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Horse Trails and Other
Environmental Issues
By Adda Quinn
Abstract
Groundwater, surface water, stream crossings,
wildlife, invasive plants and impacts on trail surfaces are discussed.
Bacteriological and nutrient effects on water bodies from trail horses
are not detectable. BMPs exist to prevent trail horses from eliminating
in water bodies. Horses are recognized by wildlife as prey animals by
their look, smell and sound. Wildlife is less afraid of horseback riders
than hikers. The role of the horse as a disperser of weed seed is not
known. Most food passes through the horse gut within 48 hours. While
seeds can survive longer, they face formidable odds of survival. Two
studies of horse impacts on trails are reviewed.
Horse Manure on Trails and Water Quality
Compared to other large livestock, horse manure is
relatively "dry" and "hot" due to its high nitrogen
that makes it capable of fermentation (Hill web ref.). Once deposited,
it can achieve total mineralization in as short a time as 21 days (Ajwa,
et al 1994). Because it is so dry at excretion, nutrients tend to
volatilize rapidly into the atmosphere. One of the challenges in
preserving nutrients in horse manure is to get them turned into the soil
as rapidly as possible before the nutrients are lost to the air. See
EnviroHorse papers Horse Manure Aging and Nutrient Content and
Implications for Trails, and Horse Urine and Implication for Trails
Groundwater
There are no studies that we have been able to find
implicating equids in groundwater contamination. Horses primarily
eliminate in their pastures and paddocks (Meyer 1997). Manure left in a
loose heap in deposits on trails loses its nitrogen rapidly (New
Hampshire 1990). It is inconceivable that trail horses making dispersed
deposits could possibly impact ground water. Most contamination of this
sort occurs from areas associated with feedlots where thousands of
commercially harvested animals are confined at one time, or from
excessive fertilization added to soils.
Surface Water
Again, there is very little data about impacts from
horses. Bacteriological and nutrient effects (on water bodies) are
seldom detectable except next to stables. (Williams et al, 1998). As
part of the 319 (h) grants from the Clean Water Act, new data has
recently become available here in the SF Bay Area. In addition, some
data is now available from watershed monitoring efforts. Here is some of
what we know:
- USDA RCD Demonstration Project at Black Dog
Ranch, Half Moon Bay
This property is bisected by Pilarcitos Creek.
Horse paddocks were literally on the edge of the creek bank. Water
sampling taken Spring of 2000 showed no nitrates, ammonia, P, or K
above allowable limits. While sodium levels were elevated, their
source was somewhere upstream of the property. The property owner is
undertaking voluntary pollution prevention measures by pulling fence
lines back from the creek and planting vegetative buffer strips. See
San Mateo County RCD for data.
- San Francisquito Creek CRMP. This watershed
monitoring effort for the San Francisquito and Los Trampas Creeks
has taken periodic water samples. The pollutants that would have
been expected from equine sources were generally not present at
levels of concern. Near the Piers Ranch some samples showed nitrate
spiking which was originally thought to be the horses, but turned
out to be a nursery upstream that was storing compost adjacent to
the creek. They were asked to move the compost piles away and water
quality improved subsequently. In the meantime the Piers Ranch
voluntarily undertook vegetative buffering on their property to
assure that they would not become a source of pollution. A
comprehensive watershed monitoring program is being designed for
long-term monitoring purposes by SFCCRMP.
- In February 2000 surfers at the Fitzgerald
Marine Reserve reported high levels of coliform pollution in the
ocean near where the "creek" (which is in a rusted iron
pipe) dumps into the ocean. Immediately, the two commercial horse
properties up stream on San Vincente Creek were accused of being the
source. Both of these properties had already undertaken berming,
creek revegetation, vegetative buffer stripping, rocking and other
measures to assure that their operations were environmentally benign
so it seemed unlikely that they could be the source. Both
proprietors welcomed County inspectors to sample on numerous
occasions. Sewage was found to be going directly into the creek from
ABOVE their properties. That source was corrected. Subsequent
monitoring has provided data that indicates that the creek water is
leaving cleaner than when it enters these properties, and at
acceptable levels of coliform. However, coliforms are still high at
the ocean apparently implicating the broken and rusted sewage pipes
serving residences in Moss Beach downstream of the horse property.
See San Mateo County Health data from 2000.
- Family Farm Road, in the heart of horse
property, flooded badly in the winter of 1996. Coliform levels again
were involved. The source was quickly traced to leaking aged human
septic systems upstream. Data from Stanford Property Management.
- Coliforms are ubiquitous to the environment.
