Weed-Free Window & Yield Cost of Spraying Late

The CPWC is the window in a crop's life when weeds must be kept under control to avoid losing more than an acceptable amount of yield, usually 5%. It is bounded by the critical timing of weed removal (how long early weeds can be tolerated) and the critical weed-free period (how long the crop must then be kept clean). For maize this window is roughly 14 to 42 days after emergence; controlling inside it protects most of the yield, and the cost of waiting climbs sharply afterward.

Published corn CPWC studies (Hall et al. 1992; Knezevic et al. 2002) put the critical weed-free period at about 14 to 42 days after emergence, ending near the V6–V8 stage, at a 5% acceptable-yield-loss threshold. Keep the crop weed-free across that window and season-long loss stays near 5%; each extra week of delay beyond it adds roughly 6% more yield loss in this tool's model.

The tool models loss as staying near the 5% acceptable threshold at the window's close, then rising by the crop's reported loss-per-week coefficient for every week of delay. For maize that is about +6% per week, so spraying one week late costs ~11%, two weeks late ~17%, climbing toward the season-long maximum (≈50% for corn) if weeds are never controlled. Slide the spray marker on the chart to read your own figure.

The last safe day is the close of the critical weed-free period — the end of the window shaded green on the timeline. Spraying on or before it keeps loss near the acceptable threshold; spraying after it is where yield starts to slip measurably. The exact day depends on the crop and your weed pressure, both of which the calculator accounts for.

The critical timing of weed removal (CTWR) is the window's start — the latest you can leave crop-emergence weeds before they must be removed. The critical weed-free period (CWFP) is the window's end — how long the crop must stay clean before later weed flushes no longer cut yield. The span between them is the critical period during which control is non-negotiable.

Yes. Dense, competitive weeds bring the window's start earlier and push its close later, widening the critical period and steepening the yield-loss-per-week curve, exactly as Knezevic et al. (2002) describe. The calculator's low / moderate / high pressure setting shifts the begin and end days and scales the loss coefficient so the answer reflects your field.

A pre-emergence residual herbicide controls weeds from the very start of the window, removing the risk of spraying too late and buying slack at the front end. A single post-emergence pass must land inside the critical window, which is a narrower target. The strongest programs combine a residual to open the window clean with a timed post pass before the window's close.

Weeds that compete with the crop during the critical period steal light, water and nutrients exactly when the crop is setting its yield potential — losses incurred then cannot be recovered later, even if the field looks clean at harvest. That is why the model keeps loss low only while control happens inside the window and penalises every week of delay past it.

Inside the protected window, modelled loss scales from zero up to the acceptable threshold (5%) at the window's close. Past the close, loss = acceptable threshold + (weeks late × loss-per-week coefficient), capped at the crop's season-long maximum. The coefficients come from published CPWC yield-loss curves by crop, so the number reflects real competition data, not a flat rule of thumb.

Yes, but the window is measured from establishment (transplanting/emergence). Transplanted crops get a head start on weeds, so their windows tend to open a little later, but the same logic holds: there is a span during which they must be kept weed-free. The calculator carries crop-specific windows for transplanted rice, tomato, cabbage and pepper alongside direct-seeded crops.

Once the spray date is far past the window's close, the calculator flags it 'far too late' and the modelled loss is near the crop's season-long maximum — much of the damage is already done. Spraying still prevents weeds from setting seed and worsening next year's weed bank, so it is rarely pointless, but the yield for this season is largely lost. Plan earlier next season.

The windows here are fitted at a 5% acceptable yield loss, the standard convention in the CPWC literature, because that is roughly the point where the cost of weed control equals the value of yield saved. A grower willing to accept more loss would have a narrower window; one demanding cleaner fields would need a wider one. Five percent is the practical, economically grounded default.

No — it tells you when control must happen and what late control costs, which is the timing decision growers most often get wrong. Product choice depends on your weed spectrum, crop, resistance status and label, so pair this with your local herbicide-efficacy guide and a site-of-action rotation plan to pick the actual product and rate.