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Review Article
Environmental Implications of Nitrogen Output
on Horse Operations: A Review
Rebecca C. Bott
a
,
*
, Elizabeth A. Greene
b
, Nathalie L. Trottier
c
, Carey A. Williams
d
,
Michael L. Westendorf
d
, Ann M. Swinker
e
, Sara L. Mastellar
a
, Krishona L. Martinson
f
a
Department of Animal Science, South Dakota State University, Brookings, SD
b
School of Animal and Comparative Biomedical Sciences, University of Arizona, Tucson, AZ
c
Department of Animal Science, Michigan State University, East Lansing, MI
d
Department of Animal Sciences, Rutgers, The State University of New Jersey, New Brunswick, NJ
e
Department of Animal Science, Pennsylvania State University, University Park, State College, PA
f
Department of Animal Science, University of Minnesota, St. Paul, MN
article info
Article history:
Received 26 June 2015
Received in revised form 20 August 2015
Accepted 26 August 2015
Available online 31 August 2015
Keywords:
Horse
Protein
Nitrogen
Lysine
Digestibility
Environment
abstract
Nutrients such as nitrogen (N), which go unused during the digestive process, are then
excreted into the environment via urine, gas, or fecal matter. Excess N released in this
manner may contribute to a reduction of the quality of air and groundwater sources. Many
states have introduced or developed legislation mandating nutrient management plans on
livestock operations to reduce environmental N losses. Strategies for reducing the envi-
ronmental impacts of N on equine operations are twofold, including a reduction in N in-
puts and better management of N outputs. The practice of precision feeding, or feeding to
accurately meet, but not exceed the nutrients requirements of an animal is a plausible
method for reducing N inputs. This approach is not widely implemented, as feeding
protein in excess of requirements is a common practice in the equine industry. Also,
precision feeding is predicated on a body of data containing the nutrient availability and
digestibility in different feed sources; data which are not fully elucidated in the horse.
Management of N outputs on equine operations is largely based on data extrapolated from
other livestock operations as well as a few preliminary efforts on horse farms. The po-
tential impact of equine operations on N losses is explored in this review, shedding light on
areas where further research and management strategies are needed.
Published by Elsevier Inc.
1. Introduction
Feeding practices with the goal of precisely meeting the
dietary requirement of the first limiting amino acids (AA)
have been implemented in many livestock systems to
optimize performance and, recently, to minimize nitrogen
(N) excretion into the environment, for example, the swine
industry [1]. Paradoxically, feeding protein in excess of
requirement has been a common practice in the horse
industry [2]. Fecal and urinary N from livestock contributes
to ground water contamination and decreased environ-
mental air quality [3]. With increasing public awareness
and emerging regulations to limit N losses to the environ-
ment, nutrient management plans may be mandated in the
future to ensure sustainability of equine facilities, espe-
cially in increasingly urbanized areas. Strategies to mitigate
the impact of equine-feeding practices on the environment
are contingent on knowledge of factors that impact protein
utilization of common feeds for equids. Because of the
complexity of the equine digestive tract and the vast dif-
ference in feed types, including forages and cereal grains,
prediction of feed protein digestibility and N output
*Corresponding author at: Rebecca C. Bott, Department of Animal
Science, South Dakota State University, Brookings, SD 57007.
E-mail address: Rebecca.Bott@sdstate.edu (R.C. Bott).
Contents lists available at ScienceDirect
Journal of Equine Veterinary Science
journal homepage: www.j-evs.com
0737-0806/$ –see front matter Published by Elsevier Inc.
http://dx.doi.org/10.1016/j.jevs.2015.08.019
Journal of Equine Veterinary Science 45 (2016) 98–106
remains a challenge [4]. The goals of this review are to
provide an overview of the current knowledge on N utili-
zation in equids in relation to the environment and to
identify knowledge gaps that preclude the progress in
designing prediction models of N excretion in equids. The
specific objective of this review is to assess the potential
impact of N excretion from equids on the environment.
