No Standing in the Shadow

Word Alert allelopathy

By: Charles Geddes

Competition between crops and weeds begins sooner than most of us think!  

Growers are all too familiar with the impact of weeds on their farm. Crop and weeds compete for essential resources, like water, nutrients or sunlight; which can have a direct effect on crop yields. We call this resource direct competition and it is the most well-known mechanism of yield loss. 

However, less-well understood are the impacts of resource indirect competition. With the large impact weeds can have on net farm income through crop yield reduction, research into this is valid and warranted.
Resource indirect plant competition results when a neighboring weed impacts the crop without depleting essential resources. Research in this field is growing and scientists are beginning to understand some of the mechanisms of indirect weed competition. There are a few mechanisms of indirect competition that can result in crop yield loss. One mechanism is allelopathy, which I will address in a subsequent article. 

Another mechanism results from a plant's ability to sense its immediate environment. On-going research suggests that plants can sense the quality of light reflected off of neighboring plants, the soil surface or even surface residue. Incoming light reflects off neighboring plants including weeds. By sensing the change in light quality a crop plant can know whether it might have to compete with a neighboring weed in the near future.

A crop plant can sense weeds even when they are too small to have a significant impact on resources available to the crop. When a plant senses its neighbor, it can have a shade avoidance response. Shade avoidance results in an alteration of crop morphology, usually observed as an increase in plant height (among other changes) in anticipation of competition with a neighboring weed. This alteration of crop morphology can use energy that would commonly go towards producing seed, resulting in a yield reduction.

Yield loss can result from direct or indirect plant competition, however the contribution of each mechanism in the field is still under investigation.