Giant ragweed (Ambrosia trifida) seedling

emergence for 2007

by Brian Schutte, Emilie Regnier, Kent Harrison

OSU Department of Horticulture & Crop Science

Weed Ecology Laboratory

Introduction

In Ohio agricultural fields, giant ragweed seedling emergence starts in early spring and continues into July.   We have found that giant ragweed seedlings emerge in a predictable pattern based on temperature and rainfall.  This allows us to monitor the progress of giant ragweed seedling emergence and make emergence forecast from anticipated weather.     

2007 giant ragweed seedling emergence model results

As indicated in the figure below, giant ragweed seedling emergence in 2007 is nearly complete.  Looking back at this year's pattern of emergence produces three interesting observations: 

•  First, the cold spell in early April stalled the progression of giant ragweed seedling emergence.  This indicates the importance of soil temperature on giant ragweed seedling emergence.      

•  Second, few seedlings emerged in late-April to early-May.  This mid-spring lag in emergence is characteristic of giant ragweed in Ohio agricultural fields.

•  Third, seedling emergence continued into July despite periods of little rainfall in May and June.  However, the number of seedlings per day decreased as the season progressed.     

These observations reflect the general character of giant ragweed seedling emergence and will be pertinent in subsequent years.   

Our predictive model was produced with information collected at the OARDC Western Agricultural Research Station in South Charleston, OH, so it should be representative of giant ragweed populations in west-central Ohio agricultural fields.  This information is meant to supplement local knowledge and observations of giant ragweed emergence so that growers can target their weed control practices more effectively.

Utility of emergence model

Seedling emergence models are necessary for well-timed and efficient weed control.  Knowledge of the giant ragweed seedling emergence progression can be useful in both conventional and organic crop production systems.

Conventional farming

An example for how this seedling emergence model can assist in deciding when to apply post-emergent herbicides is provided in the C.O.R.N. Newsletter article available at the following link:  Issue 2006-08

Organic farming

In a site where severe yield reductions result from heavy ragweed infestations, the seedling emergence model can assist with planting date decisions.  For a field of this sort, it is wise to delay planting until seedling emergence is nearly complete.  Allowing more seedlings to emerge prior to planting will reduce the need for control after planting.  For example, consider giant ragweed seedling emergence in 2005:

By June 5, 85% of giant ragweed had emerged.  Planting on June 5 would mean that 15% of the giant ragweed would have escaped pre-plant control measures.  If planting was delayed by two weeks, 95% of the giant ragweed would have been eliminated by pre-plant management. 

Delayed planting until ragweed emergence is nearly complete is especially important where giant ragweed control is difficult after crops have emerged.  However, frequent pre-plant weed control measures will be needed to prevent emerged ragweed plants from becoming too large.  Over time, delayed planting will diminish the giant ragweed population since fewer and fewer plants are allowed to mature and produce seeds.

Acknowledgements

•  Frank Forcella and Kurt Spokas of the USDA ARS North Central Soil Conservation Research Lab at Morris, MN provided expertise in seedling emergence modeling and soil environment modeling.
•  Jerron Schmoll monitored giant ragweed seedling emergence and maintained research plots from 2000 to 2004.
•  Funding for this project was provided by the Wittmeyer Knox Scholarship.

Questions can be sent to schutte.23@osu.edu