Why did my weedicide spray fail to kill the weeds?

When a herbicide fails even though the product and dose seem right, the cause is almost always in the application or the conditions, and the common reasons are worth checking one by one. Spraying too late, when weeds are big and hardened, is the top reason, because small actively growing weeds are killed easily while large ones survive. A dry, moisture-stressed field is the next, since stressed weeds take up little herbicide; spray on a moist field. Too little water volume or the wrong nozzle (using a cone nozzle meant for insecticides instead of a flat-fan) gives poor, patchy coverage. Rain too soon after spraying washes off the chemical before it acts. Hard or muddy spray water can reduce the activity of some herbicides. Wrong stage for the herbicide type, like using a pre-emergence product after weeds are already up, simply will not work. And if the right product at the right time and setup still fails repeatedly, suspect herbicide resistance and rotate to a different mode of action. Check these before blaming the chemical. Use only herbicides recommended for your crop at the label dose and timing, set up the sprayer correctly, and confirm with your KVK if the problem repeats.