Hidden costs in weed control

Haidee VandenbergheCountryman

Growers should consider the hidden costs of harvest weed seed management, according to Department of Agriculture and Food WA researcher Peter Newman.

At the 2012 Crop Updates, Mr Newman joked that although he had spent the past 12 years urging growers to burn their windrows at harvest to manage weed burdens, he would now be spending the next 12 years encouraging them not to.

In a 2011 study involving several growers, Mr Newman looked at the efficacy of the Harrington Seed Destructor, towing a chaff cart and windrow burning in terms of weed seed management.

But importantly, for the first time, Mr Newman included the cost of lost nutrients in his calculations on the running costs of each system.

With a Harrington Seed Destructor costing about $150,000 and a chaff cart in the range of $50,000 to $70,000, spending just a couple of hundred dollars on a windrow chute looked attractive, Mr Newman said.

"If you look at the running cost of these things, and that includes running costs and depreciation, interest and fuel, the Destructor is somewhere around $12 on a 3t/ha crop," he said.

"A chaff cart is a few dollars per hectare and the windrow costs basically nothing. If we look at the overall cost, why do it for $12 when you can do it for $2 by burning a windrow?"

While each of the methods was effective, achieving about a 57 per cent rate of control, Mr Newman said the real differences were apparent when you started looking at the value of the nutrients they took out.

The researcher measured nutrient rates in chaff heaps, windrows and in residue on the ground and found there were about six units of potassium per tonne of residue.

Growers were asked to take measurements of the amount of residue collected in the chaff cart per tonne of grain harvested.

"You're getting about 300kg of chaff per tonne of grain," Mr Newman said. "Growers were removing anywhere from 11 to 37 per cent of residue from the paddock and putting it in the chaff cart.

"That's $3.70 worth of nutrients just for the nitrogen and potassium per tonne of grain they harvest.

"(On average) the chaff charts were removing $11 worth of nutrients and the windrow, if we move the header across one metre every year, it works out at $13 of nutrients - that is just the nitrogen."

The study concluded that windrow burning effectively removed about five times as much potassium as a chaff cart.

The Harrington Seed Destructor had no nutrient costs because the residue was left in the paddock.

With the nutrient value included in the costs, the ballpark cost of running the systems worked out to be roughly the same - that is if growers using windrows committed to moving their windrows every year.

"If you auto-steer your header and put the windrow in the same spot every year, it's a blow out in cost," Mr Newman said.

The conservative value of nutrients burnt in wheat windrows if an auto-steer harvester followed the same path each year was $42.75/ha and significantly, about 80 per cent of farmers are using auto-steer. This is why I've said I'm going to spend 12 years talking people out of windrow burning," Mr Newman said.

However, he conceded windrow burning had its place in some cropping systems, particularly those on high potassium soils with lower rainfall. "I think putting windrows in the same spot every year is going to create a really big problem," Mr Newman said.

Instead, they should consider moving the windrow one metre each year to ensure the spread of nutrients.

But harvest weed seed management was one aspect of the war on weeds and Mr Newman said using crop competition to control weed seed set was his next "personal crusade".

Using several unreleased cultivates bred by CSIRO, Mr Newman tested whether using a competitive variety could help control to ryegrass.

The wheat lines, which included UA39 and UA40, were selected in CSIRO's breeding lines only for early vigour and not yield or disease resistance.

They were compared against several commercial varieties, including wheat varieties Wyalkatchem and Magenta and Baudin barley.

Conducted at Eradu and Wongan Hills, the trial concluded that variety selection could have a huge impact on ryegrass growth and eventual seed set. Biomass was measured at anthesis, with UA40 having the highest biomass.

"We can reduce our weed seed set by 30 to 40 per cent by getting more competition," Mr Newman said.

"UA40, Magenta and Baudin all did a great job at suppressing the ryegrass biomass. Bonnie Rock and Mace were somewhere in the middle and Wyalkatchem was a very uncompetitive variety."

While the CSIRO lines aren't likely to be released commercially, growers can still work on using crop competition to tackle weed seed set.

"The competitive cultivar work is part of it, but I think with twin row sowing and higher seeding rates teamed up with a more competitive variety, we can make some big improvements,' Mr Newman said.

"If we've got a competitive variety halving the seed set and a chaff cart halving the seed set and over summer you've got 50 per cent decay… the combination of these teamed up, we can be dealing with 90 odd per cent of the weed seeds right there before we've even gone near a herbicide.

"It's the layering of these integrated weed management practices where they really their straps. Adding some more tools and particularly competition at harvest is clearly where we're headed.

"I can't see a path, with the level of cropping we are doing, without using more of these tools."

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