Download Sheep Effects on Soil, Vegetation and Habitat Conditions

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Assessment of Sheep Grazing for Noxious Weed Control on
Missoula Conservation Lands –2008
Final Report by George Hirschenberger
Summary of 2008Grazing Season
The City of Missoula owns Conservation Lands north of Interstate 90 on both sides of
Rattlesnake Cr. Approximately 620 sheep (270 ewes with 350 lambs) and three goats
were pastured on these lands beginning on May 12, 2008. A mounted herder, a herd dog
and at least two guard dogs were with the sheep full time until the sheep were removed
from these lands on October 6th. Death loss, primarily from coyote predation between
mid-June and mid-July, reduced the number of sheep by about 35 during the season. In
August approximately 200 lambs were separated from the flock and taken home. Three
additional herd dogs were added in early September to reduce predation. The sheep were
always penned at night to reduce predation.
The kinds and amounts of plants grazed by sheep varied with the weather, the phenologic
stage of each species preferred as forage, and the overall composition of the plant
communities available to the animals. Under spring conditions, which began the first day
of grazing and lasted until mid-June, sheep used native forbs more than at any other time.
Summer weather patterns set in about June 15 and sheep forage preference quickly
shifted from a varied spring diet that included native forbs to a more specific diet which
consisted mostly of introduced forbs -primarily leafy spurge (EUPESU), Dalmatian
toadflax (LINDAL) and, to a lesser degree, spotted knapweed (CENMAC). The shift to
fall conditions was more gradual. Late August and September rainfalls recharged soil
moisture and that resulted in re-growth of native plants, mostly grasses. As the weather
cooled and the supply of palatable, invasive forbs was exhausted, the sheep began to use
more native grasses and, to a lesser extent, native forbs in their diet.
Chronology
Grazing began in the Waterworks/North Hills pasture on May 12th. Here the sheep grazed
in relatively small, dispersed groups and showed preference for five non-native plants. In
approximate order of preference they were tumble mustard (SISALT), Dalmatian
toadflax (LINDAL), leafy spurge (EUPESU) and two annual grasses, cheatgrass
(BROTEC) and Japanese brome (BROJAP). Consistent use on two native forbs -blanket
flower (GAIARI) and stone seed (LITRUD) –was noted in the aftermath of spring use in
this area.
On May 27th the sheep were moved to the upper south- and west-facing slopes of Mt
Jumbo. They were night penned near the hilltop and allowed to wander and graze in
small, dispersed groups during the days until June 17. Prior to June 17, the sheep’s diet
-2was similar to that noted for the Waterworks/North Hills but included many more native
forbs. This was probably due, at least in part, to greater species diversity and greater
abundance of native species. New species that were consistently grazed included sticky
geranium (GERVIS) wild onion (ALICER), silky lupine (LUPSER) and yellow bell
(FRIPED). While the sheep were not reliant on daily water prior to June 17 (the herder
said some went up to six days without going to the water trough, relying on rainwater and
snow instead) trails converging at the water source were obvious. Some of these trails are
on steep gradients and could accelerate localized erosion.
An abrupt shift to a “summer diet” and summer grazing patterns was noted on June 17th
when a water source near the East Missoula interchange of I-90 was installed. The area of
grazing use in the mornings immediately changed to the lower and mid-elevation southand east-facing slopes of Mt Jumbo. The sheep began to use leafy spurge and Dalmatian
toadflax as the mainstay of their diet. As the sheep herd trailed from the night pen across
the south-facing slopes to water, they would graze and re-graze leafy spurge (EUPESU)
and Dalmatian toadflax (LINDAL) plants. After mid-July, when spotted knapweed
(CENMAC) had begun flowering, the sheep selected this plant as well while trailing to
water. While afternoon grazing was more dispersed on the east and south facing slopes,
the sheep still used primarily these three invasive plants in their diet even though a good
variety of native forbs and grasses were available to them as they grazed upslope and
toward the night pen. The three native forbs that were consistently included in the sheep’s
summer diet were blanket flower (GAIARI), stone seed (LITRUD), and silky lupine
(LUPSER), but none of these three were used to the extent seen in the spring. The
heaviest use observed on a native forb during the summer amounted to 22% of blanket
flower stalks grazed along one transect line. Blanket flower was in full-flower when this
transect data was collected.
