Hybrid canolas to help in long-term ryegrass battle

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Photo of canola field
Research conducted by the University of Adelaide suggests hybrid canola varieties maintain better grain yield in the presence of weeds and are more tolerant of weed competition compared with open-pollinated varieties.
PHOTO: Alistair Lawson

Growing a hybrid canola variety rather than an open-pollinated variety has the potential to drastically reduce ryegrass seed-set for following seasons.

This was one of the key findings from a GRDC investment, conducted with the University of Adelaide, which set out to investigate methods to reduce the pressure on clethodim – the major post-emergent herbicide used for the control of annual ryegrass in canola and pulse crops. The project examined pre-emergent herbicide strategies and the varying outcomes stemming from canola variety selection.

According to University of Adelaide associate professor of weed management Dr Chris Preston, ryegrass has developed resistance to clethodim, causing growers to increase rates of the herbicide and prompting researchers to search for alternative control options.

“Clethodim is the main ryegrass post-emergent herbicide we have left in our break crops and part of the trouble we have is that everybody’s using it in canola and pulses to try and control ryegrass, but ryegrass has evolved resistance to it,” Dr Preston says.

“Growers have tended to increase the rates of clethodim. We have a bit of trouble with that in canola because the label for products containing 120 grams per litre of clethodim stipulates a maximum application rate of 500 millilitres per hectare. But even getting close to that label rate in canola can result in crop damage which can have yield impacts.”

Dr Preston and his team have investigated other control options such as crop competition and pre-emergent herbicide strategies.

“We started looking at other herbicides and the trouble we got with pre-emergent herbicides is that most of them don’t give us enough residual length to keep ryegrass out of canola,” Dr Preston says.

“Then when the herbicide starts to decay, the ryegrass comes in, which results in a lot of it in the crop. This prompted us to start thinking that part of our problem was that we weren’t getting enough canopy closure early in the season.

“There were a number of reasons why that was happening but one of the big reasons was the growing of open-pollinated canola cultivars, which yielded well but didn’t provide any competition against ryegrass early.”

As part of their trial work, the researchers began comparing open-pollinated varieties with hybrid varieties, only using pre-emergent herbicides for ryegrass control and no clethodim as a post-emergent control.

“What we discovered was that while we didn’t actually change the ryegrass populations in crop very much, in the latter part of the season we had reduced ryegrass seed-set by up to 50 per cent,” Dr Preston says.

“We achieved that just by growing a hybrid variety rather than an open-pollinated variety.

“By growing a hybrid canola variety with strong early vigour and greater competition against ryegrass, growers can set themselves up for following seasons by having less seed-set.

“This is exactly what we want from our break crops – to reduce the amount of ryegrass seed-set.”

The hybrid varieties maintained better grain yield in the presence of weeds and were more tolerant of weed competition than the open-pollinated varieties.

At Hart, in South Australia’s Mid North under lower ryegrass pressure (fewer than 100 plants per square metre), atrazine incorporated by sowing and applied as a post-emergent provided the greatest seed-set reduction, at more than 90 per cent.

At Roseworthy, SA, most robust herbicide treatments, such as propyzamide, or propyzamide plus atrazine, controlled ryegrass numbers under high weed pressure of more than 400 plants/m2.

“Our trials are suggesting that in yield environments above 1.5 tonnes per hectare for canola, the hybrid varieties always out-yielded the open-pollinated varieties,” Dr Preston says.

“In yield environments below 1.5t/ha, there wasn’t necessarily a yield benefit from growing hybrid varieties but there was still a weed-control benefit.”

Dr Preston says many growers still prefer open-pollinated seed to hybrid seed due to cost. Therefore, he says it is a decision for each individual grower to make as to whether there will be an economic benefit from growing hybrid varieties of canola.

The trials demonstrated the influence of seasonal conditions on crop competition, Dr Preston says.

“The impact of competition on ryegrass numbers is probably higher in a dry year as opposed to a wet year because we are relying on the pre-emergent herbicide to take out the first flush of ryegrass and then the crop competition will deal with the next flush,” he says.

“By getting canopy closure before the next lot of ryegrass germinates, growers can get a yield benefit through less competition with the growing crop but also reduce seed-set from that later ryegrass.

“By delaying ryegrass seed-set, this can increase the effectiveness of end-of-season weed tactics.

“For example, crop-topping canola will be more effective if the ryegrass hasn’t set seed. Also harvest weed-seed control tactics will be more effective because it will be easier for the harvester to capture the ryegrass seed-set.

“This project is not just about whether these tactics will do anything in particular about ryegrass in the immediate term, it’s also about managing ryegrass over the life of the crop and over the rotation.

More information

Dr Christopher Preston
08 8313 7237