Start and end with soil test: Using manure on forage crops is a balancing act.


Farms with medium to large animal herds usually have substantial quantities of manure to utilize. Cropland has traditionally been used as the best place to dispose of manure. But the use of forage cropland may allow more versatility in providing a larger window of opportunity for manure application.

There are several important reasons to apply manure to forage crops. The application of manure back to the ground within the same farm allows for a recycling of the nutrients within that farm system.

Soil test first.

In order to decide which forage fields should receive the manure, it is important to start by testing the soil in each field. Knowledge of the nutrient levels allows for good manure management, which in turn provides better plant growth and water quality protection.

Fields that are low in phosphorus (P) and potassium (K) should be chosen whenever possible. A soil test report will not only tell you the levels of these nutrients, but, in conjunction with a manure analysis, usually will provide recommendations on the rate of manure to apply.

To get accurate recommendations, it is important to make a good estimate of the yield goal. This is especially important when it comes to the use of nitrogen (N) by the crop and the corresponding availability of N from the manure.

The rate of manure may also depend on the age of the crop and on the crop that will follow in the rotation.

The soil test report will also determine the pH of the soil. It is important to maintain the soil pH between 6.0 and to 7.0 for most forage crops.

Other factors that will govern the manure application rate are the steepness of slope, nearness to water streams or ponds, and the method of application.

Nitrogen availability.

Under Ohio conditions, it is thought that about one-third of the total N becomes available to the crop during the first growing season.

It is expected that all the P and K will be available to the plant over the first growing season. These nutrients are more slowly released than from synthetic fertilizer and the rate of release depends on the kinds and populations of organisms present. These organisms are very dependent on optimum temperature and moisture conditions.

Not recommended on legumes.

The application of manure to legume forage crops usually is not recommended. However, in certain cases it may be necessary if other cropland is not available.

Even though the legume plant is in symbiotic relationship with specific microorganisms and creates its own N, legumes will take up nitrogen if it is present in the soil. In situations where the seeds were not inoculated with appropriate bacteria at planting or where the formation of nodules on legume roots has been restricted for some reason, it is beneficial to add a source of N.

It is important to understand that the addition of manure to legume crops usually promotes weed and grass growth.

It is often beneficial to apply manure to an old legume field, before it is plowed under.

Manure will also supply substantial quantities of sulfur. Legume plants have higher sulfur requirements than grass plants.

Grasses need nitrogen.

Soils supporting grass forages are ideal for manure applications. Grasses, since they have a high N requirement, and readily make use of the N released from the manure.

Also, grasses will take up substantial quantities of P and K from the manure. The recommended manure rates for grasses are usually based on the N in the manure and the expected yield of the plant.

Stay balanced.

It is important to remember that manure is low in magnesium (Mg) compared to K. Under high concentrations of K in the soil, the plant will preferentially take up more K than Mg, thus leading to possible grass tetany problems when the grass is fed to ruminant animals.


The method of manure application usually depends on the stage of the legume or grass crop. Prior to establishment, the manure can be incorporated into the soil. This is the best way to conserve the ammonia-N that may be in the manure.

In addition, it allows for more rapid breakdown of the manure and a more efficient release of the other nutrients.

It is best to apply the manure three to four weeks before planting and not to apply more than an equivalent of 300 pounds of K20 per acre. Amounts greater than this can lead to salt problems, especially in droughty times.

Try to avoid soil compaction by not applying the manure when the field is wet.

On established grass forage lands and even on legume forage lands, the manure can be applied as a topdress. In the case of established grasslands, it is best to apply in cool weather to minimize the volatilization of ammonia-N.

To avoid manure run-off into water bodies, it is best not to topdress manure on frozen ground.

A good time to top-dress manure is a few days after a cutting has been removed. However, always be careful not to smother the small plants with too much manure.

Soil testing, manure testing, and plant analysis should be employed regularly to manage manure applications properly. When using these tools, always take representative samples and follow the necessary instructions concerning the particular test.

Test the soil at least every three years. Test the manure frequently at first to establish the variation in the test parameters. Also, be sure to test if the diet or manure storage system is changed. It is best to monitor the forage crop with plant analyses every year. Don’t forget to retain the test records for each crop in each field and review the test results periodically.

(The author is fertility specialist and researcher at the Ohio Agricultural Research and Development Center.)


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