The sheep left Mt Jumbo on July 17th and returned to the Waterworks/North Hill for 6
days. Their diet here was made up primarily of the flowering stalks of leafy spurge,
Dalmatian toadflax and spotted knapweed. This treatment resulted in re-grazing of some
plants and a first grazing on others. Some occasional grazing use was observed on
blanket flower seed stalks in the seed-ripe and dormant stages. While not abundant, stone
seed flower stalks did show some summer grazing as well.
On July 23rd, the sheep were moved north to the Sunlight area west of Rattlesnake Cr.
Most of the area available for grazing was dominated by grasses, nearly all of which were
seed ripe and dormant. The sheep actively sought out leafy spurge while in this pasture
trailing nearly a mile to areas where leafy spurge was available while making minimal
use of other plants along the way. Other plants consistently grazed by sheep in the
Sunlight area included spotted knapweed (CENMAC), hounds tongue (CYNOGL), stone
seed (LITRUD) all of which were flowering, and goats beard (TRADUB), tumble
mustard (SISALT) and silky lupine (LUPSER). The herder’s opinion, borne out by
observations that some plants were grazed 3 or more times, was that the sheep were
running short of palatable forage before they left this area.
-3On August 14, the sheep were moved to the west-facing slope of Mt Jumbo, about 4
miles north of I-90. A night pen was set up near the ridge-top saddle and the sheep grazed
mostly on the west slope within about ½ mile of the night pen until mid-September. Here,
the animals showed a definite preference for leafy spurge and Dalmatian toadflax and to a
lesser degree, spotted knapweed. From mid-August until early September, most of the
native forbs and grasses in this area were dormant and little use was noted on native
species. By mid-September, some native species, especially cool season grasses, had
appreciable, new fall growth. This combined with the dry and dormant condition of the
remaining leafy spurge, Dalmatian toadflax and spotted knapweed plants resulted in a
shift in diet toward native grasses after mid-September. Between mid-September and
October 6, the sheep grazed mostly near the ridgeline and on the high elevation east
facing slopes.
Sheep Effects on Soil, Vegetation and Habitat Conditions
Soil
Most of the soils in the area that was grazed in 2008 are aridisols and molisols with a
high percentage of course fragments and good permeability. These soils generally resist
compaction and erosion well. No widespread problems with soil stability should result
from 2008 grazing levels. Site-specific adverse effects of sheep grazing during the 2008
grazing treatments include the trampling of all vegetation in and immediately around the
night pens that altogether make up less than five acres. Repeated use of some favorite
loafing/siesta areas resulted in similar but less intensive disturbance. These disturbances
could lead to the successful establishment of new, undesirable plants or at least an
increase in already established annual ‘weeds’. The accumulation of sheep feces and
heavy trampling of vegetation in areas of concentrated use may also bring objections
from the recreating public.
Sheep trails to water, in particular the source near East Missoula, were very well trodden
and noticeable to most observers. The sheep owner, his herder and employees of
Missoula Parks and Recreation did what was possible to route the animals around the
steepest slopes but, in places, the topography of this area make access to and egress from
water impractical except by steep trails. These trails provide bare ground that could result
in localized, accelerated soil erosion and a seedbed for invasive species. Rills could form
where the trails follow steep grades or are aligned with the fall line on steeper slopes
especially if repeated use occurs in future years
Recommendations: Better location of water near the “L” could reduce the concentration
of trails on erodable molic soils. Reseeding the night pens and other areas of concentrated
use just before the sheep move off with an infertile cover crop would reduce the potential
for wind and water erosion and may help restore productivity in these and other heavily
trampled areas. Expanded use of drift fencing to break up trailing patterns and, in some
instances, more active herding could help direct sheep away from areas that could
become localized problems.
