Download Overview of Phosphorus Issues in Swine Feeding

Overview of Phosphorus Issues in Swine Feeding
Alan L. Sutton, J. Scott Radcliffe, and Brad C. Joern
Professor and Assistant Professor, Department of Animal Sciences; and Professor
of Agronomy, Purdue University
Phosphorus and the Environment
Phosphorus (P) is an essential element for normal growth, development, and reproduction of
both plants and animals. However, excessive P levels can impair surface water quality. It is well
established that P is the limiting nutrient for phytoplankton production in lakes (Vollenweider,
1968; Schindler, 1974, 1977). Although less data exist for streams and rivers, research indicates
P also is a key element controlling productivity in these systems (Stockner and Shortreed, 1978;
Elmwood et al., 1981; Peterson et al., 1985). High P levels in surface waters accelerates the
eutrophication process and often results in the excessive production of phytoplankton such as
algae and cyanobacteria. The respiration of these organisms leads to decreased oxygen levels in
bottom waters and under certain circumstances (at night under calm, warm conditions), in
surface waters (Correll, 1998). These decreased oxygen levels can lead to fish kills and
significantly reduce aquatic organism diversity.
Excessive P levels in animal diets increases animal manure P excretion, and land application of
this manure to soil can increase potential P losses from fields to surface and groundwater
resources. Phosphorus loss from agricultural soils can accelerate the eutrophication of lakes and
streams (Daniel 1998; Parry, 1998; Gaynor and Findlay 1995). Sharpley et al. (1994) stated,
“Eutrophication restricts the use of surface waters for aesthetics, fisheries, recreation, industry,
and drinking, and thus has serious local and regional economic impacts.
Much of the P reaching water is from runoff, often with sediment, from cropland receiving high
rates of manure or inorganic fertilizers. While P loss to surface and ground water via P leaching
through the soil profile is generally much smaller than runoff P losses, excessive P applications
to soils over time will move P to lower portions of the soil profile, and this P can discharge into
tile drains, ditches and eventually streams (Figure 1). Significant tile discharges of P also can
occur via macropore transport of manure to tile lines after land application, especially during the
dry season when cracks form in the top soil. Additionally, sandy soils with, rapid drainage and
low anion exchange sites generally have greater P leaching potential than heavier textured soils.
Another impact of P on water bodies was identified in the coastal waters in the DELMARVA
region in the last decade. Enrichment of the Chesapeake Basin area with nutrients including high
levels of P caused an outbreak of Pfisteria pisidia, a dinoflagellate, which causes lesions on fish.
Initially, the poultry industry was cited as the cause of the increased P levels in the water, which
resulted in strict regulations forbidding land application of litter, the required use of phytase in
feeds, the export of litter and the motivation to experiment with new technologies to gain other
value-added resources from the litter. Later it was determined that there were probably several
sources of P that contributed to pollution in the Chesapeake Bay watershed. Although an
isolated case, this example highlights the potential and perceived impacts of animal manures on
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Fertilizers
Imports
Agricultural
By-products
Erosion/Runoff
Soil Processes
Clays
Al, Fe Oxides
Secondary P
Minerals
Ca, Fe, Al
Phosphates
Primary P
Minerals
By-products
P Removed with Crop
Exports
Sorbed P
Municipal and
Industrial
Sorption
Desorption
Precipitation
Dissolutio
Dissolutio
Plant
Plant
Residue Uptake
Soil Solution
P
(H2PO4-, HPO4-
Leaching
and
Drainage
Apatites
Surface Waters
(Eutrophication)
Immobilization
Mineralization
Organic P
Soil Biomass
(Living)
Soil Organic Matter
Figure 1. Phosphorus cycle in soils.
the environment, the response of regulators to an environmental situation and the changes in
legislation and enforcement along with increased management, record keeping, operational
requirements and costs to the producer.