2. Environmental Implications of Excreted N
Nitrogen in the feces and urine must be managed to
prevent consequences to the environment, such as water
contamination and decreased air quality [5]. Excess N in the
air and water will eventually reach larger bodies of water,
where it can contribute to the deterioration of fisheries and
fish habitats through harmful nutrient loading [6]. Manure
nutrients (N, phosphorus, and organic matter) can be major
pollutants in lakes and estuaries as well as rivers. Nitrogen
attached to eroded soil particles may reach waterways
through surface runoff or wind deposition. These waters,
rich in N and other manure nutrients, promote a prolifer-
ation of plant life, especially algae. This process is called
eutrophication [7]. Algae growth and the decomposition of
organic matter in water bodies reduces the dissolved oxy-
gen content of the water, which may lead to fish kills, odors,
and other negative impacts on the aquatic ecosystem [8].
Although the larger environmental concern for N is surface
runoff, N volatilization as ammonia presents other prob-
lems including nonpoint source pollution through rain, and
the effects of volatilized ammonia on human health [9].
Nitrogen in manure can be converted to ammonia
through bacterial degradation, primarily the conversion of
urinary urea to ammonia. Urease, an enzyme produced by
microorganisms in feces, reacts with urinary urea to form
ammonia. Urease activity in feces is high; therefore, urea is
rapidly converted to ammonia after excretion. It is the
physical process of combining urine and feces after depo-
sition on a floor surface, which results in ammonia vola-
tilization in the barn. Ammonia emissions (kg/LU, livestock
unit 500-kg live weight) are predicted to be lower in horses
than in cattle, pigs, poultry, or sheep [10]; however,
ammonia emitted by horses is no less able to impact the
environment on a unit-by-unit basis.
Other factors can influence ammonia volatilization in
livestock housing. They include temperature, air velocity,
pH, surface area, manure moisture content, and storage
time. For example, high pH and temperature favor
increased ammonia emissions. Horse manure pH typically
ranges from 7.7 to 8.2 when calculated from soiled stall
waste, depending on the bedding source [11]. The pH of
horse manure samples collected between 2007 and 2014
and analyzed at the Pennsylvania State University Ag
Analytical Services Laboratory ranged from 6.9 to 8.2 with
an average of 7.8, which allows for fairly rapid emission of
ammonia into the atmosphere [12]. Only a few studies have
been conducted to calculate the amount of N and/or
ammonia concentration of horse stall waste; however, the
amount will depend on a myriad of factors including
location, time of exposure, and bedding type. Horse stables
are noted to have higher ammonia concentrations than
pastures. This correlated with higher equine exhaled breath
condensate pH in stables compared to pastures [13]. One
study looked at four different bedding types for the pur-
pose of calculating composting rate [11]. The study found
that when long straw is used for bedding, it has a higher
percentage of total N before composting than a pelletized
straw and pelletized wood product. However, the amount
for long straw was not statistically different than when
wood shavings were used for a bedding source. This study
also found that ammonia did not decrease after composting
and therefore indicating that it was not completely con-
verted to nitrate (NO
3
) or that the organic matter was not
completely degraded by the composting. Table 1 indicates
N and phosphorus content of feces and different bedding
sources from soiled stalls in different studies.
The Environmental Protection Agency considers
ammonia a threat to air quality because of contribution to
surface water eutrophication, NO
3
contamination of
Table 1
Concentrations of ammonia gas (NH
3
gas), ammonium (NH
4
), total nitrogen (N), nitrate, and pH collected from various studies.
Study Reference Experimental Conditions Bedding or Treatment Types NH
3
Gas
(ppm)
NH
4
(ppm) Total N % Nitrate
(ppm)
pH
Komar et al, 2011
(mean SE)
Soiled bedding collected from
stalls for 30 d
Pelletized straw d10.1 2.65 0.71 0.04 0 7.8 0.1
Long straw d14.6 2.65 0.94 0.04 0 8.2 0.1
Pelletized wood d13.5 2.65 0.63 0.04 0 7.7 0.1
Wood shavings d17.1 2.65 0.78 0.04 0 7.9 0.1
Williams et al, 2011
(mean SE)
Feces only collected for 5 d from
horses on a low or high
protein diet
Feces only low protein diet 25.4 3.4 300 80 0.24 0.01 dd
Feces only high protein diet 37.8 3.4 700 80 0.28 0.01 dd
Fleming et al, 2008
(mean SD)
Feces and urine added to
containers under
standardized conditions with
bedding for 14 d
Wheat straw 237 118 530 168 d10 17 d
Wood shavings 207 116 805 235 d0d
Hemp 193 114 842 81 d162 58 d
Linen 178 88 783 47 d0d
Straw pellets 80 51 377 68 d0d
Paper cuttings 217 120 843 54 d0d
Garlipp et al, 2011
(mean SE)
Feces and urea added to
containers with bedding
under standardized
conditions for 19 d
Wheat straw 5.75 0.8 d1.07 0.07 d6.9 0.11
Rye straw 4.07 0.8 d1.14 0.07 d6.8 0.11
Wood shavings 2.31 0.8 d0.61 0.07 d6.4 0.11
Abbreviations: SD, standard deviation; SE, standard error.