While they are not necessarily harmful to people if ingested,
coliforms are an indicator that unwanted matter is present in the
water system. Their virulence is little understood, hence the
precautionary care to prevent human exposure to excess amounts of
them. Coliforms however have not been known to injure aquatic
organisms or wildlife. See Dr. Michael Rugg, Toxicologist,
California State Fish and Game, Yountville.
It is important to keep the fact in perspective
that these studies involved settings where horses lived 24 hours/day
next to a creek. They did not adversely affect the surface waters
immediately adjacent to them. Given this, it is difficult to conceive of
a situation where a few horses on a trail could adversely impact surface
water nearby. Again, most trails are not sited immediately adjacent to
water bodies and Mother Nature has a marvelous buffering capacity when
even as little as 10 feet of vegetation is available at the side of a
trail. www.ca.nrcs.usda.gov/rts/sec4.htm
The Pfiesteria
piscicia Hysteria: Pfiesteria is an
estuarine dynoflagellate that sometimes behaves like a plant, and
sometimes like and animal. It goes through several life stages (count 'em:
24!), some of which produce neurotoxins that attack fish along the
eastern seaboard and into the Gulf of Mexico where it has been
ubiquitous for thousands of years. It appears to thrive in nutrient rich
waters and has been responsible for some spectacular fish kills
recently. "The University of Minnesota did extensive Pfiesteria
piscicida studies in downstream riparian habitats from upstream horse
farms and found no significant increase in bacterial levels that would
harm aqua biotic life forms from mayfly larvae to fish....." is a
quote taken off of the internet and sent to EnviroHorse. We are seeking
the citation now. Science has not yet fully understood the mechanisms
involved in these sudden blooms and fish kills. Keeping manure away from
water is certainly the best policy, regardless.
Stream Crossings
There is a trend to protect stream banks from
erosion by trail use and discourage trail users from disturbing streams
that support fisheries. Bridges are being the built across historic
fords. It is known that as little as 0.025ppm of ammonia in water can
kill salmonoid species.
Equestrians are being educated not to allow their
animals to eliminate during stream crossings. Best management practices
(BMPs) have evolved such as stopping prior to a crossing to allow the
animal to rest, relax and (hopefully) eliminate PRIOR to the crossing.
Simply not allowing the equid to stop and dawdle in the water will also
help to prevent contamination. Many horses do not like getting their
feet wet and have a natural aversion to taking any more time than
necessary in water. EnviroHorse has begun to collect data on horses
eliminating while crossing streams. It is expected to take several years
before a robust database is available. But preliminary data collected in
2000 indicated very few horses eliminated during a stream crossing.
The following internet references can provide
additional information to help design stream crossing for fish passage:
http://swr.nmfs.noaa.gov/habitat.htm
for stream crossing guidelines and www.wa.gov/wdfw/hab/engineer/habeng.htm
for fish passage design at road culverts. (CBARCDs 2001).
Horses and Wildlife
Horses are prey animals. They have eyes on the
side of their head. They are herbivores and leave the trace scent of an
herbivore on the trail. Humans and dogs are predators. Their eyes are on
the front of their face. As the walk, they leave the trace scent of
omnivore on the trail that can impact wildlife.
Horses are recognized by wildlife as prey animals,
even when a person is sitting on their back. An approaching horse
passing along a trail provides sound rhythms in the cadence of a
four-footed hoofed prey animal to wildlife, which informs wildlife of a
non-threatening presence. Being warned diminishes flushing/flight
response that consumes wildlife energy. It is very common to find deer,
bobcat and coyotes that allow horses to get within feet of them on
trails before calmly moving off. Horses may, thus, be more compatible
with wildlife than other user groups.
Bennett and Zuelke (1999) undertook an extensive
review of recreation effects on birds and concluded that disturbance
from recreation has temporary effects on behavior and movement of birds.
Direct approaches caused greater disturbance than tangential approaches,
rapid movement by joggers was more disturbing than slower hikers,
children and photographers were especially disturbing, and passing or
stopping vehicles were less disturbing than human foot traffic. Horses
and riders did not disturb birds.
Sporadic human use can disturb wildlife. However,
"many animals are less afraid of horseback riders than hikers.
Riders seldom dismount to touch flora or fauna. Riders can be a
dedicated and energetic volunteer and advocacy group. The horse-rider
relationship promotes a non-anthropocentric worldview that facilitates
ecological understanding. Horses are useful for patrols and surveys.
