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Are “As Excreted Values” Valid in Phosphorus Budgets for Grazing Beef Cattle?
Pete Deal, Rangeland Management Specialist, USDA Natural Resources Conservation Service, Kissimmee, FL
John J. White, Soil and Water Technician, Osceola Soil and Water Conservation District , Kissimmee, FL
Randy Bateman, Agricultural Extension Agent, Osceola County Extension Service , Kissimmee, FL
ABSTRACT:
RESULTS
Nutrient budgets are used to support regulations aimed at reducing phosphorus leaving agricultural lands Nutrient budgets
based on “As Excreted Values” are not reliable in grazing situations where animals can select a diet composed of varied
ingredients and quality.
Traditional nutrient budgets for livestock rely on “As Excreted Values” to estimate the amount of phosphorus in manure.
These values have often been developed using data gathered from feeding operations. Values developed from feeding
operations do not accurately account for the varied dietary components selected by grazing animals. Consequently, the
results from the nutrient budget do not accurately represent the amount of phosphorus removed or retained on the
landscape.
A sampling program was conducted to estimate the amount of phosphorus excreted by grazing animals. forage quality
samples from three herds of commercial beef cattle were sampled periodically in 2007 and 2008. The amount of phosphorus
excreted was estimated using the Near Infrared Spectroscopy (NIRS) procedure and NUTBAL Pro software developed by Dr.
Jerry Stuth and his team at the Grazing Animal Nutrition Lab (GAN Lab) at Texas A&M University.
Project results indicate that the actual amount of phosphorus deposited is significantly lower than estimates based on “As
Excreted Values”. The results also indicate that actual deposition is highly variable depending upon growing conditions and
growth stage of the forage plants.
Therefore, “As Excreted Values” should not be used to develop nutrient budgets for grazing animals. Import-export nutrient
budgets that do not rely on "As Excreted Values" should be used in grazing situations.
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Comparison of Phosphorus Excretion and Intake for Grazing Cattle
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Dietary information was obtained from the University of Florida Institute for Food and Agricultural Sciences (IFAS).
Six diets were compared using a variety of feed ingredients.
Phosphorus content of the diets ranged from very high (30.91 lb/year) to very low (18.03 lb/year).
OBSERVATIONS:
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Estimates of Phosphorus excretion using "As Excreted Values" are 71% to 91% higher than estimates of dietary
Phosphorus intake.
Estimates of Phosphorus excretion using "As Excreted Values" are 123% to 149% higher than estimates of Phosphorus
excretion using the NIRS procedure.
The amount of Phosphorus excretion estimated using the NIRS procedure were 23% lower than the typical dietary intake
of Phosphorus.
The body condition score (BCS) of the animals sampled ranged from a BCS of 4 to 6. This indicates that the animals may
not be able to consume 26 lb./day of Bahiagrass.
Phosphorus excretion varied as forage conditions and stage of plant growth changed.
The source of the Phosphorus cannot be identified using “As Excreted Values”. Therefore it is not possible to
differentiate whether the source of Phosphorus was anthropogenic (feed, fertilizer, etc) or natural (soil organic matter,
inherent soil reserves, soil water table, etc.)
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Phosphorus excretion was estimated using the Near Infrared Spectroscopy (NIRS) procedure.
Results from the forage analysis were compared to “As Excreted Values” obtained from the 1992 and 2008 versions of the
Agricultural Waste Management Field Handbook. This publication was produced by the USDA Natural Resources
Conservation Service (NRCS).
Results were also compared to the estimated dietary intake of phosphorus for a typical 1,000 lb. beef cow in Florida.
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Site is managed primarily for grazing.
Commercial fertilizers products are used to apply nitrogen and phosphorus at the medium rate for Bahiagrass.
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Ranch 3 - Medium Phosphorus Regime:
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Site is managed for sod production and grazing.
A variety of commercial fertilizer products are applied at high nitrogen rates.
Phosphorus application rates typically ranged from medium to high for Bahiagrass.
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Ranch 2 - Medium to High Phosphorus Regime:
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Site is managed for grazing and sod production.
Municipal Wastewater Residuals were applied at nitrogen rates every 3rd year.
Phosphorus application rates exceeded published recommendations for Bahiagrass pasture.
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Ranch 1 - High Phosphorus Regime:
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Very High P (VHP) diet- (Dried Distillers Grains, Mineral and Bahiagrass) = 30.91 lb./year
High P (HP) diet (Fortified Molasses, Mineral and Bahiagrass) …………… = 27.72 lb./year
Medium P (MP) diet (Citrus Pulp, Mineral, and Bahiagrass) ……………..… = 23.22 lb./year
Typical (T) diet (Blackstrap Molasses, Mineral and Bahiagrass) …………… = 22.98 lb./year
Low P (LP) diet (Mineral and Bahiagrass) ………………………………….… = 22.19 lb./year
Very Low P (VLP) diet (Bahiagrass only) …………………………………….. = 18.03 lb./year
Phosphorus (lbs)
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Estimated Phosphorus Intake
13 samples were collected from each ranch.
Samples were collected at various times throughout the period.
All ranches were sampled on the same day.
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(Fecal Output (lbs) = Dry Matter Intake (lbs) X (1 – % Digestible Organic Matter (DOM) / 100))
Average DOM = 62.83% Standard Deviation = 2.29% ,DOM values ranged from 57.83% to 67.40%
Dry Matter intake was assumed to be 26 lbs. per day per animal
Three different fertility management regimes were represented in the sampling project.
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Fecal output was estimated using the formula
Thirty-nine (39) forage quality samples were collected from 3 central Florida ranches in 2007 and 2008.
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Ag. Waste Management Field Handbook, 1992 (AWMFH-1992) ……......…
= 43.8 lb./year
Ag. Waste Management Field Handbook, 2008 (AWMFH-2008) (Table 4-8) = 39.2 lb./year
NIRS Forage Analysis Average ……………………………………………......... = 17.6 lb./year
• Ranch 1 ………………………………………………………………………. = 18.6 lb. /year
• Ranch 2 ………………………………………………………………………. = 15.9 lb./year
• Ranch 3 ………………………………………………………………………. = 17.1 lb./year
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PROJECT:
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Predicted Phosphorus Excretion:
Excretion
Intake-Forage
Intake-Mineral
Intake_Suplement
CONCLUSIONS:
1. “As Excreted Values” published in the NRCS Agricultural Waste Management Field Handbook over estimate
the actual Phosphorus intake of grazing cattle.
2. Animals cannot excrete more Phosphorus than they ingest. Therefore, NRCS “As Excreted Values” are not
applicable to grazed beef cattle in central Florida.
3. Estimates of Phosphorus excretion developed using the NIRS forage quality analysis of forage are lower than
the estimates of Phosphorus intake.
4. Phosphorus retained in the tissue of the grazing animals and their offspring may account for three (3) to five
(5) pounds per year.
5. The NIRS procedure may be a valid method to estimate “As Excreted Values” when the Phosphorus retained
in animal tissue produced in a given year is added to the estimates of Phosphorus in the fecal out put.
6. Phosphorus intake by cattle may be reduced because of the limited availability of Bahiagrass forage.
7. The use “As Excreted Values” to develop nutrient budgets on grazing land will over estimate the amount of
nutrients imported onto a site because the actual source of the nutrients is not identified.
8. "As Excreted Values" should not be used to develop nutrient budgets for grazing beef cattle!
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