Editing Herbicide (section)

=== Farming practices and resistance: a case study ===
Herbicide resistance became a critical problem in [[Australia]]n agriculture after many Australian sheep farmers began to exclusively grow wheat in their pastures in the 1970s. Introduced varieties of [[ryegrass]], while good for grazing sheep, compete intensely with wheat. Ryegrasses produce so many seeds that, if left unchecked, they can completely choke a field. Herbicides provided excellent control, reducing soil disruption because of less need to plough. Within little more than a decade, ryegrass and other weeds began to develop resistance. In response Australian farmers changed methods.<ref name=s1408>{{Cite journal | doi = 10.1126/science.341.6147.734| title = The War Against Weeds Down Under| journal = Science| volume = 341| issue = 6147| pages = 734–736| year = 2013| last1 = Stokstad | first1 = E.| pmid=23950526| bibcode = 2013Sci...341..734S}}</ref> By 1983, patches of ryegrass had become immune to Hoegrass ([[diclofop-methyl]]), a family of herbicides that inhibit an enzyme called [[acetyl coenzyme A carboxylase]].<ref name=s1408/><ref>{{cite web |title=Hoegrass 500 Selective Herbicide |url=https://resources.bayer.com.au/resources/uploads/msds/file7356.pdf |publisher=Bayer CropScience |access-date=13 December 2023}}</ref>

Ryegrass populations were large and had substantial genetic diversity because farmers had planted many varieties. Ryegrass is cross-pollinated by wind, so genes shuffle frequently. To control its distribution, farmers sprayed inexpensive Hoegrass, creating [[selection pressure]]. In addition, farmers sometimes diluted the herbicide to save money, which allowed some plants to survive application. Farmers turned to a group of herbicides that block [[acetolactate synthase]] when resistance appeared. Once again, ryegrass in Australia evolved a kind of "cross-resistance" that allowed it to break down various herbicides rapidly. Four classes of herbicides become ineffective within a few years. In 2013, only two herbicide classes called [[Photosystem II]] and [[long-chain fatty acid]] inhibitors, were effective against ryegrass.<ref name=s1408/>