-4Vegetation
Generally, because the sheep used primarily forbs in their diet, it can be assumed that
grasses, sedges and other disregarded plants will benefit from the reduction in
competition from the selected forbs. Unfortunately, this would be true for now
widespread introduced species such as cheat grass and Japanese brome, as well as for the
native species. While the sheep did browse some shrubs, especially in the fall, use levels
were relatively light and not detrimental in most areas. Exceptions to this were loafing
areas near the bed ground in the Sunlight and Mt Jumbo saddle areas where shrubs and
trees were more heavily used.
The effect of grazing on perennial forbs, both native and introduced, is harder to evaluate.
The sheep sought out and heavily grazed tumble mustard until the plants were dry and
dormant. Sheep strongly preferred leafy spurge in their summer diet. They would seek it
out, graze and re-graze it and often eating all but the coarsest stems. Dalmatian toadflax
was not quite so heavily preferred but was also a consistent component of the sheep’s
diet; use was heaviest during the summer. Spotted knapweed was less preferred –stem
leaves and flowers were usually taken only when the plant was flowering. While
consistent heavy grazing can be expected to reduce plant vigor and no doubt reduces
annual seed production, spurge and knapweed have proven resistant to repeated heavy
grazing. The effect of heavy grazing on Dalmatian toadflax may be detrimental but is not
well documented. Therefore, the effect of one year’s grazing on these plants is expected
to reduce vigor and seed production but will likely have no long-term effect. If repeated
annually, grazing should be more effective in reducing populations of spurge and
possibly toadflax. The amount of grazing use observed on spotted knapweed in 2008 is
not expected to have much effect on this species. Tumble mustard, an annual, should be
reduced in density quickly since grazing removes the current years flowers.
While consistent grazing was observed on three native forbs –blanket flower, stone seed
and to a lesser extent silky lupine, it is not certain that grazing will be detrimental over a
period of years when compared with no grazing. This because the tolerance these species
have for grazing is not well documented and because there is solid evidence that nonnative plants will continue to out-compete these and other native plants if uncontrolled.
While grazing during the growing season can be assumed to reduce the vigor and density
of native forbs, this may be offset by the concurrent reduction in vigor and reproduction
in aggressive non-natives.
Long-term changes in the overall plant species composition due to sheep grazing cannot
be inferred from this investigation. Currently, there are permanent monitoring plots in the
North Hills that were installed to determine the amount of weedy species removed by
sheep grazing. But data from these plots won’t provide any direct measurement of plant
species composition over time since native forbs and grasses were lumped when the plot
data was collected.
-5Recommendation: A monitoring plan should be developed and implemented for these
lands. Permanent trend plots are recommended as a reliable way to detect changes on
these rangelands. Using Daubenmire techniques to estimate canopy coverage, frames
placed along a permanent transect line would provide site-specific information on overall
species composition, litter and bare ground. Other techniques, more sensitive to change in
individual species, can be used to monitor changes in the frequency and density of
targeted invasive species or species of special concern.
Utilization transects, monitored during the active grazing period to gauge the level of
grazing use on native forbs, could be used to set thresholds or trigger points: When
predetermined use levels are reached, grazing animals would be relocated.
Spring Grazing
No direct observations were made of sheep grazing prior to June 13. Therefore, the
following discussion and recommendations on how to reduce the probable adverse effects
of spring grazing are based on an inspection of the grazing aftermath in areas used by
sheep between early May and mid-June.
Spring use in 2008 resulted in heavy use of tumble mustard (SISALT) but relatively light
utilization on the most of the other targeted invasive plants. At the same time, the sheep
made relatively frequent use of many native forbs, especially those in flower. This calls
into question the effectiveness of spring grazing in protecting or restoring populations of
native forbs. In the absence of reliable data, the following assumptions are made: The
2008 spring diet of the sheep represents a fairly typical spring diet of these and other
sheep in other years and, that populations of native forbs that reproduce primarily from
seed will be diminished by repeated, annual grazing during the active growth and
flowering period.
