Northern Prairie Wildlife Research Center
Leafy spurge has become a very important economic pest in the Northern
Great Plains region of North America and has no real insect or disease pests
(Messersmith et. al. 1985). The extremely persistent and competitive nature
of leafy spurge has led to significant economic losses by livestock producers.
Leafy spurge contains a toxic substance that when consumed by livestock is
an irritant, emetic and purgative. It causes scours and weakness in cattle
and may result in death. The toxin has produced inflammation and loss of hair
on the feet of horses from freshly mowed stubble during haying. Cattle refused
to graze areas infested with more than 20% leafy spurge, so forage utilization
is near zero in dense stands (Lym and Peterson 1989). The relationship between
lost cattle grazing and percent leafy spurge infestation in a linear reduction
(Lym and Kirby 1987). A 20% leafy spurge infestation reduces carrying capacity
by 25% and an 80% leafy spurge infestation reduces carrying capacity by 100%.
Thus, leafy spurge infestations impact the economic return of ranchers and
landowners by reducing income from reduced carrying capacity, loss of livestock
sales, and reduced grazing land values (Sedivec 1995). Annual losses in grazing
capacity to the livestock industry in Montana, North Dakota, South Dakota
and Wyoming were estimated to be $37.1 millions in sales and $34.2 million
in annual production expenditures (Leitch et al. 1994). In North Dakota alone
it is estimated that leafy spurge occupies over 9% of the untilled land (approximately
one-half million ha), reducing livestock carrying capacity by 77,000 cows
and causing an annual direct net income loss of $23 million (Leistritz et
al. 1992). It is also a serious concern in the management of state and national
parks or other areas of preserved native grassland and woodland vegetation.
Here the loss is not identified so much as economic as ecological benefits.
Biological diversity is reduced, impacting wildlife carrying capacity and
aesthetics. While difficult to put a definite monetary value on these, it
is a loss of significant importance to a large number of people.
Heavy infestations of leafy spurge occurs in the SNG. It is estimated that
more than 50% of the public land supports leafy spurge populations of varying
densities. It is a very serious problem which has greater potential than any
other factor to significantly reduce the biodiversity of the SNG. The carrying
capacity of the SNG has already been reduced. Without an aggressive control
program the situation will only worsen.
It is extremely difficult to effect long term control of leafy spurge.
The best method of control depends upon the size and location of the infestation.
A small area may be completely eradicated by persistent application of an
herbicide (Lacey et al. 1985). The strategy is control of the photosynthetic
organs to slowly reduce the large root reserves and eventually kill the plant.
Treated areas should be examined for at least 8 years to detect any regrowth
or new seedlings. Without an early aggressive program of control, leafy spurge
can quickly spread. Once an infestation is established over a larger area,
a combination of control measures, including chemical, cultural and biological,
will likely be necessary to hold the weed in check. In large infestations,
eradication is in all likelihood impossible. The best one can hope for is
control to minimize economic loss and ecological disruption.
The most commonly used herbicides for leafy spurge control are 2,4-D, dicamba
(Banvel) and picloram (Tordon). These herbicides are selective for broadleaf
species and will do minimal harm to grasses when applied at recommended rates
(Lacey et al. 1985). Research has shown that these herbicides are most effective
when applied in mid-to-late June while the true flowers are beginning to appear
(Lym and Messersmith 1983). Fall application of herbicide, late August to
September, when carbohydrates are being transported to the roots, is also
a good time to effect control.
Picloram (Tordon) is the most effective herbicide in controlling leafy spurge.
The recommended rate is 2 lb. per acre applied in mid-to-late June. However,
this rate may not be economically feasible over large areas. Control of leafy
spurge is enhanced when picloram is used in combination with 2,4-D. Lym et
al. (1988) have shown that a tank mix of picloram at 0.25 lb/acre plus 2,4-D
at 1 lb/acre will give 85% control after 4 annual applications.
Dicamba (Banvel) is cleared for use on pastures and rangeland. Dicamba application
at four to 8 pounds per acre will give up to 85% control for 1 year, but quickly
decreases in effectiveness by the second year (Lacey et al. 1985). These rates
of dicamba are often detrimetal to forage production. Using a lower rate (2
pounds/acre) of dicamba, fall applied, with a follow-up spring application
of 1 pound per acre of 2,4-D is also effective. In general, dicamba is not
as effective as picloram. Short-term control of top growth of leafy spurge
can be attained by using 2,4-D at the rate of 1 pound per acre.
There are many occasions and sites where the use of herbicides to control
leafy spurge, or other noxious weeds, is undesirable. All of the herbicides
that are commonly used for leafy spurge control also kill other broadleaf
herbaceous plants, thereby greatly reducing the biodiversity. Picloram is
a restricted use herbicide because it is phytotoxic to most broadleaf plants,
has a relatively long soil residual and is water soluble with the potential
to move into underground water. Picloram should not be used where a sandy,
porous surface and substrata overlie groundwater 10 feet or less below the
surface, nor where it will contaminate streams, ponds or irrigation ditches
(Lym et al. 1993). Livestock should not be transferred from treated grass
areas onto sensitive broadleaf crop areas for 12 months after application
or until picloram has disappeared from the soil without first allowing 7 days
of grazing on an untreated grass pasture. Livestock which have grazed in areas
treated with picloram may contain enough picloram in their urine to cause
injury to sensitive broadleaf plants. Also, despite herbicides having been
used for many years, leafy spurge continues to spread. For these reasons research
is continuing in the search to find other methods to replace or complement
chemical control. Other means of control that have been used or are being
investigated include cultural and biological control methods (Sedivec 1995).
