Manure application rates for forage production

This simple N balance equation is most appropriate for fields that have routinely received manure for three to five or more years. This balance, sometimes called an agronomic rate approach, is an oversimplification of the real processes that govern N dynamics in the field. Yet, it is the most common approach used in N-based management plans.

A more detailed look at N dynamics is provided in Estimating plant-available N from manure, EM 8954, and Baseline soil nitrogen mineralization: Measurement and interpretation, EM 9281.

When N inputs exceed outputs, you can achieve N balance by reducing manure application rate or by improving management to increase crop yields and N removal.

This publication does not address nutrient balance in pastures that are primarily harvested by grazing animals. Consult Nutrient management for pastures: Western Oregon and Washington, EM 9224, for pasture guidance.

Forage crops in Western Oregon

This section provides information to guide your choice of “crop” (a row in Tables 1 or 2) for a nutrient management plan. Each crop has inherent characteristics, such as longevity and seasons of growth. For some crops (such as perennial grass), expected crop nutrient removal by harvest differs according to the intensity of management and harvest practices.

Red clover. Red clover is a short-lived perennial that is usually harvested several times a year and made into silage or planted as part of a mix for silage. Often, forages for silage will be planted in the fall and be highly productive for several years before the stand thins over time.

Alfalfa hay. Alfalfa is a perennial is harvested multiple times throughout the growing season. The example in Tables 1 and 2 is alfalfa hay with three harvests per year.

Perennial grass. Three examples of perennial grass management are presented below: low-, medium- and high-intensity systems. Monitoring tonnage, forage dry matter and crude protein will help you define where your fields are along this continuum.

• Low input, low intensity. Example: Native pasture that is not routinely fertilized and is harvested for hay once a year in mid-June, producing 3 tons of dry matter per acre with 10% crude protein in dry matter. Estimated crop N removal is 32 lb N per ton DM or 96 lb N per acre for the year.
• Medium management intensity. Increased management and nutrient inputs can transform a low-intensity pasture to one with increased forage production and nutrient removal. Example: Field is fertilized in March, cut for silage in mid-April (2 tons of dry matter) and cut again at the end of June for hay (2 tons of dry matter). Estimated biomass production is 4 tons of dry matter at 15% crude protein. Estimated crop N removal is 48 lb per ton of dry matter, totaling 192 lb N/acre for the year.
• High management intensity. Further intensification of production factors can increase crop yield and nutrient removal. Example: Improved grass species, active monitoring and management of nutrient inputs, timely and efficient irrigation, and a harvesting schedule that includes four or five cuttings per year. Estimated biomass production is 1.5 tons of dry matter per cutting at 18% crude protein. Estimated crop N removal is 58 lb N per ton DM. With four harvests per year, estimated crop N removal is 348 lb N/acre for the year.

Annual ryegrass. Annual ryegrass is usually seeded in the fall as a winter cover crop. It is harvested the following spring, using a one- or two-cut system. The first harvest usually takes place in March or April. If the first harvest occurs in March, then another harvest may be feasible in May. If the field cannot be harvested until April, then only one harvest is usually feasible. The two-cut system works best when grass is seeded early in fall and is well-established prior to winter.

Corn silage. Tables 1 and 2 list the planning value for nutrient removal by silage corn presented in Silage corn nutrient management guide for Western Oregon, EM 8978.

Small grains. Fall-seeded cereals are seeded in fall and harvested the following spring at boot or soft dough growth stage. When harvested at boot stage, cereals can regrow and produce a smaller second cutting. A two-cut system works best when cereals are seeded early in fall and growing conditions in spring are favorable for rapid vegetative growth.

Crop nutrient removal

Tables 1 and 2 estimate aboveground N, P and K removed per ton of harvested forage for crops grown on dairies in Western Oregon. Nutrient values in Table 1 are presented on a 100% DM basis. Table 2 expresses nutrient values on a “wet” or “as-harvested” basis. For example, when forage DM is 25%, one ton of forage dry matter equals 4 tons of “as-harvested” forage.

Data presented in Tables 1 and 2 demonstrate that N and K removal by harvest is of the same magnitude for many forages, while P removal is much lower. Crop removal values in Tables 1 and 2 represent usual management scenarios for Western Oregon. Crop nutrient removal is maximized by good management practices, including:

Keep in mind that crop nutrient removal values in Tables 1 and 2 are provided as an initial planning tool. On established dairies, field-by-field monitoring of forage yield and protein content is the best way to obtain accurate values for crop N removal.