Recommendations:
Using the above assumptions and provided that protecting and restoring native forbs is a
goal on Missoula Conservation Lands, minimizing grazing where native forbs are well
represented until mid-June should be considered. The best season of use is hard to pin
down. The phenology of preferred forage plants, both native and introduced, and weather
appeared to play an important role in the shift to a ‘summer’ diet in 2008. Another factor
in forage preference is the animal’s need to gradually become accustomed to some forage
species. For example, it may take three weeks or more of exposure to leafy spurge before
large amounts can be properly digested. In response, the sheep will likely use relatively
little of the plant until their systems are accustomed to the new forage. Considering all
these factors, some flexibility in planned grazing dates is needed to make the best use of
sheep for weed control.
If the practicality of continued sheep grazing for weed control depends on providing
forage prior to mid-June, other alternatives to repeated spring use should be considered.
Making adjacent ‘invaded’ pastureland available for spring use would present useful
options. Alternative pasture that is readily accessible could eliminate or reduce spring
-6grazing every other year, or two years out of three, on sites where plant communities
have been invaded but are still dominated or co-dominated by a community of native
plants.
While the land manager and sheep owner have had pre-grazing meetings to discuss the
location and timing of grazing, there is still an opportunity for improvement. Moving the
sheep rapidly into and out of specifically designated areas could further reduce adverse
effects of spring grazing on native forbs. For example, when a predetermined utilization
level on non-target native forbs is reached it could trigger relocating the animals to a new
area. To be sure, shorter duration grazing of relatively small areas will present challenges.
The animals cannot be forced to graze areas they find unsuitable. And the logistics of
moving the animals through residential neighborhoods, steep and forested terrain, across
Rattlesnake Creek, providing adequate water, a secure night pen and accommodations for
the herder and his dogs –all these be addressed. Still, herded sheep offer the opportunity
to finesse daily grazing management. To the extent that it is practical, predetermined
utilization levels that suspend grazing in specific areas should be considered as a way to
mitigate the possible adverse effects on native forbs.
Summer Grazing
The selective summer grazing of invaders seen in 2008, especially in areas that have a
relatively high density of leafy spurge and Dalmatian toadflax, will very likely be a
benefit to –or at least not harm -most native species. The possible exceptions to this are
blanket flower and stone seed since the sheep continued to make consistent use of these
two species in July and August when most other native forbs were only lightly used.
On some sites with dense stands of leafy spurge and Dalmatian toadflax, the sheep grazed
nearly every plant. However, these heavily invaded sites usually have a low abundance of
native, desirable plants and a good supply of cheat grass and Japanese brome seed. If the
perennial invaders are reduced, the two introduced two annual grasses can be expected to
replace them.
Recommendation:
A long-term restoration strategy that relies on more than grazing, herbicide use and
biological controls is needed to re-establish native or at least beneficial plant
communities. Some small-scale restoration efforts on Missoula Conservation Lands that
include re-introduction of desirable plant species should begin as soon as possible. Trial
and error on a small scale is likely the best way to determine what steps will be most
effective in replacing noxious and invasive plants with a community of beneficial
species.
Fall Grazing
Under the moderate stocking rates of 2008, sheep grazing after the summer heat has past
is expected to have neither a beneficial nor detrimental effect on most of the native,
-7introduced and invasive plants on Mt Jumbo. If more leafy spurge and Dalmatian
toadflax had been available to the sheep in September, they would likely have continued
to graze these species. Possible poor animal performance after mid-September because of
the lack of palatable forage is a concern of most importance to the sheep owner.
Wildlife and Wildlife Habitat
There is at least one potentially serious adverse effect of sheep grazing on these lands.
There is a small population of bighorn sheep that periodically use the same area where
the sheep were pastured. The transmission of disease and parasites, especially lungworm,
has been voice as a strong concern by some wildlife managers but discounted by others.
