Iowa State University

Importance of Timely Scouting for Weeds   
by Bob Hartzler

December 16, 2005 - Timely application of postemergence products is critical to protect corn yields, especially in fields relying solely on postemergence herbicides. Weeds can begin to impact yields as early as two weeks after crop emergence. The specific time at which weeds begin to reduce yields varies widely depending upon many factors, but the longer application is delayed the greater the risk of a yield penalty.

The initial growth of weeds is relatively slow, but after a sluggish start their growth rate increases rapidly (Figure 1). Weeds as small as 2" can impact corn yields if present at high densities ( >10 sq. ft). The crop yield lost per day increases as control tactics are delayed due to the increasing growth rate and greater impact of large weeds (Table 1). Weeds 2 - 4” tall may reduce yields by 0.5% for each day application is delayed, whereas 6” weeds can reduce yields by 1% or more per day.

Fields should be scouted within the first two weeks after crop emergence to determine the need and appropriate timing of postemergence weed control. While many of today’s herbicides are able to kill large weeds, application delays can result in reduced yield and profitability due to early-season competition.

Table 1. Giant foxtail growth rates and impact on corn yields as
affected by plant height.

(1)Adapted from Gower et al. 2003. Weed Technology.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University