Horse traffic can be used to maintain firebreaks and seldom-used
trails." (Williams et al, 1998)
Invasive Plant Species
EnviroHorse has recently undertaken a literature
survey to determine the relationship between horses and the spread of
exotic plant species. Very little exists in the literature specific to
horses. Exotic seeds appear to have been brought to this continent via
two primary routes: discharge of ballast from ships and importation of
hay bales and other forage food used to establish the cattle industry.
Many of these weeds are in fact extremely toxic to horses (star
thistles, knapweeds, etc.) and are as big a problem to horses as they
are to native plant societies because they ruin pastures and can kill
our animals.
Good horse feed is not laden with exotic weed
seed. Most horse owners would not buy junk hay for their horses. Garbage
in, poor performance out! The days of large populations of lower-grade,
inexpensive stock horses are long gone. Horses cost money, and the
purchase price is only the down payment for ownership. Horsemen cannot
afford to compromise their investments by feeding weedy hay. Responsible
horse owners are concerned about getting quality feed that has been
properly planted, harvested, cured, and marketed for their horse food
dollar. Many horses are fed processed feedstocks. This is forage that is
heated. The heat destroys weed seeds.
The horse has a very inefficient gut: it's a
one-way through-put system. Horses are physiologically incapable of
vomiting or regurgitating. If something gets stuck on the way through,
the only way to get it out is by surgery or physical intervention (at
arm's length!). As a consequence, horses must be fed carefully to avoid
the common and potentially fatal condition of colic. It is unknown to
what extent forage available contains weed seeds. The bulk of
unprocessed forage consumed by SF Bay Area horses is alfalfa (Medicago
sativa), rye grass (Lolium multiflorum or perenne), Timothy hay (Phleum
pratense) and oat hay (Avena sativa (white cultivated oats)). If horses
were a vector of seed spread, these grasses would be prevalent in our
open spaces and parks, but they are not. Nor are these forage feeds
listed on the California Exotic Pest Plant Council web site www.caleppc.org
at this time.
The Nature Conservancy lists of invasive species
of concern on a website maintained by UC Davis at http://tncweeds.ucdavis.edu/photos.html
where Dr. Barry Meyer-Rice is their
expert. He cautions that feedstocks can be weedy, and that in certain
places have become exotic invaders, notably, Yellowstone Park. A Montana
study by Tyser and Worley 1992 implicated timothy (Phleum
pratense) and bluegrass (Poa
pratensis) as species that had been
included in past roadside seeding by the local highway authority. In
California, the Department of Transportation (CalTrans) has recently
been identified as the number one spreader of yellow star thistle by its
past practices of scattering various weed-laden hays during roadside
rehabilitation projects to control erosion.
I spoke with Dr. Meyers-Rice who said there were
no definitive studies implicating horses with weed seed spread, but that
it is likely that seeds could be passed from manure and collected/spread
via tails. He felt that seeds transmission via hooves was unlikely (pers.
comm. 2000). Large herbivores carry seeds inside them, not outside them
in hair. (Jansen, 1984) Probably the best practice to assure no weed
seed is spread is one to which trail riders regularly attend already:
horse grooming. Trail horses are brushed and curried prior to saddling.
Their hooves are picked prior to mounting, and often picked two and
three times during a long ride. Hooves and hair are unlikely to be a
vector of seed transmission with standard grooming practices.
As mentioned above, the literature contains very
few references to weed seed being spread by horses. One reference
states, "The majority of seeds recovered from pastured livestock
are the common and desirable forage species of grasses and
legumes". (Dore et al 1942). EnviroHorse, the Bay Area Barns and
Trail Trust, and other sponsors have raised funds and are in the middle
of a three-phase research project. Phase 3 intends to take another look
at weed seeds in horse manure to determine if the findings from Dore
remain current.
Most veterinarians will confirm that the bulk of
food ingested by horses passes through the gut within 48 hours. This is
confirmed by many literature citations (Argenzio et al 1974). However,
seeds can survive longer as demonstrated in research by Janzen (1981,
1984). However, a seed surviving the gut does not mean that it will
propagate. Among his other findings about seeds were:
- If a seed germinates immediately in the dung,
it may be killed by the community of dung-degrading organisms.
- Severe mortality is likely through inter- and
intraspecific crowding.
- The longer the seed is retained in the gut, the
higher its likely mortality.
- "While an Enterolobium
seedling germinating in a dung pile is picturesque, its chances of
surviving the dung beetles, mice, dryness, and root exposure
characteristic of dung piles are very slim. In such a system it is
not at all certain that scarification during passage through the
mammal gut is the event that natural selection has favored during
the evolution of the Enterolobium-disperser
interaction."