The potential threat of P pollution to the waters of the United States as it relates to the livestock
and poultry industries is site specific and related to the concentration of animals producing the
manure relative the amount of cropland in the same region to utilize the P from the manure. In
certain areas of the US, more P is produced in manure than can be utilized by adjacent
productive cropland (USDA NRCS, 2000). Figure 2 illustrates specific areas in the US by
county where potential excess P from manure includes concentrated swine and poultry in North
Carolina, poultry in the DELMARVA area, poultry in several areas of the Deep South, beef
cattle in portions of the southern Great Plains, and dairy cattle in southern California, Oregon
and Washington. However, most manure P loading challenges occur at the farm or community
level.
As a result of repeated applications of manure to cropland, especially at nitrogen (N) based rates,
soil P levels increase, which leads to an increase in P loss potential from agricultural fields.
Increased soil P accumulation at the farm and regional levels has prompted efforts to regulate
manure applications on a P-basis through the recent revision the USEPA CAFO NPDES permit
rule and the promotion of comprehensive nutrient management plans (CNMPs) by NRCS to
control nutrient balance and flow on the livestock and poultry operations in the US.
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Figure 2. Percentage of phosphorus taken up and removed that could be supplied by phosphorus
from manure (USDA NRCS 2000).
Clearly, excess soil P levels resulting from livestock and poultry manure applications is a
nutrient distribution problem because P is deficient in some areas of the US. Of the manure that
is collectable in the US, it was estimated that only about 42% of the required fertilizer P for
current cropland production could be supplied by animal manure (CAST, 1996). Recent
estimates (Table 1) have indicated that collectable manure P is roughly 50% of commercial P
fertilizer consumption in the US (Hess et al., 2001).
The soil-water P cycle is illustrated in Figure 1. Both organic and inorganic P is present in soil,
but, inorganic P is the form taken up by plants. Soil P dynamics are largely influenced by soil
pH, clay content and mineralogy, amorphous iron and aluminum, and organic matter. Inorganic
P is the predominant P form in both manures and commercial fertilizers. Depending on soil pH
and mineralogy, inorganic P can be sorbed on the surface of clays and amorphous iron and
aluminum compounds or precipitated as mineral salts until utilized by plants. Organic forms of
P from crop residues, soil organic matter and manures can be mineralized by soil
microorganisms and become available for plant uptake. Conversely, inorganic P can be
immobilized to organic P forms not available for plant uptake. In addition, some organic P forms
excreted in manure may displace sorbed inorganic and increase inorganic P runoff and/or
leaching in the soil. Obviously, soil P cycling is a dynamic process.
The extent of P runoff from soils depends on rainfall intensity, soil type, topography, soil
moisture content, crop cover, and the form, rate and method of P application. Surface P
applications will result in more P runoff from soil than incorporated P applications (Nussbaum-
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Wagler, 2003). Conservation best management practices that reduce surface runoff and erosion
can greatly reduce the risk of P loss from soils.
The use of phytase in monogastric (swine and poultry) diets can significantly reduce manure P
excretion, but there has been some concern about the form of P excreted in manure and
eventually applied to cropland. While some studies have shown that phytase can increase
manure water soluble P (WSP) in poultry litter (Moore, et al., 1998), other studies have shown
reduced WSP excretion from pigs fed phytase (Hankins, 2001; Hill et al., 2003).
Table 1. Estimated quantities of manure and nutrients produced annually in the United States
compared to commercial fertilizer consumption.
Animals†
Manure‡
millions millions of tons§
65
773
18
364
61
94
1,103
36
104
13
419
17
1,770
1,297
651
N‡
P2O5‡
K2O‡
- - - - thousands of tons§ - - - Beef cattle
4,284
2,057
3,049
Milk cattle
1,825
837
1,212
Hogs
777
496
407
Broilers
463
282
221
Turkeys
180
154
77
Layers
234
181
107
All animals
7,763
4,007
5,073
Collectable manure¶
4,143
2,371
2,736
11,897
4,424
5,178
Fertilizer consumption#
† Number of animals in inventory from 1997 Census of Agriculture (USDA/NASS, 1999).
