Contamination of internationally traded wheat by herbicide‐resistant Lolium rigidum

Abstract

As herbicide-resistant weeds have spread in the agricultural fields of grain-exporting countries, their seeds could be introduced into other countries as contaminants in imported grain.The spread of resistance genes through seed and pollen can cause significant economic loss. In order to assess the extent of the problem, we investigated the contamination by herbicide-resistant annual ryegrass (Lolium rigidum) of wheat imported from Western Australia into Japan. Annual ryegrass seeds were recovered from wheat shipments and seed bioassays were conducted to identify resistance to the herbicides that are commonly used in Australia: diclofop-methyl, sethoxydim, chlorsulfuron, and glyphosate. Nearly 4500 ryegrass seeds were detected in 20 kg of wheat that was imported in both 2006 and 2007. About 35% and 15% of the seeds were resistant to diclofop-methyl, 5% and 6% were resistant to sethoxydim, and 56% and 60% were resistant to chlorsulfuron in 2006 and 2007, respectively. None was resistant to glyphosate in either year. As the contamination of crops by herbicideresistant weeds is probably a common phenomenon, the monitoring of incoming grain shipments is necessary to stem the further spread of herbicide-resistant weeds into importing countries.

Resistance to acetyl‐CoA carboxylase‐inhibiting herbicides

“…A less frequent form of resistance spread, but still worth underlining, is via seed import from regions with high infestation of resistant grass weeds. For instance, analysis of a 20 kg batch of Australian wheat exported to Japan in 2006 identified as many as 4673 L. rigidum seeds, 35% of which were resistant to diclofop‐methyl …”

“…For instance, analysis of a 20 kg batch of Australian wheat exported to Japan in 2006 identified as many as 4673 L. rigidum seeds, 35% of which were resistant to diclofop-methyl. 73 Importantly, once resistance to ACCase herbicides has been established, it is difficult to overcome even with the use of other herbicide modes of action and alternative methods of weed control combined. A black-grass study conducted over a 6-year period has shown that while different cropping systems and other herbicide modes of action can result in an overall decline in the weed seed bank, the frequency of ACCase-resistant individuals within the field did not decrease over this timeframe.…”

Establishment of Lolium species resistant to acetolactate synthase‐inhibiting herbicide in and around grain‐importation ports in Japan

“…Seeds of Lolium species were picked out from each wheat sample following the method described in Shimono et al . (). These seeds were provided by the National Institute for Agro‐Environmental Sciences to Y. Shimono at Kyoto University and were stored in an airtight container with silica gel at room temperature (20–28°C) until the experiments.…”

“…Weed seeds spread through the sharing of equipment among farmers, the use of non‐composted manure and the use of contaminated commercial seed stocks or feed (Thill & Mallory‐Smith, ; Barroso et al ., ; Shimono & Konuma, ). Recent studies have shown that many herbicide‐resistant weed seeds are contaminants in commercial grain (Michael et al ., ; Shimono et al ., ). For example, herbicide‐resistant Lolium rigidum Gaudin seeds have been detected as contaminants in wheat from Western Australia imported into Japan (Shimono et al ., ).…”

“…Recent studies have shown that many herbicide‐resistant weed seeds are contaminants in commercial grain (Michael et al ., ; Shimono et al ., ). For example, herbicide‐resistant Lolium rigidum Gaudin seeds have been detected as contaminants in wheat from Western Australia imported into Japan (Shimono et al ., ). It was found that 15–35% of the contaminant seeds were resistant to diclofop‐methyl, while 56–60% seeds were resistant to chlorsulfuron (Shimono et al ., ).…”