R.K. Koelsch, University of Nebraska and Wendy Powers, Iowa State University
Introduction
Environmental planning in animal production systems often requires an estimate of nutrient excretion.
Standard values published by the Natural Resources Conservation Service (NRCS) (SCS 1992),
American Society of Agricultural Engineers (ASAE 1999), and MidWest Plan Service (MWPS 2000) commonly have been used for this purpose. However, these current procedures do not reflect the impact of producers' animal dietary decisions on nutrient excretion. The increasing variety of feed ingredient options, changes in nutritional programs to match improving genetic potential, and feeding strategies designed to reduce nutrient excretion are influencing the amount of nutrient excretion. Standard methods for estimating nutrient excretion need to reflect animal feed strategies and integrate feeding decisions into a Comprehensive Nutrient Management Plan (CNMP).
Variation in the nutrient excretion of animal diets is critical to CNMP processes. Erickson et al. (1998) reported a 44% reduction in phosphorus (P) excretion by beef cattle for a reduction in dietary P concentration from 0.35% to 0.22%. Van Horn (1991) estimated nitrogen (N) and P excretion to be 16% higher and 48% less than ASAE estimates for feeding programs based upon recommendations by the
National Research Council (NRC 1989). Morse et al. (1994) attributed these observed differences in dairy cattle to improved genetic potential, increased feed intake, and intensive management practices. The use of phytase and crystalline amino acids is well documented to cause significant reductions in N and P excretion in swine. Similar relationships between dietary intake of nutrients and nutrient excretion have been reported for poultry.
Alternative Model for Nutrient Excretion Estimates
Powers and Van Horn (2001) and other authors have proposed an alternative model that estimates nutrient excretion by subtracting nutrient retention from feed nutrient intake (see Figure 1). Estimates of feed nutrient intake are commonly available from animal producers by calculating the product of feed intake and feed nutrient concentration and then cross checking these values with feed inventories. Nutrient retention in the animal or animal products may be estimated by

A simplified approach proposed by Powers and Van Horn (2001). This method, included as an alternative in a recent MWPS-18 publication on Manure Characteristics, is illustrated in Table 1
(see attachment). The table provides a simple calculator to relate changes in feed nutrient concentration to the associated value or cost to the CNMP program.

National Research Council methods for estimating nutrient retention. NRC procedures provide a more complex and possibly more accurate method of estimating retention for species such as beef and swine. This estimate of retention is a peer-reviewed, industry-accepted methodology.
Feed nutrient intake
Nutrients retained by animals or animal products
Animal's nutrient excretion
Figure 1. Animal nutrient balance model for estimating nutrient excretion.
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Future CNMP programs must integrate animal feed programs into CNMP processes. That integration is critical to

Recognizing the impact of feeding decisions on land requirements for managing nutrients.

Crediting producer efforts to achieve regulatory water quality standards through modified dietary strategies designed to reduce nutrient excretion. The livestock and poultry industry can potentially practice source reduction similar to any other industry.

