Weeds have a negative impact on soybean yields by competing for limited resources of light, water and nutrients, and one weed per 100 feet of row can impact yield.

Until quite recently agronomists and Extension specialists were saying that small weeds in soybeans were not a problem, and that the impact on yield was minimal. In today’s economics and weeds resistant to glyphosate and other herbicides, small weeds even limiting one bushel per acre is an economical concern, and small weeds develop into large weeds because they cannot be controlled postemerge is also a concern.

A few years ago, Bob Hartzler, Department of Agronomy at Iowa State University, noted that even when glyphosate was working, allowing weeds to grow too long before being eliminated caused yield loss. “Fields may be weed-free at harvest yet suffer significant yield losses because weeds that emerged with the crop were allowed to grow until after the critical period was reached. Once the critical period is reached, soybean yield may be reduced 1 percent for every day that weed control is delayed. Thus, controlling weeds before the critical period is critical to maximize profitability,” Hartzler wrote.

He further noted, “Unfortunately, there is no simple rule to determine the critical period for early-season competition.” Various university research has shown the critical timeframe for control of weeds to not impact yield can be as small as the V1 soybean stage in 30-inch rows, V2 soybean stage for 15-inch rows, or up to V6 soybean stage in some cases.

So, what is the best answer to achieve maximum yield potential? Don’t let the weeds get started at all. That is why pre-emerge weed control products are now being recommended extensively across the soybean growing areas of the U.S.

In addition to the early-season yield savings, not allowing the weeds to emerge keeps them from escaping post-emergence herbicide applications and growing into large weeds that definitely cut yield—if the weeds don’t completely force out the soybeans as has occurred with glyphosate resistant weeds in Mid-South fields.

The old “economic threshold” for treating weeds has gone by the wayside in general because of the fear of weeds emerging that cannot be controlled postemerge.

Most soybean farmers, up until Roundup Ready soybeans took over the market, would hoe weeds or use weed wipers when weeds hit those economic thresholds, or even before the thresholds. Based on what is known today, the cleaner the field early, the better the yield potential. The old numbers of one pigweed per 100 feet of row reducing yield by 2 percent and six pigweeds per 100 feet of row reducing yield by 10 percent, provided by the University of Illinois Cooperative Extension Service, are ignored these days. Farmers don’t want to see one pigweed or lose any yield.  

One pigweed has to be of concern today because it might be the start of herbicide resistant pigweeds taking over a field and a 2 percent yield loss is money out of a farmer’s pocket.