Chemical balancing act: use right rate to reduce residues
- Weed management for hard-to-kill weeds is a significant issue for grain growers
- Uptake of imidazolinone-tolerant varieties is becoming more common, especially when farmers are working to manage brome and barley grass, which have limited chemical control options
- Imidazolinone herbicide rates must be carefully manage to reduce the risk of residue build-up in the soil, especially in dry years
- Residues can affect plant vigour and growth of non-tolerant crops
- Incorrect use can lead to resistance so careful management is required
Effective weed management is an issue at the forefront of every broadacre cropper’s mind – which weed, how much herbicide, when and what if it doesn’t work?
Weed management, particularly for hard to kill weeds such as brome and barley grass, continues to be a major issue.
Over the past few years, chemical companies have worked in collaboration with plant breeders to develop new weed management products. One innovation was the release of imidazolinone (imi) tolerant crop varieties, and their associated weed management strategies.
Correct use of imi-tolerant technology is essential to avoid negative impacts in subsequent seasons, such as herbicide residues in soil which can impair growth in non-tolerant varieties.
Rural Directions, in collaboration with growers from South Australia’s Lower North and with support from the Grains Research and Development Corporation, led a project which looked at the effect of imi residues in cropping systems.
Project manager Brendan Wallis established a series of trials in the Lower North. Trials were sown on paddocks treated with an imi herbicide in the previous year (2012).
“Due to the dry spring and autumn, herbicide residues were likely to show up in the 2013 growing season, providing us with a great opportunity to investigate the effects of tolerant versus intolerant varieties of wheat, barley, canola and lentils,” Mr Wallis said.
The trial investigated imi herbicide efficacy on weeds by applying over two consecutive years at varying rates.
Mr Wallis says the research was of particular interest as more growers incorporated imi-tolerant varieties into their cropping systems to combat hard to kill weeds. He believes this use must be carefully assessed, particularly in low rainfall areas.
“Results showed there were no significant differences in the establishment of tolerant versus non-tolerant varieties. However, imi residues were found to reduce plant vigour, with non-tolerant plants showing stunted root growth plus some yellowing of leaves,” he said.
“We also found that the tolerant varieties out yielded non-tolerant varieties by 5 percent in wheat, 4 percent in barley, 34 percent in canola and 13 percent in lentils.
“There was obviously a large yield penalty from what we assume was herbicide residue. However, variability within the data from mice damage in one of the plots despite baiting meant that the statistical difference was not able to be shown. We hope to conduct further research into this.
“In comparison, beans and peas seem to tolerate imi residues and produced reasonable yields for the district and season at more than 2t/ha.”
The trial showed imi herbicide residues do affect non-tolerant varieties when there is insufficient rain to satisfy plant-back requirements. However, in favourable growing season conditions, they may grow away from initial damage to some extent.
Since imidazolinone herbicide is broken down by soil microbes in wet, aerobic conditions, it is likely that with significant winter rain, crops not tolerant to imidazolinone can make it through the herbicide residues. However, relying on rainfall is a huge risk.
“Our message is growers beware. Management is a must,” Mr Wallis said. “Growers should note that while some imi residues may not appear to be having an impact on non-tolerant varieties, they can reduce the crop’s tolerance levels to other herbicides, creating further management risks.
“Growers must carefully assess the rainfall they have received since applying any imidazolinone products and observe plant-back recommendations on the herbicide label when considering growing susceptible crops following imi-tolerant crops.”
Growers should rotate between different control methods when managing brome and barley grass. If one herbicide is continually used, it will lead to weeds building tolerance and eventually resistance. Imi resistance has already been identified in brome grass in the Mallee so correct management is important to preserve effectiveness for as long as possible.
“It is strongly recommended that growers using imidazolinone herbicides go back in with a tolerant variety in the subsequent year, unless rainfall has exceeded the plant-back requirement listed on the herbicide label,” he said. “Growers should maintain robust rates to manage herbicide resistance; however this must be balanced against the risks of excessive residues if another dry year occurs.
“Having the ability to use imidazolinone herbicides over tolerant crops has added a new dimension to weed control, but their use must consider that they are a soil-active, residual herbicide which can not only affect non-tolerant plant growth, but are associated with a greater risk of groundwater contamination. Proceed with caution and respect the use of these weed control options.”
More information: Brendan Wallis, 08 8525 3000, firstname.lastname@example.org
GRDC Project Code
South, National, North, West