Convincing producers and feed consultants that decisions made at the feed bunk affect water quality in their rural community.
Below, we present an example of a 1,000-head beef cattle feedlot where cattle are fed a typical industry diet (13.3% protein and 0.31% P) and a range of diets representing the current industry low (12.5% protein and 0.25% P) and high (14.0% protein and 0.50% P) levels. A dietary-based estimate of N excretion is reasonably comparable with currently accepted references. The typically observed range of dietary protein levels results in only modest changes in land requirements (Table 2). However, currently accepted references overestimate P excretion by about twice the predicted estimate based upon dietary P level. In fact, current standards estimate that excretion exceeds feed intake of P, an obvious indicator of the inaccuracy of current standards. In addition, the current range of dietary P levels produces a maximum of a 240% difference in land requirement.
Table 2. Comparison of nutrient excretion and land requirement estimates based upon standard references and a proposed nutrient balance method for beef cattle.
Nitrogen Excretion & Land Requirement Phosphorus Excretion & Land Requirement
Lbs per day per
1,000 lbs
Lbs per finished animal
Land required for 1,000-head feedlot 1
Lbs per day per
1,000 lbs
Lbs per finished animal
Land required for 1,000-head feedlot 1
Excretion estimates by standard references
ASAE
NRCS
MWPS
0.34
0.30
0.49
51.0
45.0
73.5
390 ac
350 ac
570 ac
0.092
0.094
0.083
13.8
14.1
12.5
1,450 ac
1,480 ac
1,310 ac
Nutrient balance excretion estimate. Dietary nutrient concentration at an industry
Average
High
Low
0.37
0.39
0.34
54.7
58.2
50.9
420 ac
450 ac
390 ac
0.047
0.084
0.035
7.1
12.6
5.2
750 ac
1.320 ac
550 ac
1 Assumes nutrient requirement of 150 lbs of N and 22 lbs of P per acre and an availability of manure N and P of
50% and 100%, respectively.
Changing Current Standards
To improve the current procedures for estimating nutrient excretion, a joint ASAE/FASS 1 committee of agricultural engineers and animal scientists have been assembled to review and rewrite the current ASAE
Standard on Manure Production and Characteristics. Several NRCS representatives are actively involved in these discussions with the potential for revising NRCS standard estimates based upon ASAE changes.
This committee is currently addressing the following three topics:
1. As Excreted–Feed Intake Summary: Characteristics of excreted manure will be defined based upon a mass balance approach illustrated in Figure 1. Predictive equations for excretion estimates will be developed.
2. As Excreted–Average Summary: Existing average ASAE standard values of excreted manure will be redefined based upon estimates from Topic #1 for current average feed programs.
1 Federation of Animal Science Societies
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3. As Removed–Average Summary: An estimate of average as-removed manure characteristics for common animal housing and manure storage systems would be assembled, based in part on the MWPS-
18, Manure Characteristics handbook.
Approximately 30 individuals are involved in this manure excretion review process with the following seven work groups and individuals providing leadership for those workgroups:
As-Excreted–Feed Intake Summary for
Beef: Galen Erickson, University of Nebraska
Dairy: Deanne Meyer, University of California-Davis
Poultry: Todd Applegate, Purdue University
Swine: Phil Westerman, North Carolina State University
Equine: Laurie Lawrence, University of Kentucky
As Excreted–Average Summary: Charles Fulhage, University of Missouri
As Removed–Average Summary: Jeff Lorimor, Iowa State University
The authors of this article are providing overall leadership.
Summary
Opportunities exist to improve our estimates of nutrient excretion using an animal nutrient balance model.
The attached tool (see Table 1) provides one method of estimating nutrient excretion based upon current feeding practices and evaluating the impact of changes in the feed program on the overall nutrient management program. Such procedures will also be of value to producers adopting animal feeding programs designed to reduce nutrient excretion and achieve compliance with public policy goals for nutrient management planning. NRCS and ASAE standards for nutrient excretion will need to reflect these alternative procedures.
References
ASAE. 1999. Manure production and characteristics. ASAE Standard ASAE D384.1. In ASAE Standards
1999 . ASAE: St. Joseph, MI, 663-665.
Erickson, G., T. Klopfenstein, D. Walters, and G. Lesoing. 1998. Nutrient balance of nitrogen, organic matter, phosphorus and sulfur in the feedlot. In 1998 Beef Cattle Report . Agricultural Research
Division, University of Nebraska-Lincoln, 86-87.
Morse, D., R.A. Nordstedt, H.H. Head, and H.H. VanHorn. 1994. Production and characteristics of manure from lactating dairy cows in Florida. Transactions of the ASAE . 37(1): 275-279.
Lorimor, J., W. Powers, and A. Sutton. 2000. Manure Characteristics. MWPS-18 Manure Management
Series. Iowa State University, Ames.
NRC. 1989. Nutrient Requirements of Dairy Cattle. Sixth Revised Edition.
Powers, W.J. and H.H. Van Horn. 2001. Nutritional implications for manure nutrient management planning. Appl. Engng. Agric . 17(1): 27-39.
SCS. 1992 Agricultural Waste Management Field Handbook. Part 651. Published by US Department of
Agriculture, Soil Conservation Service.
Van Horn, H.H. 1991. Achieving environmental balance of nutrient flow through animal production systems. The Professional Animal Scientist . 7(3): 22-33.
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a.
Animal Group
Example: 1,000 beef finishers b.
Group Daily
Feed Intake
(lbs/day)
27,000 lbs
DM/day
Feed Nutrient Concentration (%) c.
Crude
Protein
13.5% d.
N 1
(c ÷ 6.25)
13.5 ÷ 6.25
= 2.16% e.
P
0.35%
Total Nutrient in Feed (lbs/day) f. g.
Daily N Fed Daily P Fed
(b x d ÷ 100)
27,000 x 2.16 ÷ 100 =
(b x e ÷ 100)
27,000 x 0.35 ÷ 100 =
583 lbs/day 94.5 lbs/day
a.
Animal Group
Example: Beef h.
Maximum
One-time
Capacity
(# of animals)
1,000 i.
Average Daily
Gain/Animal
(lbs/day)
4.1 lbs/day
Beef
Live Weight Nutrient
Concentration j.
N
0.026 k.
P
Daily Nutrients Retained by Animal l.
N
(lbs/day)
(h x i x j)
0.0070 1,000 x 4.1 x 0.026 =
107 lbs/day
(h x m.
P
i x k)
1,000 x 4.1 x 0.0070
= 28.7 lbs/day
0.026 0.0070
Dairy
Pork
Hens
0.027
0.024
0.028
0.0075
0.0050
0.0058
Broilers 0.028 0.0058
Turkeys 0.028 0.0059
a.
Animal Product n.
Daily Production
(lbs/day)
Nutrient Concentration of
Animal Products o.
N (lbN/lb prod.) p.
P (lbP/lb prod.)
Nutrients Retained by Animal Products
(lbs/day) q.
N
(n x o) r.
P
(n x p)
Milk 1
Eggs 1
0.0050
0.0166
0.0010
0.0021
a. Animal
Group/Product
Example: Beef finisher s.
Days Fed Per
Year (days/yr)
350 days/yr t.
N [s x (f – l)] or
[s x (f – q)] (lbs/yr)
350 x (583 – 107)
= 167,000 lbs/yr
Annual Nutrient Excretion u.
P [s x (g – m)] or
[s x (g – r)] (lbs/yr)
350 x (94.5 – 28.7)
= 23,000 lbs/yr v.
P
2
O
5
2
(u x 2.3) (lbs/yr)
23,000 x 2.3 =.
53,000 lbs P
2
O
5
/yr
Total w. x .
1.
N in feed = Protein ÷ 6.25, N in milk = Protein ÷ 6.38. Nitrogen content of 0.0050 assumes 3.2% protein in milk, N in eggs =
Protein ÷ 6.25. Nitrogen content of 0.0166 assumes 10.4% protein in eggs.
2.
Lbs P
2
O
5
= lbs x 2.3
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