Weed Control? Timing!

The trend towards greater reliance on post-emergent herbicides continues. These programs are effective weed control tools, but require a greater level of management, for both conventional and herbicide tolerant weed control programs. Don't be fooled by the marketing hype!

The perceived simplicity of herbicide tolerance does not account for the added management required. Management time is still required with a herbicide tolerant crop weed control program.

Most post emergent weed control programs have little or no residual activity. Timing is critical to make these programs successful. Critical "weed free" periods have been determined for many crops. However, these are general in nature. Each situation needs to be scouted and evaluated individually.

The critical weed free period guidelines were developed from research across Ontario. The critical weed free period is dynamic, typically from the 3-8 leaf stage in corn and the 1^st-3^rd trifoliate leaf stage in soybeans. In any given year or field situation the critical period may vary. The time when it starts and its duration are dependent on many factors. Climatic conditions, time of weed emergence, weed density, weed species, soil type and crop canopy all contribute. Scouting and good record keeping allows weed control timing to match the critical period in any field situation.

Certain trends can be predicted. On light sandy soils, especially with dry conditions early in the growing season, weed competition impacts occur at an earlier stage of crop development than on medium or heavy textured soils. If weeds emerge with the crop, regardless of soil type, herbicide application will need to be earlier. This ensures no crop yield loss due to weed competition.

Figure 1 looks at timing of roundup application on corn yield, on different soil types, with no-till or conventional tillage production. The average of the 4 trials (bar graph) showed little impact of herbicide timing on yield. However, on the sandy soil (line graph), with heavy lambsquarters and barnyard grass pressure, significant yield losses occurred if the weed control was not applied at the optimal timing. Crop yield declined rapidly when weeds were controlled after the 5^th leaf stage of the corn crop.

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The economic impact of this yield loss is huge. The optimal timing for weed control to achieve the best yield was the 5^th leaf stage. At the 6^th leaf stage, yields declined from 136 to 124 bu/ac. A 60% loss in net profit per acre. If herbicide timing was delayed until the 7^th leaf stage, yield had declined from 136 to 112 bu/ac. A 111% loss in profit (some assumptions apply)! The difference between the 5^th leaf stage, and 6 or 7^th leaf stage is only 4-10 days. This highlights the need to scout fields to determine the best timing of your post herbicide programs.

Figure 2 shows no significant reduction in corn yield at any timing of application. The trend is to reduced yields as herbicide timing is delayed, but more variability exists in the data. This emphasizes that large yield losses from delayed application field do not always occur. Each field situation is unique. But it re-inforces the importance of scouting. Determine the correct timing for your herbicide program before yield robbing weed competition can occur. A "one size fits all" approach could be setting you up for significant losses.

Be aware of the pitfalls of relying on a single weed control timing strategy. Spread the risk: use all the application timing and weed control tools available. With post-emergent systems, the research indicates that earlier application has a greater return, even when earlier application fields look weedier at the end of the season. Early removal of weeds that emerge with the crop, the most competitive weeds, pays big dividends. Waiting for those last few weeds, those "not competitive" weeds can be very costly!

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