‡ Excretion values adapted from Midwest Plan Service (MWPS, 2000).
§ To convert to metric tonnes, multiply values by 0.907.
¶ Assumes 100% of manure from beef cows and dairy replacement animals (heifers and heifer
calves) and 15% of manure from pigs, poultry and other cattle is not collected.
# Average 1990-1999 U.S. commercial fertilizer consumption (AAPFCO-TFI, 1999).
Swine Manure
Swine manure N, P and potassium (K) composition is not properly balanced for plant uptake by
typical crops grown in production agriculture. The relative ratio of N, P, and K in manure from
pigs fed commercial diets after storage and applied to cropland is generally 1:1:1. Corn grain
production requires a 3:1:1 ratio and if corn is grown for silage, then a 2:1:2 ratio of N, P and K
is desirable. Therefore, if manure is applied to meet the N requirement of corn grain production,
P is over applied by 3 times crop P removal. Ideally, if the ratio of N, P and K in manure could
be altered by nutritional means to meet specific crop nutrient requirements, it would alleviate a
significant problem currently facing many pork producers utilizing manure as a crop nutrient
resource.
Current regulations are forcing pork producers to apply manure at agronomic rates based upon
the most limiting nutrient, which in most cases is P. However, there is the potential that
producers can “bank” P for short periods of time if there is sufficient land available to rotate the
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fields for manure application in subsequent years. A common practice may be to apply the
manure to meet the N requirement of the crop, but only apply the manure to the field every 3
years. A rotation for manure application to crop fields must be established for manure
applications to meet crop P needs for the crop rotation grown on specific fields.
Phosphorus in Swine Diets
Phosphorus is an essential nutrient in swine diets serving important functions as part of structural
compounds in bone and in cell membranes, as a source of high energy bonds in nucleotides, as a
structural component of nucleic acids, as a component of many enzyme cofactors, and as a
component in many metabolic pathways (Jongbloed, 1987; Ziegler and Filer, 1996; Berner
1997). Approximately 85% of total body P is found in bone, with 14% in soft tissue and muscle,
and 1% in blood (Berner, 1997). Phosphorus is stored in bone as a crystalline structure, along
with Ca, known as hydroxyappetite (Ca10(PO4)6(OH)2). Deficiencies in bone P lead to rickets
and osteomalacia.
Phosphorus absorption occurs throughout the small intestine with the largest portion of P
absorption occurring in the jejunum (Korwaski and Schachter, 1969; Walling, 1977). There are
two primary mechanisms involved in phosphate (PO4) absorption, an active transport system and
a passive transport system. Active transport of PO4 occurs primarily in the proximal small
intestine and is linearly related to lumenal Na+ concentration (Danisi and Straub, 1980).
Phosphate absorption is blocked by calcitonin (Juan et al., 1976), while 1,25(OH)2-D3 has a
stimulatory effect (Danisi and Straub, 1980). Passive transport occurs primarily in the jejunum
and ileum and is related to lumenal PO4 concentration (Danisi and Straub, 1980). Therefore,
when PO4 intake is low, the active transport mechanism is dominant, and when the intake of PO4
is high, the passive transport mechanism is dominant.
More research has addressed the P requirement of pigs than probably the requirement of any
other mineral. However, even with this abundance of research, nutritionists still struggle to
decide what the correct P level to feed is. In the past, over-formulation of P in the diet was not
considered a major problem because P is relatively inexpensive to add to the diet, and
performance of pigs is not hindered by small over-formulations in P, provided that Ca levels are
kept at an appropriate ratio. However, with growing environmental concerns over high levels of
P in swine manure, it has become necessary to minimize over-formulations in P. Producers need
to minimize over-formulation of P for two reasons. First, it is simply a waste of money.
Assuming, a feed to gain ratio of 2.8:1.0 for pigs up to 50 pounds, and a feed to gain ratio of
3.0:1.0 for pigs greater than 50 pounds, a producer could save approximately $0.09 per hog
marketed by reducing P in the diet by 0.1 percentage-unit. Additional money would be saved
due to the extra space generated in the ration by the removal of inorganic P. Second, high levels
of P in swine manure are of growing environmental concern. Minimizing over-formulation is
the easiest and cheapest way to reduce P excretion.
