LEVERAGING THE CRITICAL WEED-FREE PERIOD IN CORN

The critical weed-free period is the time during a crop’s development when weeds must be controlled in order to preserve yield potential. For corn, this stage is generally considered to run from about V2 through canopy closure (V6-8). During this time period the crop is setting itself up for future yield by expanding its root system and leaf area, sensing how abundant resources are, and determining how many other plants are growing around it (e.g. weeds).

This means that the more weed biomass grown with the crop during this phase, the lower yield potential will be. That is why timely weed control is so important for maximizing yield. Furthermore, the bigger weeds get, the harder they are to kill with herbicides. Virtually every post emergence herbicide label only list weeds as controlled up to a certain height. If the weeds are too big, the herbicide will probably only give you weed suppression at best. To add insult to injury, late herbicide applications are also notoriously hard on the crop itself. Unless you have drop nozzles, most of the chemical gets caught by the crop leaves and must be metabolized by the crop rather than killing the weeds underneath, like it’s supposed to.

So, how do you leverage the critical weed-free period? You make sure that as little weed biomass as possible grows during this phase. If you don’t have pre-emergent products active in the soil by V2/3 or at least a burndown application of glyphosate, you’re starting to leave some yield on the table. I know it feels like you’re getting more of your money’s worth when you wait for more weed biomass to be present, but unless you have a justifiable reason to wait (like waiting for better spray weather) it isn’t worth it. Yes, there are farms out there who are making a single late application of glyphosate work OK, but that comes at the cost of yield and Roundup resistant weeds – when (not if) they show up at your farm.

The critical weed-free period is also a key point of leverage if you are interseeding your corn with a cover crop. Interseeding early in the critical weed-free period may provide the greatest interseeding success, but it can cause a substantial yield hit. The later you push it, the less of a yield hit you can expect. We have measured essentially zero yield impact when interseeding just prior to canopy closure. The only trouble is that the interseeded cover crop has to grow in the shade for the entire season, resulting in leggy plants and variable establishment. Thus, the optimal interseeding window is created by leveraging the critical weed-free period to optimize crop yield while still allowing interseeding survival.

— Allen Wilder

wilder@whminer.com