- "The seed that is so impervious that the
horse does not kill it by molar or chemical scarification may then
be so slow to be scarified by soil processes that it has
significantly fewer generations than the less impervious morph.
- " Success in the breaking of the dormancy
of seeds is not necessarily measured by the length of the period of
dormancy (inverse germination rate) but is based on a complex
interaction with habitat and weather conditions at the time of
germination."
In a Masters Thesis (1989), Benninger studied
disturbed trail edges as corridors for invasive plant species and the
role of the horse as a dispersal mechanism for weed seeds. She actually
did a greenhouse study on seeds extracted from horse manure. Seeds from
15 plants grew, but only 8 were identified. While all 8 were exotic,
NONE of them appeared in her test plots on the trail, although she
observed them elsewhere along trails. She concludes that "horses
MAY provide a means of entry of exotic species" but notes that the
horses from the stable in the park which were not allowed to graze in
the park and had controlled diets were not a problem. None of her
references include analysis of weed seed survival, notably an important
study by Harmon and Keim demonstrating actual weed death in horse
manure. Benninger says seeds can pass through horses unharmed and be
deposited in feces. Such seeds can be left in concentrated, mineral-rich
patches along trails, where they may germinate after dung is decomposed
(Janzen 1981, 1984). This has led to the assumption that manure is a
major source of exotic species (Dale and Weaver 1974, Hammitt and Cole
1987); "however, I have found no documentation of this in the
literature." Neither has EnviroHorse a decade later.
Harmon and Keim studied barn animals that were
each fed 1000 weed seeds. Chickens, sheep and horse guts seem to be able
to destroy weed seeds more thoroughly than calves and hogs. An average
of 14.2% of uninjured seeds was recovered, most within 48 hours of
ingestion. Of these, only 6.7% were viable. The uninjured weed seeds
were then buried in horse manure where a temperature of 158ºF was
achieved in 2 weeks. At the end of one month, only velvet weed,
bindweed, peppergrass, sweet bindweed and sweet clover were viable. The
rest of the weeds were partially decomposed. Only the bindweed and sweet
clover were viable at the end of the second month, but after 3 months
only one weak bindweed seedling was obtained (1% viability). All other
seeds were dead and had started to decompose at 4 months.
All trails fragment habitats and eventually
develop the "ribbon of foreigners" characteristic of trailside
vegetation, but horse trails don't have to be wider or worse than hiking
trails. Horses can carry viable seeds, but the dispersal process is
complex and the facts are largely unknown. Special feeding programs
offer a solution when real problems exist. (Williams et al, 1998)
EnviroHorse will be doing more research on this
subject in preparation for the field studies on weed seeds from horse
manure. Donations are needed for this project.
Horse Impact on Trails
This is a confusing study area. There are papers
that discuss soil compaction by horses, and others that indicated that
horses stir trails up. EnviroHorse will prepare an assessment of
literature available in the future. For now, two resources may be of
interest.
In a 5-year study, Summer (1990, 1996) concluded
that horse traffic was not the single, dominant process active on
trails. Trail degradation was also a function of landform, climatic and
catastrophic events, and geomorphic processes. Seasonal use was
important in keeping the soil exposed and vegetative cover absent on
trails. Such processes as sheetwash, rilling, gullying and soil creep
actively modified and eroded the trails and resulted in a measurable
fluctuating rate of change over time. Limited data suggested that foot
traffic produced effects similar to horse traffic in exposing the trail
to the effects of geomorphic process or climatic events. Intensive
runoff resulting form natural events can cause significant geomorphic
change in a trail from such processes as gullying and earth slumps.
Erosion from these events may overshadow effects of seasonal horse use
on trails
According to Williams et al 1998, horse hooves
loosen soil; feet compact it. Horses may destabilize hilly/sandy/mucky
trails, but may maintain permeability of level or hard surfaces. Horse
traffic may cause a washboard effect on some rocky trails. Management
can mitigate damage. As a general rule, horses wear trails three times
more than hikers (and less than bikes or ORVs). But factors other than
user type are more closely linked to trail degradation. Lightly used
trails may grow over and require more maintenance, whereas moderate
horse activity may help to maintain a multiple-use trail. Traffic on
soft or wet stretches must be managed carefully. Horses may loosen
hillside soils and initiate erosion on poorly designed or mismanaged
trails. Continued erosion is not directly related to level of horse use.
Erosion will demand maintenance on steep slopes.
The bottom line is that horse trails can be
maintained on most natural areas without unacceptably impacting
ecological values.