Cells with empty (d) values were not tested in a given study.
R.C. Bott et al. / Journal of Equine Veterinary Science 45 (2016) 98–106 99
ground water, and the associated impaired air quality [14].
Ammonia contamination has the potential for at least
short-term adverse effects on agricultural workers involved
in animal care [15,16]. Subjects in a study performed to
simulate residing in close proximity to a swine operation
exhibited headaches, eye irritation, and nausea [16].In
studies of workers at swine concentrated animal-feeding
operations (CAFOs), 25% of the workers reported at least
one of the following respiratory symptoms: asthma, bron-
chitis, acute respiratory distress syndrome, and organic
dust toxicity syndrome [15]. However, as far as the authors
know, there has never been a study evaluating the health of
equine farm workers who are exposed to high levels of
ammonia.
In a study conducted in horses fed at approximately
165% of the recommended protein amount [17], authors
showed that elevating protein levels in a horse’s diet in-
creases the ammonia and N levels excreted in manure, the
ammonia in the atmosphere, and the urea N in the animal’s
blood. More specifically, fecal N and ammonia were higher
(approximately 35 and 50% higher, respectively) in the high
protein fed horses than in the control fed horses. Nitrogen
and ammonia for control horses were 0.242 0.01 and
0.034 0.008, respectively, whereas N and ammonia were
0.278 0.01% and 0.068 0.008%, respectively, for horses
fed high protein diets (P¼.015). When atmospheric
ammonia was tested by an 8-hour accumulation via Dräger
tube, there was a significantly higher level of ammonia in
the air in the stalls of horses fed the high protein diet. The
high protein horses averaged 37.8 3.4 ppm, whereas the
control horses averaged 25.4 3.4 ppm (P¼.029) [17]. This
could also lead to the question of horse health because
these Dräger tubes were placed on the noseband of the
horses’halters over the course of an 8-hour period while
stalled. These levels could potentially be a problem for the
horses’health if left exposed for an extended period of
time. A study in Japan found that a horse inhaling only 2–
17 ppm of ammonia (range of ammonia in an enclosed
trailer) over the course of 40 hours created excessive nasal
discharge and slight swelling of the nasal cilia [18]. This
same study found that another horse inhaling 40–130 ppm
(maximum exposure for humans) had a much more severe
nasal discharge, swelling and irregular distribution of
tracheal epithelium and edema of the submucosa, loss of
nasal cilia, and more severe swelling than the horse
exposed to 2–17 ppm. This study indicates that inhalation
of these concentrations of ammonia is detrimental to the
respiratory health of horses. Future research should further
investigate the possible negative consequence for long-
term exposure to ammonia using larger sample sizes.
Another study compared sawdust-bedded stalls to
paper bedding (recycled phone books) to investigate the
ammonia differences and horse health over a 14-day period
[19]. Ammonia samples were taken from an air pump
placed at horse nose height for a period of 30 seconds and
found the ammonia levels were higher with the sawdust
bedding (1.42–2.54 ppm) compared the paper bedding
(0.21–0.88 ppm). However, horse health variables (e.g.,
endoscopic examination of the nasal passage and trachea,
tracheal wash cytology, tracheal inflammation, and general
health observations) did not differ between bedding types.
Pratt et al [20] found that when using straw bedding
instead of rubber mats, ammonia concentrations rose to a
peak of 14 ppm after 14 days in the same stalls; stalls were
cleaned each day. This study did not find any respiratory or
health complications with horses exposed to this level of
ammonia. However, it is possible that diet may effect
nutrient excretion in the context of environmental
contamination, air quality, and horse health. More research
is needed to evaluate the effect of different feeding stra-
tegies on nutrient excretion in the context of environ-
mental contamination and air quality.
