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Measuring the value of weed control expenses

Weed control is expensive, and it is often hard to measure the return on your investment. WeedSmart’s southern extension agronomist, Chris Davey, is determined to show that his advice to growers will put money back in their pockets through improved crop yield and fewer weeds to control in future seasons.

Several years ago, a nationwide survey indicated that the overall cost of weeds to Australian grain growers is $3.3 billion, or over $140/ha in expenditure and losses. Keeping this cost to a minimum will increase profits, but calculating the costs and benefits on the farm or paddock scale is complex.

Chris suggests three ways to reassure yourself that a weed control tactic is worthwhile.

The first is to assess yield maps to show the impact of weeds on yield. This is an easy way to put exact numbers on what poor weed control is potentially costing you,” he says. “The second is to look into the results of local, replicated trials, and the third is to leave untreated strips in the paddock and see the effect – sometimes this happens unintentionally anyway and is usually a good reminder about how important weed control is!”

Chris works primarily with growers in the medium to low-rainfall districts of South Australia and has collected numerous economic analyses of weed control scenarios over many years.

Yield map interrogation

“In one example, using a yield map from a lentil crop, we measured a $866/ha loss from 11 ha within a 64 ha paddock due to high annual ryegrass infestation,” says Chris. “When averaged across the whole paddock, the loss was $148/ha, without including other patches of the paddock with low level ryegrass infestation.”

This scenario features lentils, a high-value crop that is a poor competitor against weeds. In another example, Chris has measured a yield loss in wheat of 2.12 t/ha due to annual ryegrass pressure.

“When using yield maps to calculate losses, make sure you always ground-truth the low-yielding areas to ensure weeds are the cause of the yield loss,” says Chris. “Often, other factors, such as soil constraints suppress crop growth or allow weeds to gain an advantage over the crop. Addressing these constraints is actually a non-herbicide weed control tactic from the WeedSmart Big 6 strategy.”

Local replicated trials

Local, replicated trials provide a wealth of information for growers to see the effect of various herbicide treatments or the impact of crop competition strategies. One replicated trial on the Yorke Peninsula in 2018 clearly demonstrated the value of competitive cropping choices against a highly resistant annual ryegrass population.

“Herbicide resistance testing revealed the ryegrass population at the trial site was 100 per cent resistant to trifluralin (e.g. Treflan), 50 per cent resistant to triallate (e.g. Avadex) and 30 per cent resistant to prosulfocarb + s-metolachlor (e.g. Boxer Gold),” says Chris.

“We showed that just changing the row direction from north-south to east-west gave a 0.5 t/ha yield improvement in wheat. In barley, stacking a premium pre-emergent herbicide mix onto an already-competitive variety boosted yield by 1.1 t/ha and reduced ryegrass plant numbers ahead of the following seeding by over 80 per cent, compared to the least competitive, nil pre-emergent barley treatment.”

This trial was very valuable in quantifying the impact of some crop competition tactics on crop yield in this region.

Where it is practical to implement, changing row orientation is free and the 0.5 t/ha yield improvement equates to approximately $150/ha additional income. And – changing barley varieties is nearly free. If we add the extra cost of the premium herbicide (about $50/ha), the net benefit of the 1.1 t/ha yield response is about $280/ha of pure profit. Multiply that over the crop area and it is clear to see the value in these incremental changes.

In another trial featuring lentils, Chris has demonstrated that choosing a variety with a herbicide-tolerant trait can pay off in a weedy situation, even if the variety comes with a yield penalty compared to other lentil varieties.

“In a paddock with high numbers of herbicide resistant Indian hedge mustard, the Metro lentil variety with metribuzin tolerance allowed the weeds to be well-controlled, and seed set was minimal,” he says. “Although the yield penalty was 20 to 25 per cent compared to the highest yielding variety in a weed-free situation, growing a lower-yielding variety in a high weed-pressure situation maximised grain yield and avoided a huge addition to the weed seed bank for future seasons.”

On-farm trials and observations

The third method of testing cost-effectiveness is to conduct your own demonstration or side-by-side paddock trial. One of Chris’ clients inadvertently conducted a strip trial where the outside laps of a barley crop were missed during an application of a new herbicide, bixlozone (Group 13, e.g. Overwatch). The herbicide application generated a 1.82 t/ha yield improvement in the treated versus non-treated areas, valued at $533/ha. When the herbicide cost of $38/ha plus an application cost of $8/ha were included in the calculation the premium herbicide had generated a return on investment of $11.59 benefit for every dollar spent.

Another way to compare the return on investment would be to apply a premium herbicide alongside a conventional one, for example, trifluralin plus triallate.

Similarly, having a weed population tested for herbicide resistance and susceptibility can save the costs associated with a failed herbicide application. Even if the herbicide is less expensive than other options, applying it to a weed population with resistance to that mode of action will fail to control the weeds, probably resulting in yield loss in the current crop, and add to the paddock’s weed seed bank, increasing future crop losses and weed control costs.

“For example, we had a population of annual ryegrass tested, and the results showed 85 per cent resistance to Group 2 [B] herbicides and 30 per cent resistance to Group 1 [A] herbicides and glyphosate (Group 9 [M]),” says Chris. “We were able to use this information to generate a crop rotation and herbicide plan that would be effective on this population and reduce weed numbers for the following seasons.”

These examples demonstrate that effective weed control tactics are very cost-effective. However, there are often multiple options, and it may be useful to test the value of new tactics against ones you have relied on in previous years.

The WeedSmart Big 6 is an integrated weed management strategy that growers and agronomists can use to implement as many weed control tactics as possible to keep weed numbers low and maximise farm profitability. 

More resources

What are the economic benefits of diverse rotations?

What’s the best way to out-compete resistant annual ryegrass in cereals?

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