December 2024 Update of the Cover Crop Demonstration Plot

Field Update

This December, Polk SWCD staff visited the cover crop demonstration plot to report on current condition of the cover crops. To provide some context, the field was split into eight different test plots, each using a unique combination of species and mixes, which were planted in August 2024.

Cold temperatures this month have had a noticeable impact on the cover crops. So far in December we have had 11 days with an average temperature under 32 degrees. The most significant effect has been on the oats, radishes and turnips, which have been killed by the below freezing temperatures.  Oats, radishes and turnips are all winterkill species, which means they will not have to be terminated in the Spring with herbicide or tillage.  While these three species are no longer viable, their roots will continue to hold the soil and reduce erosion.   In addition, the nutrients stored in these cover crops plants will slowly return to the soil as they decompose this Spring.

The remaining cover crops (Rye, Winter wheat, Triticale and Vetch) have gone dormant for the winter.  When spring comes around, they will be ready to continue growing.

The Cover Crop Plot is open to the public, so feel free to visit this winter and see how the cover crops are doing!  The Polk SWCD staff will be checking in on the plot regularly and providing updates.

Benefits of Winter Kill Species

Winter-terminated cover crops, such as oats and radishes, provide significant benefits for central Iowa’s corn-soybean rotations, particularly for improving soil health and reducing nutrient loss. These crops grow in the fall and naturally die off in winter, leaving residue that protects and enhances the soil. They reduce erosion by shielding the soil surface from wind and water, while their roots improve soil structure and water infiltration. The decomposition of cover crops also adds organic matter, boosting long-term soil fertility and tilth.

A key benefit of winter-terminated cover crops is their ability to manage nutrients, especially nitrogen. They act as nitrogen scavengers in the fall, capturing excess nutrients and preventing nitrate leaching into waterways. When they decompose, nutrients are gradually released, making them available for spring crops, potentially reducing fertilizer needs. This is particularly important for Iowa’s tile-drained fields, which are prone to nutrient loss into streams and rivers.

These cover crops also help protect water quality by reducing runoff and nutrient loading. Their residue slows surface water movement, minimizing phosphorus and sediment loss. Additionally, they suppress weed growth in early spring by creating a mulch layer that shades the soil and prevents weed germination, reducing the need for herbicides.

Winter-terminated cover crops simplify spring management. They decompose early, leaving fields drier and more accessible for planting compared to spring-terminated cover crops. This reduces the need for termination efforts such as herbicide applications or tillage, saving time and input costs while improving field conditions for planting.

By enhancing soil health, protecting water quality, and improving field access, winter-terminated cover crops are a cost-effective, low-maintenance conservation practice. Oats and radishes are popular options in central Iowa for their nutrient-scavenging abilities and rapid decomposition. When integrated into a farming system, these cover crops support both environmental goals and long-term farm productivity.