The primary reason for controlling weeds is to protect crop yields from the negative effects of weeds on yields. Weeds compete with crops for limited resources such as light, water and nutrients, but it is also known that other interactions occur between plants growing in close proximity that may influence productivity. A recent paper in Weed Science (Rajcan et al. 2004) presents information indicating that changes in the quality of light following reflection off weed foliage may negatively impact the productivity of corn in the absence of competition.
All plants have a photoreceptor that is able to detect slight changes in the ratio of red and far red wavelengths. This photoreceptor (phytochrome) is responsible for many light induced responses, including stimulation of germination of seeds after exposure to light and shade induced changes in plant growth. Plant leaves absorb a high percentage of red light, thus lowering the ratio of red to far red light. The Canadian scientists designed an experiment to determine whether changes in light quality due to the presence of low-lying weeds could impact corn growth in the absence of competition. Corn was grown in growth chambers in a manner that simulated growing in the presence or absence of weeds (Figure 1)a. The light reflecting materials were placed along two sides of a row of pots containing a single corn plant. The experimental design prevented competition for light, water or nutrients between the corn and sod, thus any changes in corn growth were due to the quality of light reflected by the surrounding surfaces. Corn was harvested at the V6 stage of growth.
Corn growing in the presence of other plants (sod treatment) was taller and had greater leaf area than corn surrounded by a surface that reflected red light (Table 1). The tendency of plants to grow taller when surrounded by other plants is well documented, and is believed to be a shade avoidance mechanism. While the increase in height and leaf area may allow the corn to reduce competition for light by getting its leaves above those of adjacent weeds, this alteration in resource allocation may have long-term negative consequences. The increase in shoot growth was accomplished by a reduction in root growth, resulting in a increased shoot:root ratio. Changes in leaf orientation were also observed between the two treatments. The pots containing the corn were placed so that corn leaves grew towards the light reflecting surface (perpendicular to the row). Corn growing with sod at its side altered its growth habit so that a high percentage of the plant's leaves were oriented parallel with the row of pots, whereas in the red light treatment the majority of plants had leaves perpendicular to the row.
Table 1. Effect of quality of reflected light on corn growth.
| Treatment | Red:Far Red Ratio | Corn height (inches) | Leaf area (cm2) | Shoot: Root ratio |
|---|---|---|---|---|
| Sod (Far red reflecting) | 0.67 | 16 | 631 | 2.06 |
| Soil (Red light reflecting) | 1.37 | 11 | 442 | 1.80 |
The negative impact of early-season weed competition on corn yields is well documented. The yield losses often are much bigger than one would expect from competition since the corn's demand for light, water and nutrients are limited early in the season. The authors speculate that reallocation of resources to favor shoot growth at the sacrifice of the root system when corn is growing in the presence of weeds may be detrimental in the long run. This would especially be critical in situations where environmental conditions favor an extensive root system (water and heat stress) or when the root system is damaged by insects or diseases. This research illustrates the complexity of crop-weed interactions that make it very difficult to accurately predict when weeds will begin to impact crop yield potential. Due to this uncertainty, we feel it is best to take a conservative approach to weed management by controlling weeds early to minimize the risk of weed-related yield losses.
Reference:
Rajcan, I., K.J. Chandler and C.J. Swanton. 2004. Red far-red ratio of reflected light: a hypothesis of why early-season weed control is important in corn. Weed Sci. 52:774-778.
aAuthor note: Some people involved in managing weeds might question the applicability of this research to the real world, and think this is what happens when a) Roundup Ready crops eliminate the need for weed scientists to spend their time evaluating new herbicides, or b) the cancellation of the NHL season causes Canadian scientists to lose touch with reality. However, I think the research provides an interesting explanation for the types of responses many of us have observed with total post programs in corn.