Nutrient Management Using a Phosphorus
Risk Index for Manure Application Fields

Anne McFarland, Larry Hauck
Texas Institute for Applied Environmental Research (TIAER), Tarleton State University

As part of a voluntary demonstration project to decrease phosphorus (P) runoff into the North Bosque River, the NRCS and TIAER are working with dairy farmers in the Goose Branch micro-watershed to identify and implement phosphorus control practices for manure application. The Goose Branch micro-watershed (3,500 acres) is located in the headwaters of the North Bosque River within Erath County, the number one milk-producing county in Texas. Nutrient enrichment of the North Bosque River is a concern, particularly in regard to high phosphorus concentrations and loadings leading to the eutrophication of downstream waterbodies.

To aid in decreasing phosphorus runoff from manure application fields, a Phosphorus Risk Index was developed specifically for manure management on dairy operations in this region. While developed regionally, the P Risk Index is quite adaptable to other types of livestock and regions and is based on work by Lemunyon and Gilbert (1993). The index is being used to help in manure management planning on seven dairies within the Goose Branch micro-watershed which involves the assessment of 112 individual fields. Continued field assessments on an annual basis, as well as in-stream monitoring in Goose Branch, are planned for the next 4 years allowing an interactive evaluation of phosphorus control practices with the P Risk Index.

The objectives of the Phosphorus Risk Index are to:

1. Be systematic and consistent in evaluating the risk of P movement from fields,
2. Identify "sensitive spots" or areas with a high potential for phosphorus runoff,
3. Determine the cause of unacceptable P risk in identified fields to help in the selection of ways to lower that risk, and
4. Work as an interactive educational tool for field managers to measure the change in risk over time with changes in management practices.

The Phosphorus Risk Index uses nine parameters to obtain an overall rating for a field. Each parameter is assigned an interpretive rating along with a numerical value to identify the associated risk category for phosphorus runoff as follows: Very Low (1), Low (2), Medium (4), High (8) and Very High (16). A base-2 risk rating provides a distinct separation of the risk categories.

The nine indexing parameters were also weighted to reflect the relative importance of each parameter in assessing the risk of phosphorus runoff. For example, Soil Test P (weighting factor=2.5) was considered more important in assessing the potential for P runoff than the P Application Method (weighting factor=1.0). The weighting factors presented were based on the professional judgment of the scientists and managers involved in the Goose Branch demonstration project. Weighting factors as well as the parameters used in the P Risk Index should be adjusted to consider conditions and practices specific to other locations. The nine indexing parameters and associated weighting factors used in the Goose Branch watershed are listed below:

A brief description of each parameter and suggested guidelines for determining appropriate risk categories within each parameter are discussed below.

Phosphorus Application Rate considered the total amount of P₂O₅ in manure, effluent and/or commercial fertilizer applied to a field in a given year. The application rate was compared to generalized plant uptake needs for commonly grown crops in the area. About 140 lbs P₂O₅/acre/crop-yr is considered a maximum application rate for meeting crop needs given ideal growing conditions, while a range of 40 to 60 lbs/acre/yr P₂O₅ is commonly recommended given normal growing conditions. Phosphorus application rate was considered in lbs P₂O₅/acre/crop-yr and placed in the following rating categories: 0-20 (Very Low), 20-60 (Low), 60-100 (Medium), 100-140 (High) and > 140 (Very High).

Soil Test P considered the extractable P concentration for a 0-6 inch soil sample based on Texas Concentrated Animal Feeding Operations (CAFO) regulations. Similar to the rating categories set for P Application Rate, Soil Test P rating categories were based on plant uptake needs, although the division between High and Very High was based on Texas CAFO regulations which indicate that manure may be applied only at a P plant uptake rate rather than a nitrogen plant uptake rate if the soil test for a field exceeds 200 ppm extractable P. Soil Test P rating categories were set as follows: <20 (Very Low), 20-60 (Low), 60-100 (Medium), 100-200 (High), >200 (Very High).

Distance to Waterbody considered the distance from the edge of a field to the nearest waterbody. This distance was set into rating categories placing fields further away from a waterbody at lower risk of impacting a water source than fields close to or adjacent to a waterbody. Rating categories were determined using the following: >1,000 ft (Very Low), 500-1,000 ft (Low), 200-500 ft (Medium), 30-200 ft (High) and < 30 ft (Very High).

Erosion Rate considered the risk of movement of sediment bound P in runoff using RUSLE (tons/acre/yr) based on current field crops and management practices. Field information needed to determine the erosion rate includes dominant soil type, soil hydrologic group, slope length, slope and information on terracing and gullies. While nation-wide most P arrives in stream systems via sediment bound P, erosion rate was given a weighting factor of only 1 for the Goose Branch watershed, because soluble P is currently a larger concern in Goose Branch than sediment bound P (McFarland and Hauck, 1997). Erosion Rate in tons/acre/yr was rated as follows: < 1 (Very Low), 1-3 (Low), 3-5 (Medium), 5-15 (High) and > 15 (Very High).

