Selecting forages for nutrient removal from animal manure
Hailin Zhang, Waste Nutrient Management Specialist, Oklahoma State University and Larry A. Redmon, Extension Forage Specialist, Texas A&M University
The production of animal wastes and their subsequent use as fertilizers and soil amendments will increase as confined livestock producers expand their market in the meat industry. Animal waste products contain valuable nutrients that are required by forage crops. These nutrients, primarily nitrogen (N), phosphorus (P) and potassium (K), can accumulate on sites with continued application. Forages managed as hay crops can be utilized to remove these nutrients via plant uptake from sites that regularly receive poultry litter, swine lagoon effluent, and dairy and feedlot manure applications. This [article] discusses the use of forages for nutrient removal from areas subject to manure applications.
Management strategies
Animal manure applications should be managed to prevent excess nutrient loading of sites from repeated applications. The preferred management strategy is to not apply more nutrients than can be effectively used by the forage system. Under most circumstances the critical nutrient to monitor will be phosphorus (P).
Phosphorus is required in lower quantities by forage plants than either nitrogen (N) or potassium (K); thus, P is removed in the least quantity by the forage system. If possible, animal manure should only be applied to meet the P requirement of the forage system. Additional N or K should be applied as commercial fertilizer.
If animal manure is applied to meet the N requirement of the forage crop, excess P will accumulate. A long-term research experiment conducted at the Oklahoma State University Agricultural Experiment Station at Lahoma documents the effect of soil-P depletion and enrichment from 27 years of annually applying zero to 80 pounds per acre fertilizer P for winter wheat production.
The soil test P index is well correlated with net P input (P applied minus P removed). About 14 pounds P2O5 per acre is required to raise or lower the soil test P by one unit for this silt loam soil. Generally, it requires 10 to 20 pounds P2O5 per acre to raise soil test P by one for a wide variety of soils. Routine analyses of animal manure will provide information regarding the level of nutrient inputs the system is receiving. An annual soil test of the application site will indicate the soil phosphorus status and whether an accumulation is taking place.
An important management strategy on sites where nutrient loading is an existing or potential problem is to use a forage crop(s) that removes a maximum level of nutrients, especially the nutrients of concern. Following are a couple of ways to accomplish this.
The first method involves using a forage crop that has a high dry matter yield potential. As dry matter yield increases, the amount of nutrients taken up by the crop increases and more nutrients can be removed. Bermudagrass is an example of a forage crop that has a high yield characteristic.
Another forage-use strategy involves using forages that have higher nutrient uptake for specific nutrients even though dry matter yield may be less than some other forage crops. Orchardgrass, for example, does not yield as much dry matter as bermudagrass, but its uptake of P is almost twice that of bermudagrass. Concentrations of nutrients in different species can vary depending on plant available nutrients in the soil, yield and maturity of the forage when cut.
Alfalfa has both a high yield potential, if given adequate moisture, and a higher P and K requirement than that of bermudagrass. Although a legume is not dependent on applied N, alfalfa receiving N, P and K as swine lagoon effluent has the potential to remove a large quantity of applied nutrients.
The most efficient nutrient removal forage system will use a combination of both warm- and cool-season forage species to enhance nutrient removal on a year-round basis.
It should be noted that nutrients are not effectively removed from the forage system by grazing livestock. Nutrient removal is accomplished only by removing forage as a hay crop and transporting the nutrients away from the application site. Grazing, for the most part, recycles most of the nutrients back into the forage system. Continued use under grazed-only systems will lead to increased nutrient accumulation at that site.
Efficiency of nutrient removal using forage
Some fields have already built up high P levels in the soil due to continuous application. New regulations set a soil test P limit for additional application. For example, under current Natural Resource Conservation Service nutrient management guidelines, no animal manure can be land-applied if soil test P is over 400 in Oklahoma. Therefore, some efforts have been made to reduce P in those fields with high-yielding forage crops and legumes. However, it may take a long time to see a significant soil P reduction. It is a slow process to use forages for lowering the soil phosphorus level.
Summary
Animal manure contains valuable nutrients and organic matter that can and should be used as a source of plant nutrients and soil amendments in forage production systems. However, continued applications of critical nutrients such as P, in excess of the level that can be effectively removed by the forage system as a hay crop, can lead to an accumulation of nutrients. Additionally, on sandy sites, relatively mobile nutrients, such as N, can be leached through the soil profile into groundwater in the underlying aquifers, thus posing a potential environmental hazard.
Forages with a high yield potential can be used effectively to remove nutrients from application sites, if application rates are kept within reasonable extents. Bermudagrass and certain warm-season annual grasses have the potential to produce large dry matter yields and thus remove large quantities of applied nutrients. Cool-season grasses and certain legumes have a higher uptake of certain nutrients (such as P) and may remove more specific nutrients than higher-yielding forage species, although the yield potential is not as high.
Producers should seek to use a combination of both warm- and cool-season forages in a production system that attempts to remove a maximum level of nutrients from sites that regularly receive poultry litter or types of animal manure applications. However, it is a slow process to use forages for lowering the soil phosphorus level. Grazed-only systems will not effectively remove nutrients from an application site since most of the applied nutrients, especially P and K, are recycled to the land during the grazing process. ANM
References omitted but are available upon request at editor@progressivedairy.com
—Excerpts from Oklahoma State University Extension website
