a] Herbicides must be applied at the correct stage of weed growth : young actively growing weeds are more easily affected by herbicides at the normal rates than older weeds which tend to become resistant. Less chemical is used when weeds are sprayed young.
b] Must apply at the correct stage of growth of the crop. Different crops differ in their susceptibility to different weedkillers depending on stage of growth.
c] Avoid spraying during windy weather to prevent drift getting onto susceptible crops which may be growing in the neighboring fields. Spraying during windy weather makes it difficult to spray uniformly.
d] Do not spray if it looks like it is going to rain soon after spraying. For most trans-located and contact herbicides you require a minimum of 2-3 hours without rains after spraying for the weedkiller to have any effect at all. For root herbicides, which are normally soil-applied, a slight shower after application might be welcome to wash the chemical into the root zone.