Cooler temperatures and maturing crops indicate the start of harvest season. For those growers using cover crops to protect soil and suppress weeds over the winter, it also means the time to establish fall-planted cover crops is imminent. When it comes to cover crops that are used for the suppression of weeds, one species stands alone in effectiveness, affordability, and simplicity of management. Cereal rye is the most popular species planted in the state and in the Midwest for these and many other reasons. Increasingly unpredictable fall weather can delay harvest, and rye can tolerate later fall planting in comparison with some other more frost sensitive species. Rye germinates and grows in lower temperatures than other species and resumes growth with robust biomass production in spring. We know that for the suppression of weeds by cover crops, there are two main drivers – ground cover and biomass production – both of which rye excels at. Beyond planting time and method, rye seeding rate is another factor that requires some consideration when planning establishment. But what is the effect of seeding rate on weed suppression?
If biomass production and ground cover are the main drivers of weed suppression, it would be logical to assume that increased seeding rates would optimize both of these factors and increase the weed suppression potential of a cover crop. Studies have shown that increased seeding rates often lead to higher levels of biomass production. However, the data are less clear in how that translates to differences in weed suppression. When compared to other factors such as spring termination timing, the seeding rate of rye tends to have less of an effect on weed density. Consider the following:
- A study in Ohio comparing spring marestail density in rye planted at 0, 45 or 90 lb/A found an increase in rye biomass at the higher seeding rate and higher marestail density where no rye was planted. However, there was no difference in marestail density between the two seeding rates of 45 and 90 lb/A.
- Similar marestail suppression was provided by a wheat and cereal rye cover crop drilled at 60 and 120 lb/A before no-till soybeans in a Michigan study.
- In Missouri, researchers saw no difference in biomass among rye seeding rates of 30, 50, 70, 90, and 110 lb/A, and only incremental increases in waterhemp suppression at the higher rates, which they contributed to increases in ground cover.
Results of these and other studies in the Midwest suggest that when cereal rye is used to suppress weeds, increases in seeding rate above 50 lb/A may have less influence than other factors such as spring termination timing. Rates lower than 50 lb/A may also suppress weeds well, but the uniformity of the rye stand and biomass can be more variable. Weed suppression may therefore also be more variable.
For more information on cover crops for weed suppression, visit: https://iwilltakeaction.com/news/cover-crop-fact-sheet-series. This series of four fact sheets covers species selection, establishment, herbicide persistence and carryover, and termination, and how these different factors influence the weed suppression potential of cover crops.