3. Defining a Need for Managing N Input and Output
on Horse Operations
Animals acquire N from protein in the feedstuffs they
eat. A substantial amount of organic N is excreted in urine
or feces, as it has been estimated that less than 45% of
consumed protein is used and made into animal protein
[21,22]. It has been shown that horse owners commonly
over feed protein, up to 150% of the recommended re-
quirements [2]. In a survey conducted by Harper [23], horse
owners fed their horses 161% of National Research Council
[24] crude protein (CP) requirements on average. Mainte-
nance horses were associated with greater overfeeding
than working horses. Maintenance horses require fewer
nutrients than working horses [24]; however, they often
received the same type of concentrate as their working
counterparts on any given farm, just in smaller amounts.
Those concentrates often have higher concentrations of
nutrients than required for maintenance, which leads to
excessive nutrient supplementation and therefore nutrient
excretion. Usually, maintenance horses can be sustained on
forage only diets [25]. Excess protein in the horse diet is
then excreted in sweat, feces, and urine (Fig. 1).
There are several situations when excessive protein or
an imbalance can occur. A forage ration consisting pri-
marily of excellent quality hay typically has excessive levels
of protein and soluble protein. Formulating feed rations
with higher than required protein levels, to ensure all
essential AA requirements, have been met often com-
pounds the problem. It is possible to meet minimum re-
quirements for CP while being deficient in single AAs [24].
When this occurs, whole body protein synthesis might be
limited [26]. Depending on the actual performance level of
the horses, this can result in excessive levels of N being fed
and excreted. Furthermore, in many operations, a single
diet is fed to all horses without consideration of activity
levels or physiological states of individual animals. This
practice could lead to oversupplementation or under-
supplementation of individual animals and poor overall N
management.
A few studies have looked at reducing the CP concen-
tration of horse rations through supplementation of
purified limiting AAs. Lysine added to linseed meal and
brewer-dried grain diets with hay has increased foal weight
gain and feed conversion efficiency [27,28]. Staniar et al
[29] reduced the CP level of a supplement to 9% while
fortifying with lysine and threonine to growing Thor-
oughbreds on pasture. For the overall observational period,
no differences in physical growth measurements were
R.C. Bott et al. / Journal of Equine Veterinary Science 45 (2016) 98–106100
observed in the crystalline AA supplemented group (9% CP)
compared to the control (14% CP).
Feeding excess dietary N not only is of consequence to
the environment, but also to the animal and cost of the diet.
There is a metabolic energy cost associated with excreting
excess N in the urine, making diets with excess protein
inefficient and not cost effective. Additionally, higher pro-
tein diets can affect acid–base balance [30], heat produc-
tion [31], and water requirements [30]. All these factors are
of particular concern to the athletic horse.
4. Nitrogen Content of Common Equine Feedstuffs
Digestibility of a protein and absorption of its AA con-
stituents are the main determinants of protein quality and
the predictor of utilization for maintenance or productive
functions by the animal as compared. Domesticated equids
are fed a wide variety of feeds ranging from herbage to
seeds. The composition of proteins and their availability for
digestion varies extensively across feeds, thus rendering
prediction of N excretion difficult.
Nitrogen in equine diets comes from forages, grains, and
oil seed meals. Legume forages generally have higher CP
concentration than grasses, w20% versus 11%–13% CP dry
matter (DM) basis [32]. Within grasses, cool season grasses
generally have higher CP concentration (average of 15% CP
on a DM basis) than warm season (11% CP DM basis) [32].
Although the CP concentration of grains does not vary as
greatly as that of forages, it can differ between cultivars.
Wheat cultivars can range from 13%–15% CP [33]. Oilseed
meals have greater CP concentration than grains. Soybean
meal, cottonseed meal, and linseed meal are about 51%,
44%, and 36% CP on a DM basis, respectively [32].
Several factors influence the protein content of plants.
Plant maturity plays a role with immature forage plants
having greater protein content than more mature plants as
reviewed by Pagan [34]. For instance, CP content of alfalfa
decreased from 15% to 9% after 20 additional days of growth
[35]. Fertilization and management practices additionally
influence the N content of forage with greater amounts in
grasses receiving fertilizer and regular clipping [36].
Fertilization can also increase the CP concentration of
grains [37,38]. Crude protein and NO
3
concentration of
common horse feeds are outlined in Table 2.
