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Thoughts on Cover Crops for Nitrogen

Traditionally, we have used cover crops for two main purposes, which are erosion control and additional spring forage. Today, however, the uses and benefits of the various grass and legume species is varied and numerous.

Just this past week (Aug 4-8) I received two calls from growers who would like to be able to “grow” a significant proportion of the nitrogen that next year‟s corn crop will need.

Cover crops add organic matter to the soil enhancing the natural productivity and fertility of our soils, they help to scavenge and hold nutrients so that they are available to the following crop. They also protect the soil surface and structure from rainfall impacts, and are an essential tool in successful no-till production systems. Species selection and use is an exciting area of farmer interest and industry research. New and less commonly used species do however come with some risk and tend to be more difficult to manage than good 'ol rye or another cereal.

Species which are now becoming more common within the capital region are: Crimson clover, Hairy vetch, Annual ryegrass, Forage radish and other brassica species. Much of the interest in non-legume species is for the purpose of trapping and holding nutrients, such as from fall and early spring manure applications. Legumes such as Crimson clover and Hairy vetch will be able to capture 70-150 lbs of nitrogen and then release it for the following crop. In my mind a legume would be the most useful in situations with no manure or limited and uneven manure use and application. In essence you are growing your own nitrogen fertilizer.

Harvest date considerations:

One rule of thumb to remember for all cover crops is that the amount of top growth present in the spring is proportional to the amount of nutrients contained within the roots and the entire plant. It‟s common sense but very important to consider. As the cover crops decompose they will release much of these nutrients back into the soil—It‟s what my grandpa called “green manure”. This is especially true of the nitrogen in legumes. It‟s true in grasses as well; however, if your grass‟s top growth has started shooting heads or becomes “stemy” and lignified, a portion of the N and other nutrients will be tied up and not available to the next crop.

As stated above, more biomass generally means more nutrients and organic matter returned to the soil. In order to obtain greater N benefits you will want to consider delaying legume termination and corn planting until early-mid May. Cover crops burned down in April have significantly less biomass. This is especially true of legume species. See table below*:

Dry Matter Production Graph

Dry matter production of various cover crops.

*Average of the last 5 years.
**Vetch did not establish well in two seasons and therefore has a lower than expected yield.

Planting date considerations:

Keep in mind that grasses can be seeded late into the season, often up through the end of October. Legumes should be started in late August until no later than the end of September (the 15th is preferred).

My observations have been that Hairy vetch, Crimson clover and even annual ryegrass can smother if they become excessively rank. If you are growing one of these species and have heavy fall growth consider taking a high cutting to avoid winter kill. A little manure applied pre- or post planting will help all cover crops establish quickly. However, avoid heavy manure applications which can stimulate excessive fall growth and can worsen winter kill in legumes and ryegrass species.

When planting a legume, I would also consider adding a small amount of a nurse crop such as triticale, ryegrass or wheat (rye will head too early). A bushel or less should bring you the added benefits of a grass species without being to competitive with the legume.

Summary

From fall establishment to spring termination, I cannot begin to cover all the issues associated with successful cover crop use. So, let me leave you with some questions about your situation.

What is your primary goal/need? Forage? Cover? N from a legume? N scavenging from a grass? Soil building? Erosion control?

What are your constraints? Planting Date? Harvest Date? Equipment?

The Penn State Agronomy Guide, other printed resources along with many crop scouts and extension staff are available to you. Use them to develop a cover crop strategy and avoid disappointing results.

Whether to improve your no-ill success, grow more your soils with living roots year round, cover crops are playing an increasingly important part of crop production in the capital region.

Contact Information

Jeffrey S Graybill
  • Extension Educator, Agronomy
Email:
Phone: 717-394-6851