Postemergence herbicide programs in corn should make growers and agronomists think hard about the question of “When does weed competition start to affect corn yield potential?”  A few bushels lost to weed competition can easily result in a $10/a or $15/a loss or more. Certainly, this is enough money to be concerned about. Of course, the answer to the question varies depending on the weed species, their density, and their size. Based on years of field trials, our rule of thumb is that weeds need to be controlled before they exceed 4 inches in height or by the time corn is about the V3 (3-collar) stage to prevent significant yield loss. 

 

It is interesting that there is a debate on what factor is primarily responsible for corn’s yield loss from this early-season competition. Is it competition for water, nutrients, or light? Competition for water as a primary factor certainly seems reasonable in dry springs or on coarse soil, but competition for water seems less important on medium soils in seasons with average rainfall in Wisconsin. Competition for nutrients most likely would relate to nitrogen. Do 4-inch tall weeds remove a significant amount of nitrogen to affect corn growth? We (Laboski and Boerboom) are in the second year of trials to answer this question. The third option is competition for light. Early in the season, it seems unlikely that corn is truly in competition for light because the corn is typically taller than most weeds. 

 

However, another mechanism might be affecting corn growth and competition with weeds. Plants can detect if other plants are growing nearby because the spectrum of light changes. Light reflecting off plants has more far-red light and less red light, so the ratio of red to far-red (R:FR) light decreases. One hypothesis is that corn detects the presence of weeds when the light spectrum changes and then corn growth shifts to more shoot growth at the expense of root growth. Over time this would limit corn’s yield potential if true. 

We (Stoltenberg and Markham) field tested this idea during the past two summers by measuring corn growth and yield when grown with “normal” light conditions (weed-free corn) and low R:FR light (simulated weed competition). Weed-free corn was grown at 22,000 plants/a for the normal light treatment (Figure 1).  For the low R:FR light treatment, corn was grown at 44,000 plants/a which simulated the light spectrum associated with weed competition,(Figure 2). Plots were irrigated and fertilized so that these factors would not limit corn growth. When corn plants started to shade each other at V6-7 in the low R:FR (high density) treatment, one-half of the corn stand was removed such that both treatments had the same plant density and spacing for the remainder of the season.  

 

 

 

The amount of sunlight, soil moisture, and soil nitrogen were similar between the normal and low R:FR light treatments through the V6-7 growth stage. However, the higher density of corn shifted the light spectrum such that the R:FR ratio was about 50% lower than the normal light treatment by V6-7. In other words, the extra corn plants changed the light spectrum to simulate weed competition. 

 

Did the corn’s growth differ because of the shifted light spectrum? For 18 characteristics that included early-season leaf, shoot, and root growth, corn did not differ between the normal or low R:FR light conditions in 2005 (only a few of these results are shown in Table 1). In 2006, corn plants in the low R:FR conditions (the simulated weed competition treatment) were taller, had longer leaves, and had less tillers than corn plants in normal light conditions. The root-to-shoot ratio did not differ between the light quality treatments in either year, which means the corn did not shift its growth to the shoot at the expense of root growth. Hand-harvested corn grain yield was also similar between these treatments in each year.

 

 

 

Some of the previous research to determine the effects of light spectrum (R:FR) on corn  growth has been conducted under controlled conditions such as in growth chambers. However, the results from these Wisconsin field experiments suggest that the effect of early-season light quality had little effect on early corn growth and no effect on corn grain yield. 

 

Is light quality a critical factor affecting corn growth and a significant component of early-season weed’s interaction with corn? Perhaps not. A simple answer would be nice to explain and predict weed competition. However, the interactions between the weeds and the corn are probably more complex and may be driven by a mixture of water, nutrient, and light factors.