Sources of N in feedstuffs include protein N and
nonprotein nitrogen (NPN) compounds. Stage of maturity
may affect the AA profile of alfalfa [39], but red clover and
grass AA profiles may not be as influenced by maturity [40].
Some grain cultivars have altered AA characteristics.
Breeding for specific characteristics can change the relative
proportions of proteins present in the plant resulting in an
altered AA profile. One such example is a high fat oat
cultivar [41]. There are also high lysine corn cultivars that
have been specifically developed to improve feed value
[42,43].
One of the most common forms of NPN in plants is NO
3
,
and much of the N absorbed from the environment and
therefore used by plants is in the form of NO
3
. Factors
affecting NO
3
accumulation in plants including level of N
fertilization, forage species, maturity, herbicides, and light
intensity have been reviewed by Crawford et al [44] and by
Wright and Davison [45]. Some forms of NPN are
commonly added to ruminant diets as reviewed by Hun-
tington [46]. Urea is a type of NPN that is minimally used by
equids [47–49]. Unlike ruminants, NPN compounds are not
added to practical equine diets; because most microbial
Fig. 1. Nitrogen utilization by the horse as adapted from Tanner (2014). The dotted arrow line indicates that the importance and proportion of AA uptake in the
hindgut after the digesta has interacted with the largest portion of the microbial population remains largely unknown. AA, amino acid.
R.C. Bott et al. / Journal of Equine Veterinary Science 45 (2016) 98–106 101
population and bacterial protein synthesis is distal to the
small intestine, horses are not as efficient at using these
forms of N [50].
Selecting feedstuffs containing appropriate protein and
N profiles to meet the horse’s dietary needs may help to
reduce N outputs through strategic selection of inputs.
5. Nitrogen Balance
A Whole Farm Nutrient Balance [14] assessment is a tool
that can be used to determine generation of excess nutri-
ents on farm and can be useful in developing plans to
manage nutrient buildups. This idea may be helpful for
reducing N losses in equine production. The challenge is to
manage the animals, crops, and other farm components to
efficiently use available manure N and reduce potential
losses to the environment [51]. Nutrient accumulation oc-
curs when nutrient inputs exceed nutrient outputs [52].
Whole farm balances of inputs and outputs can be used to
assess the risk of nonpoint source pollution and identify
pollution reduction strategies [53]. Nutrient balances can
be calculated on several levels including regional, whole
farm, and field scales [54].
Whole farm or “farm gate”nutrient balances compare
the nutrients of concern, which come in and go out of the
farm gate [55]. Products typically going onto a farm
through the gate include purchased hay and grain or con-
centrates, mineral fertilizer, manure, bedding, and live an-
imals [55]. Products typically leaving a farm through the
gate include crops, eggs, milk, meat, fiber, live animals, and
manure [55].
A Pennsylvania State University study [23] which fol-
lowed 14 Mid-Atlantic horse farms for an entire year found
that whole farm balances on Mid-Atlantic horse farms
averaged 73% for N, which is similar to findings in other
species. Kohn et al [56] reported that whole farm N balance
averaged 66% on high efficiency dairy farms. Percentage is
an indicator [57] of the proportion of N inputs (feed,
fertilizer, animals, and legume N) not accounted for in the
outputs (crops, animal products, and manure). As a point of
reference, a lower percent would mean a higher or poorer
balance for the farm. On horse farms, there may be a wider
range of N balance values due to the amount of manure
horse farms export. In the Pennsylvania State University
study [23], horse farms exporting all their manure averaged
39% balance for N; and they reported that all farms im-
ported some feed. The mean farm balance for those that did
not export any manure was 100%, indicating that all N
imported onto the farm remains on the farm. Determining
N balance will always be more of a challenge on horse
farms because there are a variety of types of operations,
including breeding, boarding, performance and pleasure,
and so forth [58] often having a high turnover rate of
horses; a Rutgers University study [59] found that an
average of only five of 13 horses remained on 20 farms
sampled after 1 year (most left the farm, some were
removed due to age, disease, physical defect, and so forth).
Manure that remains on pasture when horses are grazed
can be substantial and will also influence balance calcula-
tions and is complicated if rotational pastures are poorly
maintained. Finally, the amount of manure exported can
either help or hinder a farm’s balance depending on the
amount exported.
