Iowa State University

Get The Most Out Of Your Total Post Program
by Bob Hartzler
 May 21, 2004 -  Total post programs in corn are increasing with greater adoption of RR and Liberty Link corn.  Determining when to apply the post treatment typically is the most difficult management decision in this type of program.  The optimum application time will vary from field to field based on cultural practices, weed infestation, and environmental conditions.  This variability makes it important to have some level of understanding of the interactions between application timing and crop yields and weed control.  Researchers at The Ohio State University conducted eight experiments in Roundup Ready corn investigating this control strategy.  Timing of glyphosate application was based on the height of giant foxtail, with initial applications being made at the 2, 4, 6, 9 and 12 inch height.  In addition to the one-pass programs, sequential treatments were included with a second glyphosate application made two to three weeks after the first treatment. 

An effective weed control program must consistently provide a high level of weed control and also protect crop yields.  To evaluate the success of the treatments included in these experiments, the percentage of experiments in which greater than 85% weed control (a number often used as commercially acceptable) and the percentage of sites where yield losses from weed competition were prevented was calculated (Table 1). One-pass programs made to giant foxtail 6" in height or less protected corn yields 88% of the time (7 out of 8 experiments).  Delaying application until 9 and 12 inch foxtail resulted in yield losses at 50 and 75% of the experiments, respectively.  The 2 and 4" sequential treatments provided 100% protection of yields, but the 6" sequential treatment resulted in reduced yield at one site.  The yield loss in the 6" sequential treatment likely was due to competition between the corn and weeds prior to the initial glyphosate application.

Although most of the total post treatments were effective at protecting corn yields, the one-pass programs were inconsistent at providing acceptable levels of weed control.  The consistency of giant foxtail control increased as application was delayed, whereas there was less of a benefit to delayed application with lambsquarters and velvetleaf.  This is probably because foxtail is very sensitive to glyphosate and thus late applications still provided effective control of large plants while reducing problems with late-emerging weeds.  The velvetleaf and lambsquarter may have been too large for  effective control with the later applications, therefore resulting in poor control ratings.

 Table 1.  Consistency of total postemergence program in Roundup Ready corn. 
                  Application timing was based on size of giant foxtail.

Adapted from Gower et al.  2002.  Weed Technol.  16:488--494.

Another way to evaluate the data is to look at the range of control provided by individual treatments (Figure 1).  The line inside of the box represents the median giant foxtail control in the experiments, whereas the 'box and whiskers' provide information on the range of responses.  Within the box fall the mid-50% of the responses by a treatment, whereas the whiskers show the total range of values (e.g. the 2" treatment provided anywhere from 26 to 85% control).  Since consistency is the key to success, the smaller the box and whiskers the better.  The sequential treatments and late applications (9 and 12" foxtail) had the least variability in control; however, the late applications were ineffective at protecting crop yields.

The results of this research reinforce the importance of timely control of weeds that emerge with the crop.  Although early applications (2 and 4" foxtail) resulted in poor weed control due to late flushes of weeds, the late-emerging weeds only affected yields in one of the eight experiments.  Similar to other studies, this research indicates that allowing weeds to reach 6" in height before treatment may result in yield losses due to early-season competition.  The influence of preemergence herbicides on weed control and yield was also evaluated.  Although the preemergence herbicides did not consistently increase performance of the post treatments, one of the major advantages of using an integrated approach is that a combination of  preemergence and postemergence herbicide provided much greater flexibility in application timing than a total post program.

Gower, S. A., M. M. Loux, J. Cardina and S. K. Harrison.  2002.  Effect of planting date, residual herbicides and postemergence application timing on weed control and grain yield in glyphosate-tolerant corn.  Weed Technol. 16:488-494.

Related articles:

Critical periods of competition
Realistic expecatations

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