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Gain Greatest Value from Manure Nutrients

Natalie Rector
Extension Manure Nutrient Management Educator

As soon as the crops come off fields in the fall, manure applications begin in earnest for most dairy producers. Surface applications are fast and convenient, but run the risks of odors, runoff and loss of nitrogen into the air. Capturing this nitrogen for the next crop can make incorporation cost-effective. More nitrogen would be retained if manure was applied onto cover crops or after temperatures dip below 50F. Granted, it isn’t always prudent to wait for cooler weather to haul manure. 

Plant available nitrogen is highly dependent on application rates, method of application, timing and type of manure. The chart below shows a “typical” dairy manure test from the milking herd, and describes the difference in nitrogen value from surface and injected applications.

Ammonium (NH4-N)
•  The ammonium form of N in manure is readily available to plants.
•  Most of the ammonium will volatilize when surface applied during hot, dry conditions (e.g., on wheat stubble in late summer).
•  Under cool, damp soil conditions (such as spring and late fall) more of the ammonium-N is retained in the soil. 
•  Incorporation into soil within hours or injection greatly improves the retention of the ammonium fraction, as will a light rain after applications.
•  Manure injected in the spring should retain almost 100% of the ammonium in the manure. 
•  Add up both the estimated ammonium fraction of manure with the amount of mineralized nitrogen for a total nitrogen credit to the current crop. 
•  A pre-side dress nitrate test (PSNT) is the best method to measure the total nitrogen from manure applications. It will measure both the ammonium and organic fraction from manures as they are converted to nitrates in the spring.
•  Fall stalk tests can be an indicator of over or insufficient nitrogen as a double check on manure and nitrogen rates on corn.

Try combining manure and inorganic fertilizer applications so that the future crop’s needs are met and not exceeded. Reducing or eliminating commercial fertilizer use on any fields where soil tests indicate that these nutrients are not needed, or where manure has already been applied to supply the necessary nutrient amounts, reduces the risk of nutrient loss. Matching the combination of commercial fertilizer and manure will save money, without yield loss. To gain the greatest value from manure nutrients, it must be applied to fields that need the phosphorus and potassium.

If surface applying manures, select fields with no adjacent surface waters when possible. When there are surface waters nearby, assess slopes and employ management practices to decrease the risks of runoff, such as:
•  incorporating manure within 24 hours when possible;
•  basing application rates on the ability of the soil to accept and store; water and the ability of plants to utilize nutrients;
•  observing a spreading setback from surface waters and from drain inlets; surface application of 150 ft is recommended;
•  knowing the slopes in fields, and installing conservation practices to control runoff and erosion;
•  planting permanent vegetative buffers by creeks and ditches;
•  monitoring the outlets in tile-drained fields prior, during and after manure applications;
•  applying manure uniformly and at known rates;
•  leaving as much previous crop residue as possible and managing tillage to leave the field surface rough;
•  planting crop rows perpendicular to slopes;
•  planting cover crops ahead of planned manure applications;
•  avoiding manure application during rain or when rain is impending; and,
•  combining several of the practices listed here.

Winter-time spreading increases the risks of runoff during melting and rain events. Field-by-field assessments should be conducted on all fields to select those most appropriate for winter spreading. Liquid manure may be applied in winter on fields with slopes of 3% or less. Solid manures may be applied on fields with slopes of 6% or less. Slopes above 6% should not be surface applied during frozen and snow- covered seasons.

Managing Manure Nutrients for Profitable Crop Production
•  apply manure to lowest-testing fields for greatest benefit from all nutrients in the manure;
•  soil test every 3 years, in less than 20-acre increments;
•  take manure samples annually or frequently enough to monitor change over time;
•  adjust application rates of manure to supply crop needs and water holding capacity of soils;
•  calibrate manure spreaders to apply the desired rate and to monitor accuracy;
•  train farm employees and family members to maintain desired manure nutrient rates are consistently applied to the fields; and,
•  spread manure evenly and consistently, treat it like the fertilizer it is. Do not apply manure in a manner that creates ponding or soil erosion.

Finally, apply, monitor, evaluate and RECORD manure applications.

See www.animalagteam.msu.edu for more information on manure nutrients.

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