Crop damage caused by residual acetolactate synthase herbicides in the soils of south-eastern Australia

Abstract

Grain growers in south-eastern Australia have reported unexpected crop failures with theoretically safe recropping periods for acetolactate synthase herbicides in alkaline soils. This experience has led to the concern that these herbicides may degrade very slowly in alkaline soils, and herbicide residues have at times been blamed for unexplained crop losses. To address this issue, we established 5 recropping trials across Victoria and South Australia with 5 acetolactate synthase herbicides (chlorsulfuron, triasulfuron, metsulfuron-methyl, imazethapyr, and flumetsulam). The herbicides were applied to separate plots in years 1, 2 or 3, and sensitive crop species were sown in year 4 to measure the impact of herbicide residues. We observed that the persistence of the sulfonylureas (chlorsulfuron, triasulfuron, metsulfuron-methyl) varied between herbicides, but all persisted longer in alkaline soils than in acid soils, and were, therefore, more likely to damage crops in alkaline soil. Imazethapyr persisted longer in clay soils than in sandy soils and was, therefore, more likely to damage crops in clay soils. All herbicides persisted longer when rainfall was below average. Canola was more sensitive to imazethapyr than either pea, lentil or medic, but was less sensitive to the sulfonylureas. In contrast, lentil and medic were the most sensitive to sulfonylureas. Despite some damage, we found that safe recropping periods could be predicted from the product labels in all but one situation. The sole exception was that metsulfuron-methyl reduced dry matter and yield of lentil and medic sown 10 months after application in a soil with pH 8.5. We hypothesise that the real cause of crop failure in many situations is not unusual herbicide persistence, but failure to take full account of soil type (pH and clay content including variation in the paddock) and rainfall when deciding to recrop after using acetolactate synthase herbicides.

Phenotypic and molecular characterisation of novel Vicia faba germplasm with tolerance to acetohydroxyacid synthase‐inhibiting herbicides (AHAS) developed through mutagenesis techniques

“…Although dry matter response was not influenced by site, grain yield was more responsive and in particular Nura showed significantly higher yield loss from some of the treatments at the Pinery site. This is most likely due to differences in soil type and rainfall affecting herbicide persistence and subsequent root uptake affecting grain yield, taken at maturity . Conversely dry matter measurements were taken much earlier in the season and are likely to be more strongly linked to damage from foliar herbicide uptake.…”

“…This is most likely due to differences in soil type and rainfall affecting herbicide persistence and subsequent root uptake affecting grain yield, taken at maturity. 8 Conversely dry matter measurements were taken much earlier in the season and are likely to be more strongly linked to damage from foliar herbicide uptake. Overall, IMI-3 exhibited a higher level of field tolerance compared with IMI-1, and showed no visible damage, dry matter reduction or grain yield loss at any rate of imidazolinone herbicide applied at either site, including at four times the field rate used in current commercial imidazolinone-tolerant crops.…”

“…However, weed competition is a major constraint on Australian faba bean production, with a limited number of safe or suitable herbicide control options available, and in particular, no safe in‐crop herbicide control options for broadleaf weeds . In addition, current faba bean varieties exhibit sensitivities to some herbicides commonly used in other crop phases, such as the acetohydroxyacid synthase (AHAS) inhibitors, which can further restrict their inclusion in the cropping rotation for up to 24 months or longer in dry soils with a pH > 6.5 . The use of herbicides is the primary method of weed control in modern‐day broad‐acre cropping systems worldwide, with increasing reliance on chemical control methods as farming systems become more mechanised .…”

“…5,6 In addition, current faba bean varieties exhibit sensitivities to some herbicides commonly used in other crop phases, such as the acetohydroxyacid synthase (AHAS) inhibitors, which can further restrict their inclusion in the cropping rotation for up to 24 months or longer in dry soils with a pH > 6.5. 7,8 The use of herbicides is the primary method of weed control in modern-day broad-acre cropping systems worldwide, with increasing reliance on chemical control methods as farming systems become more mechanised. 9 In addition, there has been a decreasing number of new herbicide modes of action being introduced into the market, and the development of herbicide tolerance traits to existing chemistries has become an innovative way to expand weed control options in minor crops.…”

Assessment of management options for Salsola australis in south‐west Australia by transition matrix modelling

“…However, residual herbicides cannot be used in all systems, because of crop compatibility issues. Unfortunately, on alkaline soils where S. australis is abundant, residues of residual herbicides are of particular concern (Mitchell & Wilcox, 1988; Hollaway et al. , 2006).…”