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Do cover crops need fertilizer?

Christine Gelley for Progressive Forage Published on 27 November 2019

Why cover crops?

Each farm is different, and therefore the way you use cover crops can differ too.

Whether you are a row-crop farmer, a fruit and vegetable grower, exclusively in the hay business, a livestock manager or involved in a combination of pursuits, cover crops can be an added benefit to your system.

Although the economic benefit of cover crops is difficult to quantify, the environmental principles associated with their growth are influential. Cover crops are selected for use based on their growing seasons, ability to reduce soil erosion and build soil organic matter, scavenge excess nutrients from the previous crop, relieve soil compaction and for their nutritional value as feed for animals. The way we incorporate cover crops into our systems should complement our current crops, fit our soil types and allow for termination of the stand when their useful period ends.

It just so happens many cover crops are excellent forage crops for livestock and wildlife too. Most are ideal as grazed forage rather than harvested forage, although preserving them for feed at a later time is possible if you have the right tools.

Cover crops in a forage system

Cover crops can be very useful when switching crops. When switching between two spring-harvested cool-season crops, warm-season annuals can bridge the gap between harvest and planting. When switching between fall-harvested warm-season crops, cool-season annuals can fill the void.

When switching from a perennial to annual crop, annual to perennial or perennial to perennial, cover crops can be useful for smothering weeds in between old-stand termination and new-stand establishment.

Grazing cover crops is by far more beneficial for nutrient distribution, machinery costs and reduced handling of the forage than mechanical harvest. Strip grazing cover crops can allow access to highly nutritious forage in controlled quantities while allowing even distribution of solid and liquid animal waste across the field.

One of the most common barriers to grazing cover crops is a lack of perimeter fencing around row-crop fields. Temporary fence is easy to install and move, but a perimeter fence is still advisable to keep animals off adjacent roadways in the event of an electrical failure.

Mechanical harvest can be used for most grass and legume cover crops, but drydown and feeding facilities can be limiting. If feeding facilities are in place, cover crops can be fed as greenchop in combination with high-fiber forage. If silage or baleage storage is possible, these feeds can be safely stored for extended periods of time.

In a forage system, the choice to graze or mechanically harvest can be the deciding factor on whether fertilizer application is necessary or not.

Don’t guess – soil test

The only way to accurately determine what your specific fertility needs are is by conducting a soil test. This should be the first step of any soil fertility program. The general recommendation for soil test frequency is every three years. However, if you have not accounted for the crops you are actually growing and will be growing in the next three years, you may not have all the information you need to make a good decision. Likely, a better suggestion is to test when switching crops, indicate your rotation plans and include cover crops on your soil test forms that go to the lab.

When the answer is ‘no’

Some situations where the answer to the question, “Do cover crops need fertilizer?” may be “no” include:

  • The transition of an inoculated legume crop to an annual grass crop – Due to the relationship between rhizobia and legume plants, enough available nitrogen may be present to eliminate the need for nitrogen fertilizer on the grass crop.

  • The transition out of an intensely fertilized annual crop – Excess nitrogen or phosphorus may remain in the soil in plentiful amounts for the cover crop to thrive. This would be an ideal situation to use cover crops as nutrient scavengers to draw down the buildup of phosphorus on a site or prevent off-target movement of nitrogen into waterways.

  • The cover crop will be rotationally or strip grazed by livestock – As livestock graze, they will redeposit nutrients from the cover crops back to the soil, replenishing the nutrient availability over time.

When the answer is ‘yes’

Some situations where the answer to the question, “Do cover crops need fertilizer?” may be “yes” include:

  • The transition into a legume cover crop – Many crops are luxury consumers of potassium, meaning that if potassium is readily available, the plants will take up more than they actually need. (Example: consuming more food in one sitting because it was served in a large dish versus a small dish.) Potassium is needed in greater quantities for legume growth than other common crops. Therefore, potassium could be a limiting nutrient for successful legume establishment.

  • The cover crop will be harvested and fed off-site – If the majority of the cover crop will be removed and relocated, the nutrients in the plants go with them. This could be desirable in some cases or lead to greater nutrient needs when the next crop is established. Most likely if the cover crop is designed to be harvested, conservative nutrient application at the correct time may be advisable. The likelihood increases with multiple harvests, in which split applications between harvests would be best. The manure produced after animal feeding could be spread back on the field, but this will increases time and labor investments in the system.

  • Cover crops are part of a remediation program for a low- fertility site – On locations where soil stability is a primary concern and soil fertility or pH are low, nutrient application may be necessary to allow successful growth of the existing cover crop and subsequent crops to hold soil in place and build organic matter.

One way to simplify this thought process is to use a decision tree to organize the priorities of your system and management capabilities (Figure 1).

Decision tree for cover crop fertilization

Making the decision

If reducing fertilizer use is one of your primary goals, choose cover crops that thrive in the soil conditions you currently have on your farm and still provide residual benefits for your crops and livestock. Incorporate regular soil tests into your management routine to best gauge your fertility needs.  end mark

Christine Gelley, M.S., is the Ohio State University Agriculture and Natural Resources Extension educator for Noble County. Her specialization is forage and pasture management.

Christine Gelley
  • Christine Gelley

  • Extension Educator, Agriculture and Natural Resources
  • Noble County Extension
  • Email Christine Gelley

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