Managing the maturity of your cover crops is important to achieving your desired goals, especially as it relates to forage quality, nutrient cycling, and lasting ground cover. The ratio of carbon to nitrogen (C:N) in the cover crop is a good indicator of maturity and will be a reflection of how fast the plant material decomposes. As noted in the chart below, a high C:N ratio is an indicator of maturity and increasing amounts of lignin, which is a high carbon complex organic material within plant cell walls. The woodiness of lignin gives plants strong, lasting residue and the ability stand through the winter. A high C:N ration is a good thing if the goal is to produce long lasting residue but can be detrimental if fast nutrient cycling is important. Some benefits to maintaining longer lasting residue include reduced erosion, increased water infiltration, superior weed suppression, habitat/food for a balanced biological environment, and cooler soil temperatures during the summer.
Cover crop production with a low C:N ratio will have excellent forage quality and will release nutrients efficiently, but will also decompose quickly and leave bare soil. The best cover crops often have a moderate C:N ratio between 25 and 30. Our exclusive SMARTmix® calculator will make a prediction about the potential C:N ratio of the cover crop mix that you build. The predicted ratio is based on the species selected and how many growing degree days the mix potentially has.
As plants mature the C:N ratio will increase and quality factors such as crude protein, digestibility, nutrient density, palatability, and forage quality tend to decrease. The extent to which they decrease varies between plant families, species, and even down to the variety level. In general brassica and non-woody legumes will show forage quality decreasing slower than cereals or annual warm season grass species. Through proper grazing management, most cover crop forages can be maintained in the mid to late vegetative growth stage, thus maximizing regrowth potential and ensuring high forage quality characteristics. As your soil biological health develops, crop residue will be cycled faster and the need for more carbon will become increasingly important. The beauty of a custom cover crop mixture is that species can be blended together to target your desired C:N ratio.