The concern that sheep grazing on Missoula’s Conservation Lands could spread disease
to the big horn sheep should be balance by the fact that these same bighorn sheep have
contact with other domestic sheep in other parts of their range.
Adverse effects on wildlife habitat are not obvious. Elk, deer, bighorn sheep and possibly
other wild ungulates all use Missoula’s Conservation Lands. Coyotes, fox, numerous
rodents and a host of birds all use this area. A normal array of insects is assumed to be
found here along with some insects that have been introduced specifically to feed on
invasive plants. There may be some reduction in spring/summer/fall forage for local deer
populations since the diets of sheep and especially whitetail deer overlap. Removal of elk
winter forage is not expected to be an important effect since the forage preferred by sheep
in the spring and summer is quite different from that preferred by elk during the late fall,
winter and early spring. The effect on coyote and to a lesser degree fox and other
predators is an immediate but seasonal increase in their food supply. The same can be
said of birds, insects and other organisms that scavenge dead animals.
Further Discussion, Observations and Suggestions
Kind and Class of Livestock
Sheep were effective in consuming the targeted plants, especially after mid-June. Goats
might perform as well. While cattle and horses can be expected to eat little, if any, of the
invasive plants targeted species, it has been demonstrated that, under very intensive
management, they can reduce cheatgrass and Japanese brome.
Distribution and Stocking Rate
With the exception of areas where the sheep concentrated repeatedly –night pens and
afternoon loafing/siesta areas –only small areas within each pasture could be described as
heavily grazed. On the other hand, and using the Sunlight area as an example, tumble
mustard and other invaders that supply good sheep forage went unused because it was not
available to the sheep when it was palatable.
-8Overall Effects of Grazing on the Native Plant Community
Provided there is a good representation of desirable plants in the community being
grazed, there is reason to believe that the grazing observed in the summer of 2008 will
increase the vigor and productivity of the overall native plant community, in the short
term. In Stillwater County, Montana, even relatively light spring and summer sheep
grazing of leafy spurge has resulted in a marked increase in native grass productivity.
Also, much of the 2008 grazing on leafy spurge and Dalmatian toadflax removed the
flowers before seed was set or had ripened. Therefore, areas not currently infested by
these invasive species may benefit from reduced seed production. One troubling reality in many places where use on the targeted invaders was the greatest there is little native
vegetation present. With few desirable species able to respond to defoliation of the
invaders, annual invasive grasses –cheat grass and Japanese brome –are likely to be the
species that fill in behind the invasive forbs. These invaders can change fire frequency
and intensity and can have other long-term adverse effects on native communities.
Possible Modifications to Future Vegetation Treatments
Grazing of spotted knapweed during 2008 resulted in moderate use levels at best. In the
aftermath of grazing, most knapweed-infested areas still had 50% or more of the flowers
in tact. This level of seed production is easily enough to replenish knapweed seed in the
soil in perpetuity. While grazing may damage individual knapweed plants, a better
strategy and better tactics are need for knapweed control. Hand pulling by pedestrians,
especially when the soil is moist and knapweed roots will yield, is an ongoing effort and
should continue. Breaking off stalks with flowers/seeds and removing them from the site
can also help. Biological controls, while painfully slow, may be the most effective way to
reduce knapweed populations in the long haul. Localized herbicide application should be
continued or expanded, if possible, where spotted knapweed is at densities were pulling is
not practical and native species represent less than 60% of the plant cover. Because of the
sheep’s strong preference for leafy spurge, with Dalmatian toadflax not too far behind,
discontinuing herbicide use to control of these plants and prioritizing herbicide use to
control/contain primarily spotted knapweed and new invaders should be considered if
grazing for weed control continues.
Some small ‘islands’ of relatively intact, native vegetation surrounded by invaded
communities were observed in the course of fieldwork. Since these sites appear to be
more resistant to invasion than the surrounding plant communities, paying close attention
to these islands may provide insight into to how to stabilize other native communities and
how to better design restoration projects. These sites and others like them in western
Montana could also provide opportunities for graduate level research or at least some
‘careful tinkering’.