Crop Rotations

How Does Crop Rotation Suppress Weeds?

Crop rotation is the practice of planting different crop species across years. This cultural practice is known to suppress weeds, diseases, insects, and nematodes. But how much does crop rotation suppress weeds? One long-term study found that crop rotation itself only explained a minimal amount of the difference in weed density while  most of the changes in weed density were driven by weed management strategy (i.e. tillage or herbicides). While the overall effect on weed density can be minimal, crop rotation has a large impact on diversity of weed species. Differing crops offer differing levels of competition for weeds. For example, corn is generally planted earlier, grows taller, and is harvested earlier than soybeans.

Diversity is Key

The most important thing that crop rotation allows is the opportunity to use diverse weed management strategies. These differences between cropping systems that impact weed diversity include:

  • Planting timing
  • Row spacing
  • Crop competition
  • Fertility
  • Weed management options
    • Herbicides
    • Tillage or cultivation
    • Potential to include a cover crop
    • Potential for use of harvest weed seed control

Increase the Number of Crops and Length of Rotation

When crop monocultures are used (i.e. continuous corn), weed species that thrive in the conditions created for optimal crop growth become the dominant weed species in the fields. These weeds are often problematic because they are difficult to control in these cropping systems with the tools available. Over time, populations of these weed species build up, sometimes to disastrous levels.  Diversifying crop rotations, which leads to diverse weed management tactics, allows for more weed diversity and keeps weed populations under control.

As the diversity of the rotation increases, the weed management benefits increase (Figure 1).

 

Figure 1. Crop rotation effects on weed management. 

Moving from a monoculture (i.e. continuous corn) to alternating summer annual crops (i.e. corn-soybean rotation) is the first step. But thinking about crop diversity across a longer rotation will have a greater impact on weed populations. Adding in a small grain during the winter, if possible, will shift planting times and disrupt weed germination.  If the rotation also includes a multi-year or forage crop, that will lead to greater changes than rotating summer annual crops alone. Also, perennial forage systems are very competitive against germinating weeds because of the thick crop canopy that allows for year-long competition against weeds.

Conclusion

Crop rotation in itself is a cultural tactic for suppressing weeds but even more so allows for alternative weed management strategies to create a diverse environment where weeds won’t be able to thrive.

Resources

Doucet C, Weaver SE, Hamill AS, Zhang J (1999) Separating the effects of crop rotation from weed management on weed density and diversity. Weed Science 47:729-735.

Mohler, Charles L. The Role of Crop Rotation in Weed Management. Sustainable Agriculture Research and Education.

Liebman M and Dyck E (1993) Crop Rotation and Intercropping Strategies for Weed Management. Ecological Applications. 3:92-122.

 

Written by: Kara Pittman and Michael Flessner . VA Tech