Iowa State University

Seed production following mowing in a summer annual
(or, How a Weed Beats Crop Rotation)  
by Bob Hartzler

December 16, 2005 -  Waterhemp is a prolific seed producer and can produce abundant seed under adverse conditions.  A simple study was conducted in 2004 to determine seed production by waterhemp plants that emerged in oats and survived oat harvest in late July and a mowing in mid-August.  On Aug 20 fifty waterhemp plants were tagged shortly after being mowed at a height of four inches.  Ten plants were harvested at weekly intervals and viable seed per plant was determined.  Only 50% of the plants had produced seed on the first harvest date, resulting in an average of one seed per plant (Table 1).  However, further delays in mowing resulted in a rapid increase in seed produced.  Waterhemp plants averaged nearly 2000 seeds on four weeks following mowing.

Table 1.  Seed production of waterhemp growing in alfalfa following late-season
mowing.

Arce and Hartzler.  2004.  Iowa State University.

The implications of waterhemp's ability to rapidly produce seed was demonstrated on an on-farm study in central Iowa.  There is a general consensus that diverse rotations are beneficial to weed management.  The simplistic explanation for the rotation benefit is that weeds are best suited for survival in crops with similar life cycles.  Thus, when a field is planted to summer annual crops such as corn or soybeans, summer annual weeds such as foxtail and waterhemp will increase, but perennial and winter annual weeds will be at a disadvantage and will decline in severity.  Rotations therefore help prevent any one particular weed species, or type of weed, from increasing to densities that overwhelm control tactics.  However, as with most things, it's not quite that simple.  Researchers followed changes in the seed bank on a central Iowa farm using a 5-year rotation (Figure 1).  The seed bank was sampled at the end of the growing season during each phase of the rotation, and in all years the seed bank was dominated by waterhemp.  Contrary to dogma, the seed bank declined during years the field was in corn and soybean, but skyrocketed when the field was planted to oats underseeded with legume.  The farmer had developed a very effective system for managing weeds in corn and soybeans, and was able to drive down the seed bank during this phase of the rotation even though herbicides were not used.  The weak link in the rotation occurred during the year legumes were established with an oat companion crop.  Waterhemp was able to survive oat harvesting, and the underseeded legume was not an effective competitor.  Although the farmer intended to to prevent weed seed production with mowing, the rapid increase in the seed bank during this phase of the rotation indicates this field operation was delayed too long. 

There are distinct advantages to rotation in managing weeds, however, this study illustrates that good management is required in any cropping system to stay ahead of weeds.

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