Whole farm nutrient balance is critical for managing N
losses in the environment. As this issue has gained public
awareness, several states have developed legislation to
manage balance on farms. Depending on the individual
state’sdefinitions of “farming,”many equine operations are
not factored into the nutrient balance equation. Many
hobby or small farms, which may have high nonpoint
source pollution potential, may not be accounted for if they
do not hit the threshold of “farming”definitions. In New
Jersey, any facility with less than eight horses is encour-
aged, but not required to establish and follow an animal
waste management plan [60], while Vermont’s new small
farm regulations accounts for larger farms, not regulating
Table 2
Concentration range (average) of crude protein (CP), nitrates (NO
3
), and nitrate-nitrogen (NO
3
-N) in common equine feedstuffs
a
.
Feedstuff % of CP % of NO
3
ppm NO
3
-N % of Nitrate
b
Dry forage
Legume hay 18.6–23.9 (21.3) 0.005–0.35 10.56–788.2 0–0.19
Grass hay 7.0–14.6 (10.8) 0–0.69 0–1,563 0–0.24
Bermudagrass hay 8.2–13.6 (10.9) 0–0.25 0–567.8 0–0.16
Straw 2.8–7.6 (5.2) 0–1.48 0–3,341 0–0.04
Wheat hay 6.8–14.3 (10.5) 0–0.53 0–1,202 0–0.17
Fresh forage
Mixed mostly grass pasture 10.9–25.1 (18.0) 0–0.23 (0.038) 0–511 (85.7) 0–0.57 (0.193)
Grass pasture 7.9–22.8 (15.4) 0–0.15 (0.052) 0–341 (116.4) 0–0.44 (0.153)
Grains
Barley 9.6–14.1 (11.9) 0.01–0.01 (0.01) 22.17–22.17 (22.17) 0.003–0.003 (0.003)
Beet pulp 7.5–11.1 (9.3) 0–0.05 (0.022) 0–116.9 (48.94) 0.004–0.02 (0.012)
Corn 7.5–10.6 (9.0) 0 0 0.008–0.008 (0.008)
Oats 10.5–14.6 (12.5) 0 0 0.02–0.02 (0.02)
Rice bran 10.6–18.7 (14.6) 0 0 0
Soybean Meal 46.4–56.0 (51.2) 0–0.17 (0.004) 0–36.9 (9.63) 0.004–0.004 (0.004)
Soybeans 34.9–44.8 (39.9) 0 0 0
Wheat 11.0–16.1 (13.6) 0–0.01 (0.003) 0–28.5 (6.60) 0
a
Data obtained from Equi-Analytical Laboratories, Ithaca, New York, Common Feed Profiles. http://equi-analytical.com/interactive-common-feed-profile/.
Accumulated crop years of 2000–2014.
b
Old Equi-Analytical method of analyzing NO
3
.
R.C. Bott et al. / Journal of Equine Veterinary Science 45 (2016) 98–106102
any farm using less than 10 acres, and/or those where
farming is not considered a business [61]. It is important to
realize that current legislation may help to mitigate some N
losses on targeted operations, although livestock farms
including horse farms of any size can have an impact on the
environment. Farms with compacted soil, bare patches of
pasture, or with manure storage located near ground water
sources should be managed to mitigate potential environ-
mental impacts [62].
The Netherlands was the first country to research and
develop methods to reduce environmental impact of live-
stock operations. Their programs focused on three solu-
tions (1) reduce nutrient inputs in when fed in excess of
requirements, (2) apply practical management solutions to
reduce nutrient usage of outputs on farm, and (3) increase
manure that is exported off farm [5]. These are all programs
that have been previously tested and implemented on
many concentrated animal-feeding operations in the
United States [51] and have plausible implications on horse
farms. Hauling manure off the property can improve farm
balance although neighboring crop farms have to be willing
to work with the producer [57]. However, equine research
and implementation is much slower to follow. It has been
reported that most horse farms in the Chesapeake Bay area
export nearly 50% of their manure [58,63] found that 58% of
New Jersey horse farms disposed of some or all manure off-
site, whereas 54% of survey farms spread some manure on
their farms.