To accomplish this, the P requirement of pigs needs to be accurately defined. The most recent
Swine NRC (1998) cites 151 references that in some way address P nutrition. Of these, 37 are
from references in the 1990s. Of those 37, 18 are from studies investigating the effects of
phytase and nine are from studies investigating the effects of porcine somatotropin on P balance.
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Of the remaining 10 studies, five directly address P requirements and one addresses the
bioavailability of P from different dietary sources. Therefore, recent research surrounding
dietary P has focused on phytase and other dietary additives that may alter P utilization and
metabolism rather than the P requirement. Even with the importance of studying new dietary
additives on P utilization by swine, there is a need to question the reliability of P requirements
based on data that in many cases is more than 20 years old. During that time, plant and animal
genetics have changed drastically, analytical methods have changed, and the criteria for
evaluating P status have changed. Thus, P requirements need to be determined for specific
genetic lines at different phases of the life cycle of the pig. This would allow for more precise
formulation of feeds for P in phase feeding programs and compliment the efforts of maximal N
utilization that has been an emphasis in the past decade.
Availability of Phosphorus in Feeds
Historically, P requirements were reported as total P. In more recent years, it has been realized
that the bioavailability of P differs among feedstuffs, and therefore we have moved to defining
the P requirement on a digestible P basis. The P requirement is listed in the most recent swine
NRC (1998) as the amount of P required per kilogram of diet and as the amount of P required per
day. Obviously, feed intake will affect the amount of P consumed per day. Therefore, in diets
with higher energy contents, it may be necessary to raise the concentration of P. Ultimately, P
requirements for market animals should probably be based on the amount of P required per
pound of lean tissue growth. Because of different feed intakes and lean growth potential for gilts
compared to barrows, specific diet formulations are needed for gilts and barrows.
A limiting factor in the use of digestible P in diet formulations is the accuracy of P
bioavailability estimates for feedstuffs. The use of apparent P digestibility as an estimator of P
bioavailability in feedstuffs has been used frequently in recent studies where microbial phytase
was added to the diet (Jongbloed et al., 1992; Dungelhoef et al., 1994; Mroz et al., 1994;
Kornegay and Qian, 1996; Yi et al., 1996, Radcliffe and Kornegay, 1998; Robbins et al., 2000).
This is supported by the conclusions of Dellaert et al., (1990) who compared eight P supplements
added to supply 0.6 to 2.2 g/kg of additional P to a basal diet which contained 3.0 to 3.2 g/kg of
P. In addition, a comparison of techniques used to evaluate P availability from feedstuffs was
made. They concluded that apparent P digestibility was the most sensitive indicator of P
bioavailability, followed by bone parameters, with blood parameters showing an insufficient
response to P. However, Peo (1991) concluded that apparent Ca and P digestibility data had
little value in estimating Ca and P bioavailability from feedstuffs. In the studies of Kornegay
and Qian (1996) and Yi et al. (1996c) where the effects of phytase were investigated, the
apparent digestibility of P did serve as a good measure of the bioavailability of P from the
feedstuffs. This was further supported by the work of Radcliffe and Kornegay (1988), Skaggs
(1999), and Zhang (1999).
The bioavailability of P from inorganic P sources used in swine diets is generally considered to
be quite high. However, there has been a substantial amount of variation reported in the
literature. Soares (1995) suggested a relative bioavailability value of 90% for defluorinated
phosphate, 95% for dicalcium phosphate and 100% for monocalcium phosphate when
monosodium phosphate was used as the standard and given a value of 100. A relative
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bioavailability of 87% is suggested for defluorinated phosphate and a value of 100% is suggested
for dicalcium phosphate when monocalcium phosphate is given a value of 100 (NRC, 1998).
