BREAK crops and long-term herbicide strategies are key to driving down brome grass numbers, according to University of Adelaide weed management associate professor Chris Preston.
Speaking at the recent Mallee Sustainable Farming Field Day at Lowaldie, Dr Preston said brome grass had become a serious issue across SA due to a number of reasons, including herbicide resistance, delay in emergence, the grass’s persistence and an inability to manage the seed bank.
“All the strategies we’ve been developing for ryegrass in the Mid North have been focused on how to stop weeds setting seed,” he said.
“Most of them don’t work as well on brome.”
Dr Preston said while post-emergent herbicide Intervix was “by far and away” the best herbicide to tackle brome grass, evidence of resistance to the chemical had begun to emerge.
This problem, as well as rotational strategies to reduce brome grass seed numbers, have been investigated at a long-term trial site at Balaklava.
Two herbicide strategies were used on lupins, and Clearfield and non-Clearfield sites of wheat, barley and canola – one with a crop top and one without.
Dr Preston said the best rotation for brome grass control was lupins followed by TT canola – two break crops – with the herbicide strategy not influential because enough control was achieved with a pre-emergent and post-emergent spray.
“Crop topping was a little better, but nothing significant,” he said.
The brome grass seed bank was reduced from more than 600 brome grass seeds a square metre to 20 seeds/sqm using two consecutive break crops.
Dr Preston said when pre-emergent herbicides were relied on solely in cereals, seed numbers went up, with the best strategy the one that used Intervix.
“The trouble we have is, when we lose Intervix, you don’t have any post-emergent herbicides for cereals,” he said.
A break crop followed by a cereal showed promising results, particularly when Intervix was used, with a cereal/cereal rotation the worst.
Dr Preston said if farmers wanted to manage brome grass more effectively, they would have to do it outside the cereal phase and use Intervix sparingly to avoid resistance.
He suggested a three-year rotation of a legume, using an alternative herbicide to Intervix and crop topping, followed by a Clearfield wheat using Intervix, and then following that with Clearfield barley using an alternative herbicide again.
“That sort of strategy might get your brome numbers down and allow you to have the frequency of cereals you want to have in your rotation,” he said.