6. Challenges to Precision-Feeding N in the Horse
Industry
In a Pennsylvania SARE Project Report [64], farm man-
agers reported it was very difficult for horse owners to
balance horse rations. Most farms used a commercial
mixed feed concentrate that could not be adjusted. How-
ever, one farm with 70 horses reported that all horses were
on individual diets. Most farms reported feeding hay from
different weekly truckloads and did not produce hay on
farm. In most cases, farm managers are unable to make
major feed ration adjustments due to these reasons. Con-
centrates often include more CP than is required to ensure
that individual AA and CP requirements have been met
when the concentrate is fed with a variety of forages with
different CP and AA contents. However, caretakers of ani-
mal species should be able to incorporate best manage-
ment practices (BMPs) that help to reduce N excretion. A
major deterrent in reaching a nearly perfect balance is feed
cost and profit[51]. However, in the horse industry, the
deterrent appears to be management traditions, conve-
nience, and a lack of independence. In the equine industry,
feed and supplement cost is less of an issue, especially on
noncommercial or pleasure horse operations. This is an
important consideration because most horse farms fall in
this category. For example, the average size of horse farms
in Minnesota participating in a pasture management pro-
gram was 10 acres with five horses [65].
When attempting to procure a valid figure on equine N
excretion in light of multiple quantities and types of feed-
stuffs provided at any given equine operation, the most
logical places to evaluate or attempt to calculate prediction
equations are going to be based on either feed consumed or
in fecal N levels excreted. Zeyner and Kienzle [66] used data
from over 250 digestion trials to establish a predictive
formula to estimate digestible nutrients in equine feed ra-
tions. They were able to confirm a strong uniformity of CP,
demonstrated through linear regression of digestible CP by
total consumed CP. Using this information and resulting
formula calculation (digestible crude protein ¼2.27 þ
0.917 CP) allows equine nutritionists to calculate a
reasonable N excretion value based on feed intake of horses
on farms. Mesochina et al [67] evaluated in vivo di-
gestibility data from horses fed 27 different forage diets to
create prediction equations for CP and other dietary pa-
rameters for horses grazing on pasture or rangeland. They
reported that fecal CP and dietary fiber were the best pre-
dictors of diet digestibility. However, the most accurate
estimates of diet digestibility were determined when di-
etary variables such as dietary CP and fiber were included
in the calculations. Readers should be cautioned that these
approaches may not be reliable predictors in horses that
are fed low protein or excessively high protein diets.
7. Programs and Regulations
In addition to the myriad of intrinsic reasons for man-
aging horses to minimize nitrogenous impacts on the
environment, horse owners, especially in populated areas
near watersheds, recognize regulations imposed by gov-
ernment. Although all horse farms fall under the jurisdic-
tion of the Federal Animal Feeding Operation, which is
administered and regulated by the Environmental Protec-
tion Agency, many states have begun developing and
passing laws which require higher standards than the
federal requirements. Most of these recommendations and/
or regulations pertain to limiting nonpoint source pollution
of state waters and fall under the jurisdiction of the state
level agricultural agencies. Whether they use terms such as
Animal Waste Management, BMPs, and Accepted Agricul-
tural Practices, most contain requirements and limit for
direct animal access to waterways, manure storage and/or
application, and grazing near wells, neighboring property,
and state waters. Recently, Vermont passed legislation [61]
which extends requirements to small farms. In the past,
Vermont required permitting for medium (150–499 hors-
es) and large (500 or more horses) farm operations. This
water quality bill includes a certification for small farm
operations, which are currently defined as farms “on which
10 or more acres are used for farming,”and have no more
than 149 horses and the lower limit has yet to be deter-
mined by the Secretary of Agriculture, Food, and Markets.
Although farms falling under these guidelines are required
to submit annual certification of compliance, the law ex-
tends to any farm, which potentially provides a threat to
water quality through potential threat of discharge or
contamination of groundwater. In addition, there are sec-
tions which will allow for compliance inspection at any
time and require water quality training for 8 hours over
each 5-year period on topics including prevention of dis-
charges, mitigation and management of storm water
runoff, land application of manure or nutrients, nutrient
R.C. Bott et al. / Journal of Equine Veterinary Science 45 (2016) 98–106 103
management planning, and other farm compliance rules
and regulations.