Kornegay and Radcliffe (1997) compared four sources of defluorinated phosphate and one
source of dicalcium phosphate against a monocalcium phosphate control. They found no
differences between any of the phosphate sources. The relative bioavailabilities ranged from
95.1 to 105.3%. The availability of each P source in this study can be estimated by dividing the
increase (digested P in diet - basal digested P) in digested P (g/kg) when P is added by the
amount of added inorganic P added and multiplying by 100. With the exception of one of the
defluroinated phosphate sources (bioavailability = 58.5%) the estimated bioavailabilities of P
from all sources used in the study by Kornegay and Radcliffe (1997) ranged from 71.9 to 79.3 %,
with the average being 75.1%. This is in good agreement with a recent review by Kornegay et
al. (1998) in which the estimated bioavailability of inorganic P across 52 experiments was 76.7%
for swine.
Ultimately, diets should be formulated on an available P:available Ca basis, but there is not
enough data available to allow for this at the present time. In addition, the use of enzymes such
as phytase, alter the availability/digestibility of both P and Ca, which further complicates the
issue. In the end, P nutrition becomes a risk management situation; with the nutritionist trying to
decide how low the dietary P level can be dropped without affecting performance.
Phosphorus in Feeds
Phytate (Figure 3), the salt of phytic acid (myoinositol 1, 2, 3, 4, 5, 6 hexa, dihydrogen
phosphate; IUPAC-IUB, 1975), serves as the primary storage form of P in plants, accounting for
60 to 80% of the total P in most grains. Reported values in the literature for the total P content
and the phytate-P content of various plant ingredients are shown in Table 2.
OH
OH
++
- Ca -O P O
O P O
O
O
Starch
O
Ca++ -O C CH2 Protein
-
O
O P O
Protein
H
CH2
H
H
O
H
H
-
OH
NH3+
-O
O
P O
-O
O P O Zn++
O
H
-O
O
P O
OH
CH2OH
-O starch O-
Figure 3. Phytate phosphorus molecule.
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Table 2. Total and phytate P content of various feedstuffs.
Feedstuff
Corn
Corn
SBM, 44%
SBM, 48.5%
SBM
SBM, 44%
SBM, 48%
SBM
Total P, %
.28
.26
.65
.69
.61
.66
.61
Barley
Barley
Barley
Barley
.35-.36
.34
.37
Wheat
Wheat
Wheat
Wheat
.35-.39
.30
.33
Wheat middlings
Wheat middlings
.93
.80
Canola meal
Canola meal
1.01
1.12
Sorghum
Sorghum
Sorghum
.29
.27
.31
Meat and bone meal
4.98
Phytate P,
% of total
66
Reference
NRC, 1998
Nelson et al., 1968
61
53
52
51-61
NRC, 1998
NRC, 1998
Nelson et al., 1968
Eeckhout and De Paepe, 1994
Eeckhout and De Paepe, 1994
Pointillart, 1994
56
60
51-66
NRC, 1998
Nelson et al., 1968
Eeckhout and De Paepe, 1994
Pointillart, 1994
67
67
60-77
NRC, 1998
Nelson et al., 1968
Eeckhout and De Paepe, 1994
Pointillart, 1994
66
NRC, 1998
Eeckhout and De Paepe, 1994
36
NRC, 1998
Eeckhout and De Paepe, 1994
70
68
NRC, 1998
Eeckhout and De Paepe, 1994
Nelson et al., 1968
NRC, 1998
Monogastric animals do not secrete phytase in sufficient quantities to breakdown the phytate
molecule, hence, most of phytate P is not available for absorption. Therefore, large amounts of a
highly available inorganic P source must be added to meet the P requirement of pigs. While
most phytate P is not available for absorption, much of this phytate P is mineralized to inorganic
P in the large intestine, and is excreted in manure as inorganic P and increase soluble P loss
potential when applied to soils. Estimates of the bioavailability of P from various plant
ingredients as reported in the literature are shown in Table 3.
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Table 3. Estimates of P bioavailability from various feedstuffs for pigs.