When different methods are used in various combinations it is termed integrated
Cultural control methods include cultivation, mowing and the use of fire. Cultivation to control leafy spurge must be intensive. A duckfoot cultivator tilling 4 inches deep is usually recommended (Lacey et al. 1985). Cultivation should continue every 3 weeks until the soil freezes or for 2 growing seasons. Intensive cultivation is often unusable in pastures or range due to topography, trees or soil type. Intensive cultivation as noted above would completely destroy native range. Mowing has been used as a means of control in sensitive areas. It also has some of the same limitations as cultivation. Mowing can offer some level of control when done repeatedly (Dersheid et al. 1985). Mowing will prevent seed production and eliminate photosynthetic plant parts. Continued mowing for 1 to 3 or more years severely diminished leafy spurge viability (Dersheid et al. 1985).
Wolters et al. (1994) conducted a study on the use of prescribed burning
and burning in combination with herbicides as a means of reducing stands of
leafy spurge. They found fall herbicide (picloram + 2,4-D) application in
combination with a spring burn, or a spring burn alone were the most effective
treatments in reducing leafy spurge seed germination. Both of these treatments
reduced leafy spurge seed germination by over 95% compared to untreated plots.
Spring herbicide in combination with a fall burn was more effective in reducing
leafy spurge seed germination than either spring or fall herbicide application
without fire. All burn treatments substantially reduced the germination of
leafy spurge seed compared with no treatment, and spring burns were slightly
more effective than fall burns. Their study suggested that a single spring
burn is an effective method to reduce leafy spurge seed germination. Burning
did not reduce leafy spurge density. In fact, the fire top-killed the spurge
plants and stimulated vigorous sprouting. The herbicide treatments alone were
most effective in reducing leafy spurge stem density as has been reported
in other field studies (Lym et al. 1988). Wolters et al. (1994) concluded
the best all-around treatment for reducing both germination and stem density
would be a fall application of picloram plus 2,4-D followed by spring burning.
Biological control is the use of a natural agent (predators, parasites,
pathogens) to effect control of a pest population. Most usually the complete
eradication of the pest cannot be attained. The attempt is to reduce the pest
population below the economic threshold (Harris et al. 1985). A number of
studies have examined the feasibility of using biological control in the battle
against leafy spurge. These studies generally provide an optimistic outlook
for the use of biological control.
Leafy spurge, in its natural European distribution, is not a great concern
due to control by several natural parasites and pathogens. Some of these insect
parasites, namely flea beetles of the genus Aphthona, have been introduced
to North America in an attempt to establish populations that will control
leafy spurge as it is in Europe. This is a slow process which takes 7 to 10
years from the time a suitable insect is found until it can be distributed
to landowners (North Dakota Department of Agriculture 1995). Aphthona
larvae feed on roots of the leafy spurge in the fall and spring. An adult
will lay as many as 250 eggs in late June through July. The eggs hatch in
8 to 10 days and the larvae feed on the roots through the summer and fall.
Although dormant during the winter, the larvae again feed in the spring before
emerging as adults. The adults feed on the leaves and stems but cause little
damage. The 4 species of Aphthona introduced into North Dakota are:
A. nigriscutis, A. cyparissiae, A. flave and A.czwalinae. Each
species is adapted to somewhat different conditions. These introductions are
showing some promise, and more of the insects are becoming available to landowners.
This is a slow process and it will be years before we know if this form of
biocontrol is successful.
Flea beetles have not been very successful in controlling leafy spurge in
the sandhills. This may be because of the greater rooting depth of leafy spurge
(too deep for the larvae to successfully reach) in the lighter soils, or poor
overwintering of the beetles. In any case, 2 other insects are being investigated
for use in the sandhills; Oberea erythrocephala, a stem boring beetle,
and Chamesphecia hungarica, a clear wing moth (Donald Mundal, personal
commun.). Only time will tell if either of these will become a successful
parasite on leafy spurge.
Control by sheep and goats
Sheep and goats have long been used to control weeds, and they will
graze leafy spurge (Hanson 1994). Thus, sheep and goats offer an alternative
to chemical control in range and pasture. They are particularly useful in
large infestations and in places where herbicides are not well suited, or
prohibited. Furthermore, sheep are generally very cost-effective. One of the
earliest studies of using sheep as a control agent of leafy spurge was that
of Helgeson and Thompson (1939). Four years of sheep grazing reduced leafy
spurge stem density 21%. Johnston and Peake (1960) also reported that 4 years
of sheep grazing were needed to attain control of leafy spurge. After 5 years
of grazing they found a 98% decrease in the basal cover of leafy spurge. Very
few problems have been reported of sheep having leafy spurge as a large portion
of their diet. In heavily infested pasture the sheep's diet was 24% leafy
spurge (Landgraf et al. 1984). Even at this high rate of spurge grazing there
were no differences in weight gains of ewes. Chemical analysis of leafy spurge
has shown it to be the nutritional equivalent of alfalfa (Fox et al. 1991).
