that I could reduce my reliance on nitrogen while controlling weeds with grazing and weed-topping.”

Traditionally Carl has grown four to five years of subterranean clover pasture followed by a crop, but with the new serradella system he can now grow a 1:1 pasture-crop rotation on his most productive paddocks and 2:1 on less productive areas.

Livestock has always played an important role on the Moltonis’ properties with 60 per cent of arable land in pasture each year and Carl is confident that boosting legume pasture performance will also benefit the productivity of his 3500-ewe flock.

“Livestock are a risk-management tool for us – we have always had them and have never considered dropping them from our system even in the 1990s when wool and meat prices fell.”


Figure 1 Impact of the sulfonyurea (SU) herbicide triasulfuron applied at 1/10th through to 1/100,000th of the recommended dose 18 days after subterranean clover germination on nodule number and dry matter of plant tops.

SOURCE: Angelo Loi and Ron Yates, Focus Paddocks 2014 GRDC Report

Pasture nurseries

In 2012 Carl established a 130-hectare seed nursery for a range of alternative annual pasture legumes including yellow and French serradellas, rose, gland and arrowleaf clovers and biserrula.

“We have always grown and harvested our own subclover seed and will continue to do this but the serradellas handle false breaks better and their deeper roots mean they get a better start,” Carl says. “They also don’t need any specialised harvesting equipment and harvesting the new pastures places less pressure on the soil.”

While frost and a dry season led to weed issues in the 2012 pasture nursery, the MarguritaA French serradella and yellow serradella Santorini mix still yielded enough to establish 600ha of pasture in the following autumn (2013).

“I hammer milled the serradella to stimulate germination and sowed it along with subclover using a Great Plains seeder with disc openers,” Carl says.

The disc openers enable Carl to sow the pasture seed without affecting any volunteer grasses in the paddock, which act as valuable early-season sheep feed. “We spray out the grasses in July/August and then let the serradella and subclover go,” Carl says. “In September we weed wipe the radish with non-selective herbicides when the weeds are up above the pasture.”

Capeweed is controlled pre-season using the residual herbicide Spinnaker® (imazethapyr).

Nitrogen benefits

Carl’s 2015 wheat crop is the first to be sown following two years of serradella pasture so it is too early to tell whether there has been a nitrogen benefit from the legume rotation. “We will take individual header samples from the wheat-on-wheat paddocks and the wheat following pasture paddocks to assess the benefits of the serradella phase.”

A DAFWA trial (as part of the GRDC co-funded ‘Focus Paddocks’ project) on the Moltoni property in 2012-13 suggested that just one year of a pasture legume rotation could provide enough nitrogen to forgo the need for fertiliser nitrogen in the following wheat crop (Table 1).

Table 1 Yield and protein of Callingri wheat grown with varying applied rates of nitrogen fertiliser (2013) following a French/yellow serradella pasture phase in 2012.
   Nitrogen treatment (kg/ha urea)
0 (control)
50  100  150  200  lsd (p=0.05)
 Yield (t/ha)
4.6 4.0  4.2  4.5  4.6  0.9
 Protein (%)
10.1 10.2  10.5  10.9  10.3  1.2

SOURCE:  Dr Angelo Loi, Focus Paddocks 2014 GRDC Reports

Non-wetting soils

Historically, the Moltonis have used wetting agents to mitigate their non-wetting soils. However, Carl says impressive trial results at a local field day demonstrated the clear benefits of spading to treat non-wetting soils and inspired the purchase of a spader for his own property.

The Moltonis are now three years into a five-year spading/deep-ripping process to ameliorate non-wetting soils, and Carl says the results are the best return on investment he has witnessed in his 30 years of farming.

“As growers we are used to buying expensive machinery but no machine has ever given me the sorts of returns that I’ve received from spading and deep ripping.”

Carl says the $220/ha ripping and spading costs have delivered 800 to 900 kilograms/ha annual grain yields and he expects those benefits to continue for a decade.

“It’s a slow process, at 3ha/hour each for the spading and the ripping, but it’s a process we will only have to repeat every 10 to 15 years and the yield benefits pay for it all within one season.”

More information:

Carl Moltoni,


Dr Angelo Loi,
08 9368 3082,


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