Editing Weed control (section)

== Herbicide resistance ==
Resistance occurs when a target plant species does not respond to a chemical that previously used to control it. It has been argued that over-reliance on herbicides along with the absence of any preventive or other cultural practices resulted in the evolution and spread of herbicide-resistant weeds.<ref>{{cite journal | doi=10.1614/WS-03-097R | title=Resistance to ACCase inhibitor herbicides in a green foxtail (Setaria viridis) biotype in Europe | year=2004 | last1=De Prado | first1=Rafael | last2=Osuna | first2=María Dolores | last3=Fischer | first3=Albert J. | journal=Weed Science | volume=52 | issue=4 | pages=506–512 | s2cid=83918120 }}</ref><ref>{{cite journal | doi=10.3390/agronomy8040035 | doi-access=free | title=First Case of Glufosinate-Resistant Rigid Ryegrass (Lolium rigidum Gaud.) in Greece | year=2018 | last1=Travlos | first1=Ilias | last2=Cheimona | first2=Nikolina | last3=De Prado | first3=Rafael | last4=Jhala | first4=Amit | last5=Chachalis | first5=Demosthenis | last6=Tani | first6=Eleni | journal=Agronomy | volume=8 | issue=4 | page=35 | hdl=10396/17371 | hdl-access=free }}</ref> Increasing number of [[herbicide resistance]] weeds around the world has led to warnings on reducing frequent use of herbicides with the same or similar modes of action and combining chemicals with other weed control methods; this is called 'Integrated Weed Management'.<ref>Plant Production and Protection Division: What is Integrated Weed Management (fao.org)</ref>

===Farming practices===

Herbicide resistance recently became a critical problem as many Australian sheep farmers switched to exclusively growing wheat in their pastures in the 1970s.  In wheat fields, introduced varieties of [[ryegrass]], while good for grazing sheep, are intense competitors with wheat. Ryegrasses produce so many seeds that, if left unchecked, they can completely choke a field. Herbicides provided excellent control, while reducing soil disrupting because of less need to plough. Within little more than a decade, ryegrass and other weeds began to develop resistance. Australian farmers evolved again and began diversifying their techniques.<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>

In 1983, patches of ryegrass had become immune to Hoegrass, a family of herbicides that inhibit an enzyme called [[acetyl coenzyme A]] [[carboxylase]].<ref name=s1408/>

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. Farmers sprayed inexpensive Hoegrass year after year, creating selection pressure, but were diluting the herbicide in order to save money, increasing plants survival. Hoegrass was mostly replaced by a group of herbicides that block [[acetolactate synthase]], again helped by poor application practices. Ryegrass evolved a kind of "cross-resistance" that allowed it to rapidly break down a variety of herbicides. Australian farmers lost four classes of herbicides in only a few years. As of 2013 only two herbicide classes, called [[Photosystem II]] and [[long-chain fatty acid]] inhibitors, had become the last hope.<ref name=s1408/>