Source
Corn
Corn
Corn
Corn
Corn
Corn
Corn
Bioavailabilitya, % Standardb
14
MSP
14
MSP
15
12
29
48
29
MSP
Reference
NRC, 1998
Cromwell, 1992
Pierce et al., 1977
Calvert et al., 1978
Pointillart, 1984
Pointillart, 1987
Huang and Alle, 1981
SBM, 44%
SBM, 48%
SBM, 44%
SBM, 44%
SBM, 44%
31
23
31
27
36
MSP
NRC, 1998
MSP
Cromwell, 1992
Tonroy et al., 1973
Huang and Alle, 1981
Barley
Barley
Barley
30
30
28
MSP
MSP
NRC, 1998
Cromwell, 1992
Calvert et al., 1978
Wheat
Wheat
Wheat
Wheat
50
49
46
51
MSP
MSP
MSP
NRC, 1998
Cromwell, 1992
Pointillart, 1984
Huang and Allee, 1981
Wheat middlings
Wheat middlings
41
41
MSP
MSP
NRC, 1998
Cromwell, 1992
Canola meal
Canola meal
21
21
MSP
MSP
NRC, 1998
Cromwell, 1992
Sorghum
Sorghum
20
20
MSP
MSP
NRC, 1998
Cromwell, 1992
MSP
Meat and bone meal
67
MSP
Cromwell, 1992
Meat and bone meal
90
MSP
NRC, 1998
a
Bioavailability is expressed as a percentage relative to the standard which is assumed
to have a bioavailability of 100%.
b
If no standard is listed then bioavailability is expressed as a percentage of apparent
absorption
At a neutral pH phytic acid can carry one or two negatively charged oxygen atoms in each
phosphate group (Erdman, 1979), giving it a total of up to 12 negative charges. Because of these
negative charges, phytic acid has the ability to bind a variety of di and tri-valent cations
including Ca, Mg, Zn, Cu, Fe, Co, and Cr, in the small intestine of the pig and amino acids or
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peptides rendering them unavailable for absorption (Maga, 1982; Reddy et al., 1982; Morris
1986). The mineral complexing potential of phytic acid is shown in Figure 3. Phytic acid has
the greatest affinity for Zn and Cu and a fairly low affinity for Ca. However, due to the much
larger concentration of Ca in diets fed to pigs relative to Zn and Cu, an effect on Ca is possible.
It has also been shown that the anionic phosphate groups of phytate possess the ability to bind
proteins (Prattley et al., 1982) and amino acids; having its greatest affinity for the basic amino
acids: lysine, arginine, and histidine (Reddy et al., 1982).
Byproduct Feeds
Byproduct feeds can serve as a source of nutrients in pig diets. Often, byproduct feeds, such as
distiller’s grains, corn gluten meal, wheat middlings, etc. are included in the diet if they are
readily available and economically justified, especially when there is a shortage or increase in
prices of conventional feed sources. Also, due the processing methods employed, nutrients in
the byproducts become more biologically available and can potentially reduce nutrient excretion
if the byproduct nutrients can be balanced in the diet.
Distiller’s dried grains with solubles (DDGS) which contains from 0.62 to 0.87% P, have a
higher concentration of available P than corn, other cereal grains, and cereal co-products,
averaging 77% for DDGS compared to a range of 12 to 30% for corn, other cereal grains, and
cereal co-products. Studies by Spiehs et al., (1999) showed that when formulating diets on a
total P basis, the percentage of P retained tends to increase when 10 and 20% DDGS is added to
grower diets compared to a control corn-soybean meal diet (63.9%, 66.3%, and 59.1%,
respectively). Similar results were observed when feeding finisher diets containing up to 30%
DDGS. These results suggest that the P in DDGS is more available than in the corn-soybean
meal diet. Spiehs, et al. (1999) suggested that adding up to 20% DDGS to grower and finisher
diets will reduce supplemental inorganic P needs in the diet, and consequently, could reduce P
levels in the manure assuming that the diet is formulated on an available P basis.
