Profitable crop production depends on effective weed control. Weeds can reduce crop yield potential by competing for water, nutrients, and light. Some weeds release toxins that inhibit crop growth, and others may harbor insects, diseases, or nematodes that attack crops. Weeds can also reduce harvest efficiency and produce seed that can impact future crops.   


Not all weeds compete with crops equally (Table 1).1 Broadleaf weeds tend to be more competitive than grasses in corn.  For example, the predicted weed densities per 40 feet of row required to cause a 10% yield loss in corn would be 80 foxtails compared to 40 pigweeds and only 10 cockleburs.  Canopy closure of the crop can limit the competitive ability of weeds.  However, broadleaf weeds are better able to avoid the shading effects of the canopy and compete longer into the growing season.  Early-germinating weeds are generally more competitive than weeds that emerge later in the growing season.  Fields that experience moisture stress are also at greater risk of yield losses from weed competition.  Heavier soils that hold moisture better can tolerate higher populations of weeds that may impact crop yield potential.   

Late-emerging weeds can include those that emerge after control tactics have been implemented.  The impact of late-emerging weeds decreases rapidly if weeds emerge 3 weeks or more after crop emergence.  The competitiveness of late-emerging weeds is strongly influenced by how quickly the crop canopy develops.  Stress factors that reduce crop growth can increase the impact of late-emerging weeds.  These weeds are at a competitive disadvantage to the crop due to their delayed emergence but are still capable of causing economic losses. 


Late-emerging weeds should not be ignored because they are capable of producing significant quantities of seed.  If weeds have not initiated seed set at the time of herbicide application, seed production should be eliminated or reduced.  However, if the fruiting structures are visible, it is unlikely that the application will reduce weed seed production or viability of the seeds.  The other benefit of late-season weed control is harvest efficiency, and there are situations where these applications may be worthwhile.  


Effective weed control in field crops requires the use of multiple management techniques including cultural methods as well as herbicides. Growing the same crop year after year and utilizing the same weed control techniques encourages the development of problem weeds and potentially, herbicide resistance. Utilizing crop rotation, tillage, and a diversified herbicide portfolio helps reduce this problem. 


Identifying weeds properly is an essential first step in a weed management plan. This guide provides photos and identifying characteristics of grass and broadleaf weeds common to the crop’s growing region.  



1VanGessel, M. Weed management in row crops: application to corn production - competitive index factor chart. Northeast IPM Module number 10.  

Grass Characteristics



Properly identifying grass weed species during their early growth stages can lead to a more effective management plan. Given that grass weed management is most effective when weeds are small, a hand or pocket lens with 10X magnification can improve grass weed identification. 

The ligule and collar region of the plant is where the main characteristics used for grass identification are located (Figure 1). Grass leaves are either rolled or flat within the stem. Sedges can be identified by a triangular stem. Ligules of grasses will either be absent, hairy, or membranous (Figure 2).  Auricles will either be present or absent. Below are descriptions and pictures to aid in identifying some of the common grass species found in agronomic crops in the corn growing region as well as management of these species.  For more information click below to expand.

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