Runoff Potential considered the Runoff Class as estimated from the USDA-NRCS curve number and percent slope of each field (USDA-SCS, 1994). Information needed includes land use, type of cover, condition class and hydrologic soil group. Runoff Potential was considered one of the key parameters for indicating the potential for P leaving a field and entering into a waterbody, because most soluble and sediment bound P in this area is expected to move via rainfall runoff. Wind erosion was not considered but may be a very important parameter in other regions.

Phosphorus Application Method considered the risk for P movement based on how organic and/or inorganic P fertilizer was applied to a field. In the Goose Branch watershed, most organic and inorganic fertilizer is surface applied on coastal bermudagrass fields. Surface application of P without incorporation was considered to have a High P Risk Rating as opposed to incorporation of P which was given a Low P Risk Rating. The type of irrigation method used for lagoon effluent was also rated based on the potential risk of P runoff. A center pivot system was considered a Low P Risk, while a stationery big gun was considered a High P Risk. A center pivot typically distributes effluent more evenly across the field than a big gun system that requires more operator attention to avoid over-application and application induced runoff. Other types of application systems were similarly rated.

Application Timing considered the rainfall intensity associated with each month in which organic or inorganic P was applied using 30-minute rainfall intensity data for 30 years of record at Stephenville, Texas. The Application Timing parameter assumes that the risk for P movement is more likely if P is applied during periods in which high intensity rainfall is expected. For Stephenville, the highest rainfall intensity generally occurs in May and June, so application during these months is given a Very High rating. At many sites in the Goose Branch, manure was applied year-round on an "as need" basis by the operator. Year-round application was placed in a High P Risk rating category, since manure may be applied during Very Low and Very High risk periods.

Vegetation Management considered land use and harvest method as indicators of the amount of crop residue and cover left on a field during the non-growing season to protect the soil surface from P movement. In general, if a large amount of vegetation was left on a field, the field was given a Low P Risk Rating, but if a field produced only an annual crop and did not have a cover crop in the winter, the field was given a High P Risk Rating. Grazing intensity was also used as an indicator of percent cover left on a field. Other conditions, such as rangeland vegetative cover, were assessed on a field-by-field basis using professional judgment.

Grazing Management considered the manure added to a field from grazing animals as a function of supplemental feeding and grazing pressure separate from the impact of grazing on residual vegetation and cover. While most dairy operations in the Goose Branch micro-watershed are confinement operations, one operator grazes his milking herd and several of the operators graze their dry cows and heifers. A field that was not grazed or only occasional grazed, such as clean-up grazing on a winter wheat field after harvest, was given a Very Low P Risk Rating. The other extreme, a Very High P Risk, was considered a pasture in which 80 to 100 percent of dry matter intake was provided as supplemental feed. Proper grazing use (PGU) with 30 to 80 percent of feed supplemented was considered a Medium Risk, while PGU with less than 30 percent of feed supplemented was considered a Low Risk.

To make a field assessment using the P Risk Index, each parameter rating value was multiplied by its suggested weighting factor. These values were then summed across all parameters for an overall P Risk Value to determine the vulnerability of a site for P movement into nearby waterbodies. Total rating scores for Goose Branch varied from 15 to 248 and were placed in overall risk categories as follows: 15-23 (Very Low), 22-46 (Low), 47-93 (Medium), 94-186 (High) and 186-248 (Very High). These overall risk ratings were used to identify fields where P management was most needed. Management plans were then developed in cooperation with dairy owners to minimize the risk for P movement from their fields and to consider P in developing sustainable manure waste management plans as part of the long-term goals for each dairy. Many of the recommended changes in management involved more intensive soil and manure testing for nutrient management and the opening of new land for waste application. A common finding was that fields nearest the confined dairy area and lagoons were more likely to be at higher P risk than other fields due to the build-up of P in the soil from repeated manure or effluent applications. Filter strips and/or buffer zones were also widely recommended as the Goose Branch micro-watershed's dendritic stream pattern boarders many fields.

Acknowledgments

The authors wish to acknowledge the many people involved in this project, particularly the land owners within the Goose Branch watershed. Without the voluntary cooperation of these landowners, this project would not have been possible. The project is funded by the USDA-NRCS and involves cooperating efforts from the Texas State Soil & Water Conservation Board, Cross Timbers Soil & Water Conservation District, Texas Agricultural Experiment Station, Texas Agricultural Extension Service, USDA-NRCS and TIAER.

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