The relative availability of P in corn gluten feed and wheat middlings are 59% and 41%,
respectively, compared to a standard monosodium or monocalcium phosphate (NRC, 1998). In
addition, byproducts from dairy processing and high protein meals from animal origin are high in
available P ranging from 90 to 97%. However, there is considerable variation in total P contents
of these byproduct sources that must be considered for inclusion rates in a balanced diet. An
example of how processing feeds can influence the bioavailability of P is fermentation of high
moisture corn (53%) compared to dry corn (12%). With the presence of bacterial derived
phytase during the fermentation process more bioavailable P is released from phytic P in the corn
(Cromwell, 1992).
Milling processes to degerm and dehull corn has created new interest as feed ingredient sources
with reduced levels of phytic P. Initial research has shown increased digestibility of dry matter,
energy and N of the degermed, dehulled corn compared to normal corn with reduced fecal dry
matter and N excretion (Moeser, et al., 2002; Kendall, et al., 2003). Indigestible P was reduced
by 15% with degermed, dehulled corn (Van Kempen, et al., 2003). Future processing techniques
may result in feed byproducts that will allow us to precision feed pigs with highly digestible
nutrients, however, economic issues may limit the implementation of these technologies.
89
Reducing Phosphorus Excretion
There has been a great deal of interest in lowering the level of phytate P in the diet, and thereby,
making more of the plant P available to the pig for absorption. There have been two primary
strategies employed to accomplish this. First, add phytase (an enzyme which breaks down phytic
acid) to the diet or second, develop plants with lower amounts of phytate P. Both methods are
successful as both increase P utilization, and therefore, decreased excretion. Since another paper
will present information on the value of phytase in swine feeding, it will not be discussed here.
Low phytic acid grains do have the potential for wide scale use in the future if crop yields can be
maintained at a similar level to conventional grains. For a maximum reduction of P, a
combination of low phytic acid grains and phytase may ultimately provide the producer with the
greatest ability to lower dietary P levels and minimize manure P excretion.
SUMMARY
Phosphorus reaching our nations waters can adversely affect the tropic status and potential uses
of the water body. If applied at excessive P rates, manure applied to cropland can increase soil P
loss potential. However, the potential risk of P loss from manured fields depends upon the
amount of P and chemical form of the P in the manure, method of manure application, and the
soil-water conditions of the application site. Currently, only isolated areas of the US have more
animal manure P generated than potential cropland P utilization.
Feed formulation and feeding programs can have a significant impact on P excretion. However,
due to the lack of current research on the P requirements of today’s pigs, and the availability of P
in feed sources, accurate feed formulation for optimum diets is a challenge, and more specific
research data are needed to reduce manure P excretion. Use of feed additives, genetically
improved feed grains, and further processed feeds, if they become economically feasible, can
assist in reducing P excretion.
REFERENCES
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Nutritionally Essential Mineral Elements. p. 63. Marcell Dekker, Inc., New York.
Calvert, C. C., R. J. Besecker, M. P. Plumlee, T. R. Cline and D. M. Forsyth. 1978. Apparent
digestibility of phosphorus in barley and corn for growing swine. J. Anim. Sci. 47:420426.
CAST. 1996. Integrated animal waste management. Council for Agricultural Science and
Technology Report 128. Ames, Iowa. 87 pages.
Correll, D.L. 1998. The role of phosphorus in the eutrophication of receiving waters: A review.
J. Environ. Qual. 27:261-266.
Cromwell, G. L. 1992. The biological availability of phosphorus from feedstuffs. Pig News
and Info. 75N-78N.
Daniel, T. C., A. N. Sharpley, and J. L. Lemunyon. 1998. Agricultural phosphorus and
eutrophication: a symposium overview. J. Environ. Qual. 27:251-257.
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Danisi, G. and R. W. Straub. 1980. Unidirectional influx of phosphate across the mucosal
membrane of the rabbit small intestine. Pflugers Arch. 385:117-122.
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