Fox et al. (1991) found leafy spurge to exceed the maintenance requirements
of sheep and goats for crude protein, phosphorus, and in vitro dry matter
digestibility throughout the season. Leafy spurge-based diets should promote
Angora goat growth and production throughout the grazing season in the Northern
Plains (Kirby et al. 1996).
Angora goats have become the subject of interest as a means of biological
control. Grazing with goats may be preferred over sheep by cattle ranchers
because the dietary overlap of sheep and cattle averaged 20 to 35% compared
to only 5 to 20% with goats (Olsen and Hansen 1977). Several studies have
been conducted by scientists at North Dakota State University to determine
the effectiveness of using Angora goats to control leafy spurge. Hanson (1994)
reported that leafy spurge and shrubs provided a large portion of the goats'
diet throughout the grazing season. The mean percent relative density of leafy
spurge in Angora goat diets ranged from 21.5% to 65.5% (Hanson 1994). In a
2-year study analyzing the botanical and nutritional composition of diets
from herded Angora goats in the SNG, Kirby et al. (1996) found leafy spurge
to comprise over 40% of the diet in over half of the collection periods. The
goats also preferred shrubs and trees and avoided most cool-season grass species.
Warm season grasses were also preferred by the goats in the early grazing
season, and the authors pointed out the need to monitor the utilization of
these in pastures co-grazed by cattle to prevent potential overuse. Sedivec
and Maine (1993) introduced Angora goats into a mixed grass range area heavily
infested with leafy spurge to determine if the goats would reduce the stem
and herbage density of the spurge. The area was so heavily infested (38.7
stems per ft2) that it had lost value for cattle grazing. After
2 years of grazing by the Angora goats, reductions of leafy spurge stem density
of 23.5%, and canopy cover of 44.1% were found. Grass production increased
due to the removal of the leafy spurge canopy, thereby improving forage for
cattle grazing and wildlife habitat.
The use of Angora goats in combination with herbicides compared to either
method alone has been examined by Lym et al. (1996). This study also compared
rotational goat grazing on the SNG with season-long grazing on the Gilbert
C. Grafton South State Military Reservation in North Dakota. Both sites had
at least an 80% ground cover of leafy spurge. Cattle were present in the SNG
allotments but were not present at Camp Grafton. They found grazing by goats
combined with an annual fall application of picloram plus 2,4-D reduced leafy
spurge density more and maintained control longer than either method used
alone. Season-long grazing at Camp Grafton alone or combined with herbicides
reduced leafy spurge density more rapidly than rotational grazing used in
the SNG. After 3 years of grazing management, season-long grazing alone reduced
leafy spurge stem density to only 1 stem/0.25 m2. Although it took
2 additional years for grazing alone to reduce leafy spurge to the level that
grazing plus fall applied herbicide reached in 1 year, it is an important
result. This means control can be achieved without the use of herbicides.
Rotational grazing alone did not reduce leafy stem density as fast or to the
low level reached by season-long grazing. However, the authors concluded the
control from the rotational system would be acceptable if other broadleaf
plants were desired in the pasture. Rotational grazing resulted in less damage
to desirable shrubs and trees but gave less satisfactory leafy spurge control.
Prosser (1995) studied multi-species grazing to determine the effect of
Angora goat grazing on leafy spurge stem density, and herbage production of
leafy spurge and graminoid species. Leafy spurge production had a significantly
greater reduction in the goats-only and cattle and goat treatment than in
the cattle-only treatment after 1 year (Table 7).
Hanson (1994) observed that while being herded, Angora goats constantly sought concentrations of leafy spurge for grazing. Grazing pressure by the goats stressed the leafy spurge plants, placing them at a competitive disadvantage with graminoids. Prosser (1995) found flowering and seed production of leafy spurge was being controlled. That the plant was being stressed was evidenced by the reduced herbage production of leafy spurge. At the same time herbage production of graminoids increased (Table 8). Prosser said the increased herbage production in all treatments may have been partially due to above average precipitation as well as less competition from leafy spurge plants stressed from grazing.
Table 7. Leafy spurge production and percent change in production by treatment at Camp Grafton South, 1993-1994. From Prosser (1995).
|Treatment||Year|| Leafy Spurge
1Percentages followed by the same letter are not different (P>0.05).
1Percentages followed by the same letter are not different (P>0.05).
Barker and Limesand (personal commun.) are currently conducting a multi-species grazing trial on the Ekre Grassland Preserve, using sheep concurrently with cattle in an attempt to control leafy spurge as well as other broadleaf weeds and shrubs. Both the sheep and cattle are rotationally grazed. At this point, 2 years of data are available. Leafy spurge canopy coverage in all 4 pastures has been reduced both years (Table 9).
|Average leafy spurge canopy coverage, %|