Horse farms may pose an environmental risk of
nonpoint source pollution because they often have higher
stocking rates than can be accommodated by natural
methods of nutrient absorption. In 2011, the Pennsylvania
Department of Environmental Protection [68] mandated
that farms with as few as one animal fitting the livestock
classification (e.g., horse, goat, and sheep) must document
the adherence to guidelines and submit a farm manure
management plan. Maryland has incorporated horse op-
erations, which bring in $2500 in gross annual income and/
or have eight or more horses under the Water Quality
Improvement Act of 1998 [69]. New Jersey has tiered re-
quirements for developing animal waste management
plans, with larger farms (>300 animal units) requiring a
Comprehensive Nutrient Management Plan, medium farms
(8–299 animal units) completing a self-certified plan, and
smaller farms being encouraged, but not required to com-
plete one [60]. Pending regulations for all horse farms
across the United States is a significant issue; Land Grant
University Extension services should be prepared to pro-
actively address through educational programs for horse
owners. This could be a major undertaking because the last
American Horse Council report indicated there were 9.2
million horses and 1.96 million horse owners in the United
States [70].
The United States Department of Agriculture Natural
Resources Conservation Service is offering an Environ-
mental Quality Incentives Program [71] to help farms in the
Chesapeake Bay area improve feed management. By help-
ing farmers formulate their rations more accurately to meet
their herd’s production requirements, these partners
(agencies and universities) are helping farmers decrease
the nutrients that are excreted while maintaining or
improving livestock production and health. These feeding
adjustments can help farmers reduce N excretions by 30%–
50%. The Environmental Quality Incentives Program in-
centives include payments and provided assistance and
cost share help to farms for ration balancing. Both Penn-
sylvania and New Jersey have begun incentivizing equine
operations/managers in these programs.
8. Conclusions
There are a myriad of factors at play in N utilization in
horses and the impact on the environment. Management of
horses and the associated N inputs and outputs is critical to
prevent negative environmental consequences such as
decreased air quality and water contamination. Precision-
feeding and waste management are strategies that can
minimize N excretion and environmental contamination.
Feed choices and combinations affect N availability and
absorption in the horse. Selection of feeds that are more
efficiently used by horses, and feeding to meet, but not
exceed the dietary protein and N requirements of a horse is
paramount. This will require an understanding of feed
choices and equine digestion, in addition to behavioral
modifications in terms of resisting the tendency to feed
more than is necessary.
In addition to feeding behavior changes, it is important
to develop some sort of integrated decision support sys-
tems (DSSs) that are more applicable to the needs of horse
operations. Karmakar et al [72] reviewed many types of DSS
programs and procedures available from academic in-
stitutions and industry, yet most of these were intended for
or aimed at more traditional livestock operations. Several
key decisions revolve around returning the nutrients to the
land to maintain N balance, yet many horse operations have
neither adequate land nor machinery to manage the
manure in this manner. An effective DSS guidance tool for
equine operators would take into consideration the com-
plexities involved in a livestock business, which often in-
cludes managing many animals on an individual basis
belonging to individual owners. Additional considerations
including potentially high stocking rates, contracted bin
removal for manure disposal, and equine business type
would need to be incorporated. Finally, calculations and
methodology are needed for creating and locating adequate
storage, covers or roofing for manure piles, drainage, and
gutter installation to divert runoff or groundwater around,
rather than through, the manure in fields and storage sites.
Because of transportation, storage, and application los-
ses, diet management is an important component of whole
farm N management in keeping water and atmospheric
losses at a minimum [51]. Reducing dietary N has been
shown to reduce both ammonia runoff and volatilized N.
Precision feeding can achieve those results without
affecting production. Further investigation into CP, lysine,
and other AA requirements of horses is also an essential
component of moving toward more efficient feeding.
Studies detailing the actual requirements of horses at
maintenance and various physiological states will enable a
movement away from exceeding requirements due to
feeding based on predictive regressions. Current legislation
in several states incentivizes or requires horse owners to
develop more precise feeding strategies or to manage their
farms to reduce potential negative consequences of N on
the environment. Although related legislation has not yet
been enacted in all states, the evidence of the potential
environmental implications of horse and other livestock
operations should provide horse owners with incentive for
incorporating BMPs in feeding and manure management
on their farms.
Acknowledgments
This project was supported in part by the US Depart-
ment of Agriculture, Multi-State project NE-1041 and
renewal NE-1441, Environmental Impacts of Equine Oper-
ations [73]. The authors thank Rozanne McGrath for
editorial assistance.
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