Nutrient analysis of manure

Nutrient analysis of manure is critical to determining an appropriate application rate. Nutrient management plans for dairies often specify manure testing frequency and which nutrient analyses are required. At a minimum, we recommend testing for total nitrogen (N), ammonium nitrogen (NH4-N), phosphorus (P), potassium (K) and dry matter (DM). Laboratories often perform these as a standard analysis package. See Sampling dairy manure and compost for nutrient analysis, PNW 673, for details on manure sampling and analysis protocols. Many dairies have more than one form of manure that is applied to fields. Adding bedding or water to raw manure dilutes the manure and decreases manure nutrient concentration. We recommend separate analyses for each manure type (such as lagoon water or manure from a reception pit).

Lagoon water and slurry manure application rates

Table 3 shows how much liquid or slurry manure is required to meet a given target for N application rate. For simplicity, the liquid or slurry application rates in Table 3 assume that all the N in lagoon water or slurry is readily available for plant uptake. See Estimating plant-available nitrogen from manure, EM 8954, for more accurate plant-available N estimates for different management scenarios. Table 3 shows that it is difficult to apply low N rates (less than 50 lb N/acre) of liquid or slurry manure, because those application volumes would be small. The lower application limit for most travelling guns is approximately 0.25–0.30 inches. Slurry manure can be applied at rates ranging from 7,000 to 30,000 gallons per acre. See Calculating manure nutrient application rates, EM 8768, for additional calibration instructions for manure application equipment.

Monitoring P and K balance via soil testing

When N inputs from manure and N outputs in harvested forages are balanced, manure almost always supplies more P and K than is removed by harvest (Table 1). Agronomic soil tests for P and K reflect the balance between manure nutrient inputs and crop removal. Consult Nutrient management for pastures: Western Oregon and Western Washington, and Nutrient management for silage corn in Western Oregon for more information on soil sampling, appropriate laboratory analyses and soil test interpretations. Your nutrient management plan may also prescribe soil sampling depth, soil testing frequency and required laboratory analyses. The Phosphorus Index for Western Oregon estimates risk of off-site P loss based on a soil sample collected from 0- to 12-inch depth.

Elevated concentrations of K in dry cow rations can lead to problems with milk fever at freshening. In most situations, forages for dry cows should contain less than 2% K on a DM basis. As soil test values increase above 200 ppm K, forage K concentrations increase, but forage yields do not. This is called “luxury consumption” and is a characteristic of plants taking up additional K beyond that needed to produce maximum forage yields.

Maintaining low K concentrations in forage is often difficult when N is supplied by manure, and the farm imports K in feeds and mineral supplements. Potassium is not lost from manure as a gas, and it is usually not lost from the soil profile by leaching. As a consequence, much of the K that is imported in feeds will accumulate in soil over time. To reduce K concentrations in forage, we recommend monitoring the quantity of K imports in feedstuffs, especially in mineral mixtures, and reducing K imports in feed when possible.

Monitoring of soil test K and forage K can also identify fields with suitable forage for dry cows. Often hayfields away from the barn that do not receive regular manure application have the lowest K concentration in forage.

For more information

Monitoring and recordkeeping

Date, Rate, & Place: The Field Book for Dairy Manure Applicators. 2017.

End-of-Season Corn Stalk Nitrate-Nitrogen Test for Post-Harvest Evaluation. 2019. Washington State University Extension Factsheet FS336E.

End-of-Season Corn Stalk Nitrate-Nitrogen Test for Post-Harvest Evaluation—A Case Study. 2019. WSU Extension Factsheet TB66E.

Nutrient management planning

National Research Council. 2001. Nutrient Requirements of Dairy Cattle: Seventh Revised Edition. The National Academies Press. Washington, DC.

Natural Resources Conservation Service (NRCS). 2013. Phosphorus Index worksheet Western Oregon. In: Appendix 1, Agronomy Technical Note 26.

Natural Resources Conservation Service (NRCS). 2020. Conservation Practice Standard Code 590. Nutrient management. In: NRCS Field Office Technical Guide.

Oregon Department of Agriculture. 2016. Animal Waste Management Plan Implementation and Compliance. Section S3.A (p.17). In: State of Oregon Confined Animal Feeding Operation Permit Program CAFO NPDES General Permit #01 and Evaluation Report and Fact Sheet.