Read time: 4 minutes

What is the value of fixed nitrogen for suppressing weeds?

with Rohan Brill, research agronomist, Brill Ag, Ganmain

Results from research on hyper-yielding crops show that crops need fertile soils, not just fertiliser. One spin-off benefit of vigorous, high-yielding crops is strong crop competition and potentially fewer weeds.

Rohan Brill, research agronomist with Brill Ag, Ganmain, NSW, says the results of GRDC-funded pulse research at sites across central and southern NSW have shown that the quantity of atmospheric nitrogen that legumes fix in the soil has been generally under-estimated.

“The old rule of thumb was 20 kg of nitrogen fixed per tonne of legume biomass,” he says. “But this only took into account the amount of nitrogen present in the above-ground portion of the plant. For pulses like faba bean, field pea, vetch and lupin, we now know the real contribution is closer to 30 kg N per tonne of legume biomass, making high biomass crops like faba beans an excellent option for building soil fertility.”

After harvest, residual nitrogen will become available to the following crops, reducing fertiliser costs while boosting yield. Improved soil fertility underpins crop competition in every crop throughout the rotation, suppressing weed growth and weed seed set.

“This is why legumes need to be a permanent part of a crop rotation,” says Rohan. “Soil nutrition is a key driver of high crop yields, and the hyper-yield research clearly demonstrated that crops respond to soil fertility that comes from increased organic matter and slow-release forms of nitrogen.”

Crop competition and diverse rotations are pillars of the WeedSmart Big 6 strategy to grow more crop and fewer weeds.

How much is the fixed nitrogen worth?

Pulses fix about 30 kgN per tonne of biomass, measured at 30 to 50 per cent podding and before leaf fall. A well-grown faba bean or lupin crop produces about 15 tonnes of biomass per ha and about 450 kgN/ha. About 160 kgN/ha is removed in the grain of a 4 t/ha faba bean crop, leaving around 290 kg/ha residual nitrogen.

If urea costs $1.50/kgN, this equates to $435/ha of residual nitrogen after growing a high biomass pulse such as faba beans.

Soil testing at the following planting time often does not show this amount of nitrogen in the soil because it is not all immediately converted into plant-available nitrogen. Instead, the nitrogen is slowly released, providing nutrients to the crop through the growing season.

The effect was clearly shown in three side-by-side canola paddocks on a client’s farm, with two paddocks sown to wheat the year before and one sown to narrow-leaf lupins. The two paddocks previously sown to wheat yielded 1.7 t/ha of canola, and the paddock sown previously to lupins (shown as blue in the NDVI image) yielded 3 t/ha of canola. About 100 kgN/ha was applied across the whole area when the canola crop was planted.

When following a pulse crop, canola is more competitive with weeds. Crop competition in the following crops is a major weed management benefit, and the direct weed control options in the pulse crop phase have also improved markedly in recent years.

How do pulses help with fallow management?

On our farm at Ganmain, we have seen poor results in our cropping program when hungry soils face any additional challenge – especially waterlogging.

We are making a concerted effort to sow around one-third of our cropping area to pulses to improve the soil fertility across very diverse soil types. For us, vetch is the best option for weedy paddocks because we can cut it for hay or silage; or sow it with other cover crop species for forage and or brown manure.

The cost is more than recouped in improved yield in the following cereal or canola crops, and we also clean up the weeds in the vetch phase. Our cleanest paddocks post-harvest are now pulse crops where we applied Reflex (fomesafen, Group 14) for broadleaf weed control. With clean paddocks at harvest, we save on fallow weed management and don’t have any large weeds like fleabane taking up stored soil moisture.

What are the best weed management tactics to use in the pulse phase?

Faba beans are the best grain legume option on our variable soils. Beans are competitive against weeds and create extra competitiveness in the following crops.

The pre-emergent herbicide options of Ultro (carbetamide, Group 23) and Overwatch (bixlozone, Group 13) against ryegrass and Reflex (fomesafen, Group 14) against broadleaf weeds, and post-emergent grass herbicides such as clethodim (Group 1) mixed with butroxydim, (Group 1, e.g. Factor) are very effective.

Faba beans and other fast-maturing pulse crops (e.g. field peas) are also well-suited to harvest weed seed control with an early harvest time to capture seed from any late germinating weeds.

Related GRDC Update papers:

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