NC State Extension Publications

Background

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Cover crops are often added to a rotation to provide soil cover during fallow periods and scavenge nutrients left behind after a crop has been harvested. Cover crops hold nutrients at the soil surface until they are terminated. Terminating the crop can be accomplished through chemical (herbicide) or mechanical (tillage, chopping, mowing, roller-crimper) methods or by natural winterkill. As the cover crop residues decompose, nutrients are released back to the soil for the next crop, reducing the amount of nutrients required to reach the realistic yield expectation for that field. This document provides guidance for acceptable Plant Available Nitrogen (PAN) application rates for cover crops within a Nutrient Management Plan (NMP) and specifies credits to be given toward the nitrogen requirements of the subsequent crop in the rotation.

Cover Crop or Double Crop

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Several cereal crops have transitioned from being primarily cover crops to part of a double crop system in which both crops are fertilized and harvested. For example, a corn crop followed by a fertilized and harvested wheat crop is a double crop. Cover crops are not harvested but terminated, and their entire biomass is left in the field where nutrients cycle to the next crop. Therefore, fertilized cover crops in an NMP require a nitrogen credit to the next crop because nutrients are returned to the soil and available for plant uptake. In a double crop system, there is typically no nitrogen credit to the next crop because nutrients have been removed from the field at harvest. Because of the biomass removal, double cropping will allow for higher PAN application within an NMP than cover cropping in the same rotation.

Allowable Application Rate for Small Grain Cover Crops within a Certified Animal Waste Management Plan (CAWMP)

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All cover crops receiving inorganic fertilizer or manure must be included in the rotation within the operation’s CAWMP. The Senate Bill 1217 Guidance Document states: “[t]he maximum amount of PAN which may be applied to small grain seeded as a cover crop not for harvest is 30 lbs [N] per acre. [Nitrogen] application to the next crop must be reduced by the amount applied to the small grain. This option must be stated in the waste utilization plan.” Therefore, a cover crop receiving the maximum amount of allowable PAN, 30 lbs N/ac, would provide 30 lbs N/ac to the next crop in the rotation. Stated another way, whatever PAN is applied to the cover crop must be deducted from the beginning allowable PAN for the next crop in the rotation.

Cover Crop Management

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When selecting a cover crop, keep in mind the anticipated harvest and planting dates of other crops in the rotation. Cover crops should be planted as soon as practical after (or prior to) harvest of the production crop and terminated as late as practical before planting to maximize nutrient uptake and biomass production. For you to qualify for crop insurance on the subsequent crop, termination of all cover crops in North Carolina must meet the United States Department of Agriculture (USDA) Risk Management Agency (RMA) guidelines, with termination of the cover crop taking place prior to emergence of the production crop. More information on cover crop selection and management is in the Winter Annual Cover Crops SoilFacts publication and the SARE Managing Cover Crops Profitably online book.

The rules surrounding nutrient application can vary depending on whether the practice has been planned by federal, state, or local agencies and the goal of the cover cropping practice. Plant species, seedbed preparation, seeding rates, seeding dates, seeding depths, fertility requirements, planting methods, and termination dates should be consistent with the applicable federal, state, and local criteria and specific soil/site conditions.

Additional Nitrogen Credits When Planting Legumes

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There should also be a nitrogen credit following legume crops that leave residual nitrogen in the soil for subsequent crops in the rotation. This credit is in addition to any cover crop nitrogen application credits (30 lbs N/ac maximum) that must be accounted for. A nitrogen credit is applied to subsequent crops within the current nutrient management planning software for soybean and peanut. Table 1 outlines the anticipated range of nitrogen returned to the soil after legume crops, including commonly used cover crops such as hairy vetch, crimson clover, and Austrian winter pea. For example, a cover crop of crimson clover receiving 30 lbs of N as swine effluent would provide 90 to 105 lbs of N (30 lbs for waste and 60 to 75 lbs for clover) to the next crop. In Wake County on a Cecil soil, the realistic N rate for corn grain is 127 lbs of N/ac, meaning the crimson clover cover crop would supply all but 22 to 37 lbs of N/ac needed to meet the realistic yield expectation.


Table 1. Range of nitrogen returned to the soil from legume crops.

Legume

Residual N Available in Soil (lb/A)

Current Nutrient Management Planning Credit within the Software (lb/A)

Soybean

15–30

20

Peanuts

20–40

20

Alfalfa

80–100

Hairy Vetch

80–100

Crimson Clover

60–75

Austrian Winter Pea

50–60


Cover Crop Mixes

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Cover crops can be seeded as mixtures of various small grains, legumes, and other species to offer diversity in plant type and provide multiple benefits to the cropping system. For example, tillage radish can be planted to loosen compacted soil, and legumes are often included to provide a source of nitrogen. Because little information is available on the amount of nitrogen produced by legumes within a cover crop mix and there is variability in germination among species within a mix, the same allowable PAN rate is approved for cover crop mixes, 30 lbs N/ac, as a single-species small grain cover. There is no additional N credit to the subsequent crop from legumes within a mix. However, single-species legume cover crops should be given the corresponding N credit (from the table) along with any manure N credit to the next crop.

Application Windows

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Manure must be applied to an actively growing crop or within 30 days of planting or breaking dormancy. This rule also applies to cover crops. For application windows, see Appendix 1.1A in the Senate Bill 1217 Guidance Document or the Nutrient Management in North Carolina website. If no application window is listed for the desired crop, ask your Regional Agronomist for guidance.

Application of waste to a crop outside of the application window designated in the CAWMP is not allowed. Contact your Regional Agronomist for assistance if you feel you need a modification to the application window listed in your CAWMP.

Authors

Assistant Professor & Extension Specialist - Nutrient Management & Animal Waste
Crop & Soil Sciences
Animal Feeding Operations Program
North Carolina Department of Environmental Quality

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Publication date: Dec. 13, 2019
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