Conclusion
The impact of the horse on trails offers very
little risk to the environment. The risk of horses eliminating and
impacting groundwater or surface water is very small. Best management
practices are available for stream crossings. Horses may be the user
class of least impact to wildlife. The role of the horse as a disperser
of weed seed is not known. Most food passes through the horse gut within
48 hours. While seeds can survive gut passage, they face formidable odds
of survival. Two studies of horse impacts on trails were reviewed.
Disclaimer
These materials have been prepared by EnviroHorse for information
purposes
only and are not legal advice. Subscribers and online readers should not
act
upon this information without seeking professional counsel. Every
attempt has been made to assure that the information contained in this
publication is accurate. EnviroHorse assumes no responsibility and
disclaims any liability for any injury or damage resulting from the use
or effect of any product or information specified in this publication.
About the Author
Adda Quinn was with a nationally known research institute for 21
years prior to her retirement. She has done research both on global
climate change and contaminated soil and groundwater issues. As a trail
advocate, she has participated in a variety of regulatory debates, both
nationally and locally. She is a founding member and Board of Director
for EnviroHorse. If you have any scientific studies that you think would
be helpful, these papers are a work-in-progress and EnviroHorse would
love to have copies of them. Please contact us at mailto:envirohorse@yahoo.com.
If you have found these citations helpful, please consider a donation to
EnviroHorse to help us find and sponsor more research. Papers are housed
at www.californiastatehorsemen.com/envirohorse.htm
References
Ajwa, H. A. and Tabatabai, M. A. 1994. "Decomposition of
different organic materials in soils". Biol. Fertil. Soils
Vol. 18. Pp. 175-182.
Argenzio, RA, JE Lowe, DW Pickard, and DE Stevens. 1974. Digestive
passage and water exchange in the equine large intestine. Amer. Jour. Of
Physiol. 226:1035-1042.
Bennett, KA and E. Zuelke . 1999. The effects of recreation on birds:
a literature review. Delaware Natural Heritage Program, Smyrna, DE
19977.
Benninger, M. 1989. "Trails as Conduits of Movement for Plant
Species in Coniferous Forests of Rocky Mt. National Park, Co.".
Miami of Ohio Univ. Masters Thesis
CBARCDs. 2001. Horse Keeping: A Guide to Land Management for Clean
Water.
Dore, W., and L. Raymond. 1942. "Pasture Studies XXIV: Viable
Seeds in Pasture Soil and Manure". Scientific Agriculture Vol. 23
No. 2. Pp. 69-79.
Harmon, G., and F. Keim. 1934. "The Percentage and Viability of
Weed Seeds recovered in the Feces of Farm Animals and Their Longevity
when Buried in Manure." American Society of Agronomy Vol. 26. Pp.
762-767.
Hill, C. www.horsekeeping.com/horse_management/manure_management.htm
Janzen, D. 1984. "Seed Dispersal of Small Seeds by Big
Herbivores: Foliage is the Fruit". The American Naturalist. V1223,
pp.338-353.
Janzen, D. 1981. "Enterolobium Cyclocarpum Seed Passage Rate and
Survival in Horses, Costa Rican Pleistocene Seed Dispersal Agents".
Ecology 62(3), pp. 593-601.
Meyer Dr. Deanne. 1997. "Horses spend most of their time in
pastures or paddocks where the majority of their excrement is deposited,
collected and managed. Horse manure is about 70-80% liquid and 20-30%
solids." Personal communication, UCDavis Manure Management
Specialist.
New Hampshire Dept. of Ag and USDA. Good Neighbor Guide for
Horse-Keeping: Manure Management. 1990.
Summer, RM. 1996. Geomorphic Impacts of horse traffic on montane
landforms 41(2): 126-128.
Summer, RM. 1980. Impacts of horse traffic on trails in RMNP. J. Soil
and Water Cons. 35(2): 85-87.
Tyser, RW and CA Worley. 1992. Alien flora in grasslands adjacent to
road and trail corridors in Glacier National Park, Montana USA.
Conservation Biology 6:253-262.
Williams, B. and L. Conway-Durver. "HORSE TRAILS IN ECOLOGICAL
RESERVES" presented at Clemson University Horse Trails Symposium,
1998. Mr. Williams holds a BS Degree in Forestry from Clemson
University. He worked for the U.S. Forest Service as a River Ranger on
the Sumter National, organized the Association of Forest Service
Employees for Environmental Ethics and is the Executive Director of the
Chattooga River Watershed Coalition.
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