Weed emergence patterns

INTRODUCTION
The weed seedbank is the source of the majority of weeds farmers face in corn and soybean fields. In recent years weed scientists have focused considerable effort at improving our understanding of the seedbank. A new extension bulletin (Weed Seeds and the Seedbank, IPM-48) provides basic information on this topic.

In 1994 Dr. Doug Buhler (USDA/ARS) and I initiated a long-term seedbank study involving four important weed species. We jokingly refer to this study as LESS (Long-term Ecological Seed Study). Our primary interest at the time was to determine the longevity of weed seeds in the soil. Many of the studies that have investigated seed persistence placed the seeds in conditions which inhibit germination (deep burial, cold storage, etc.); thus, they may not provide a realistic scenario of seed longevity under field conditions. Although it is too soon to define the longevity of the seeds, LESS has already provided some interesting insights into weed seed behavior.

METHODS
In order to determine the composition of the seedbank, seeds must be extracted from the soil - a very tedious task. In order to avoid this process, we created a synthetic seedbank by burying a known number of seeds in the upper two inches of soil. The study was initiated in an area that had been in sod for at least 20 years in order to minimize the size of the seedbank prior to our "inoculations". Seeds were placed within buried wooden frames to prevent lateral movement of seeds.

Seeds of velvetleaf, waterhemp, giant foxtail and woolly cupgrass were collected from mature plants during the 1994 growing season. Later that fall, 2000 seeds of a single specie were buried inside the wooden frames. The following season, weed emergence was monitored by counting and removing seedlings every three or four days. In 1995 a second set of seeds was collected and buried in a new set of frames.

RESULTS
Percent emergence: Large differences in emergence were observed among the species. In 1995, less that ten percent of seeds of the two broadleaf species emerged (Table 1). Emergence of the grasses was much greater, 33% of foxtail and 42% of cupgrass seed emerged during the first year. Emergence percentages from first year seed during 1996 was similar to 1995 in all species except for waterhemp, which increased from 6% emergence in 1995 to 22% in 1996.

Table 1. Cumulative emergence of four weed species during 1995 and 1996.

¹Percentage of 2000 seeds buried in top 2.5 cm of soil.

The number of seedings emerging the second year after burial was similar to that in the first year for the two broadleaf species. On the other hand, emergence of the two grass species declined by over 75% from 1995 to 1996.

After two years, less than 20% of the introduced seeds of the broadleaf species seeds produced seedlings, whereas 41% of foxtail and 53% of cupgrass seeds had emerged. We will continue the study to determine the fate of the remaining seeds.

Emergence patterns:Although the exact dates of emergence differed between 1995 and 1996, the sequence of emergence of the four species was similar in the two years. In addition, the pattern of emergence for each species was consistent in 1995 and 1996.

The emergence patterns of the two "new" weed species were completely opposite of each other (Figures 1 and 2). Woolly cupgrass was the first to emerge and reached 90% emergence in only two weeks. Waterhemp on the other hand initiated emergence approximately 2 to 3 weeks after cupgrass and velvetleaf, and didn't reach 90% emergence until six weeks after initial emergence. Giant foxtail and velvetleaf emergence patterns were intermediate of cupgrass and waterhemp.

Woolly cupgrass has been characterized as a species with multiple flushes; however, these data show that cupgrass has a much narrower primary emergence period than the other species . Although woolly cupgrass has a lower percentage of late emerging seedings than the other species, keep in mind that because of the huge woolly cupgrass seedbank found in many fields, late emergers may be of sufficient numbers to cause serious problems. Also, it is likely that some of the "late emerging" seedlings observed in fields actually are early emerging plants that were suppressed by sub-lethal herbicide doses.

The late emergence of waterhemp helps explain the rapid increase in problems with this species. In recent years weed management programs have changed so that many farmers have greatly shortened the length of control provided. This has occurred due to the movement towards postemergence herbicides and the decrease in cultivation as a weed management tool. These changes have created an ideal environment for waterhemp.

Conclusions
This study has provided interesting insights into two of the major weeds facing Iowa producers. This knowledge can be used to design integrated weed management programs that will provide more consistent weed control. We are currently expanding our efforts to evaluate emergence patterns of several more species and to develop the ability to predict weed emergence timing.

Prepared by Bob Hartzler, extension weed management specialist