Editing Herbicide (section)

===Selectivity===
Chemical structure of the herbicide is of primary affecting efficacy. 2,4-D, [[mecoprop]], and [[dicamba]] control many broadleaf weeds but remain ineffective against turf grasses.<ref name="Vats">{{cite book|last1=Vats|first1=S.|editor1-last=Lichtfouse|editor1-first=E.|title=Sustainable Agriculture Reviews 15|date=2015|publisher=Springer International Publishing|pages=153–192|chapter=Herbicides: history, classification and genetic manipulation of plants for herbicide resistance}}</ref>

Chemical additives influence selectivity. [[Surfactant]]s alter the physical properties of the spray solution and the overall phytotoxicity of the herbicide, increasing translocation. [[Herbicide safener]]s enhance the selectivity by boosting herbicide resistance by the crop but allowing the herbicide to damage the weed.

Selectivity is determined by the circumstances and technique of application. Climatic factors affect absorption including [[humidity]], light, precipitation, and temperature. Foliage-applied herbicides will enter the leaf more readily at high humidity by lengthening the drying time of the spray droplet and increasing cuticle hydration. Light of high intensity may break down some herbicides and cause the leaf cuticle to thicken, which can interfere with absorption. Precipitation may wash away or remove some foliage-applied herbicides, but it will increase root absorption of soil-applied herbicides. Drought-stressed plants are less likely to translocate herbicides. As temperature increases, herbicides' performance may decrease. Absorption and translocation may be reduced in very cold weather.