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Integrated Management of the Greater Prairie Chicken and Livestock on the Sheyenne National Grassland

Leafy Spurge Infestations and Control


Leafy spurge (Euphorbia esula L.) is a perennial noxious weed which infests 26 states and 6 Canadian provinces (Dunn 1979). Leafy spurge is a long-lived plant native to Europe and Asia. It is believed to have been introduced into North America in 1827 by water ballast from a sailing vessel. It was first reported in North Dakota in 1909 (Messersmith and Lym 1983). Leafy spurge spreads rapidly by seeds and rhizomes and forms dense colonies primarily on untilled land. Seeds are borne in pods which contain 3 gray-brown, oblong, smooth seeds. After the seed has matured, the seed pods burst explosively and throw seeds up to 15 feet from the parent plant. An average of 140 seeds is produced per stem, and seeds may remain viable in the soil at least 8 years (Lym et. al. 1993). Leafy spurge seedlings have a great capacity for vegetative reproduction, being able to reproduce vegetatively within 7 to 10 days after emergence. Leafy spurge patches may have more than 200 stems per square yard in sandy soil and higher densities in heavy soils. Patches of leafy spurge usually spread vegetatively from 1 to 3 feet per year and form dense stands which out-compete other plants for light, moisture and nutrients. As another component of competition, leafy spurge has been classified as an allelopathic plant (Steenhagen and Zimdahl 1979). Leafy spurge leaves are more toxic than the stems, and the roots produce inhibitory growth substances (Selleck 1972). Extracts of leafy spurge infested soil have been shown to inhibit coleoptile and radicle elongation.

Economic impact

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.

Chemical control

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 control.

Cultural control

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

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
(kg/ha)
Percent
reduction.
S.E.
Cattle Only 1993 3938.7    
1994 3498 11.2b 0.2
Goats Only 1993 1959.4    
1994 1294.3 33.9a 0.7
Cattle/Goats 1993 1718.9    
1994 1033.7 39.8a 0.9

1Percentages followed by the same letter are not different (P>0.05).


Table 8. Production and percent change of all graminoid species for each treatment at Camp Grafton South, 1993-1994. From Prosser (1995).

Treatment Year Graminoids
(kg/ha)
Percent
increase.
S.E.
Cattle Only 1993 3850.5    
1994 4932.4 28.1a 0.05
Goats Only 1993 4587.9    
1994 7036 53.4b 0.06
Cattle/Goats 1993 5080.7    
1994 7476.8 47.2b 0.03

1Percentages followed by the same letter are not different (P>0.05).


Dietary preferences (Kirby et al. 1996) and the degree of control of leafy spurge (Hanson 1994, Hanson et al. 1992, Lym et al. 1996, Prosser 1995, Sedivec et al. 1994) indicate that Angora goats also offer Northern Plains land managers a tool to biologically manage leafy spurge infestations. Kirby et al. (1996) noted that goats should be also compatible in an integrated program with herbicides, insects or fire in managing leafy spurge, and their greatest utility may be in rough or sensitive environments such as woodlands, waterways and subirrigated range sites.

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).


Table 9. Percent leafy spurge canopy coverage after 2 years of rotational grazing by sheep and cattle. (From Barker and Limesand, personal commun.).

Average leafy spurge canopy coverage, %
Grazed Ungrazed
Pasture 1994 1995 1994 1995
1 37 43 55 62.5
2 28.5 29.5 48.7 40.5
3 26.5 21 46 47.8
4   54.5   61


Leafy spurge can be controlled. It is imperative that measures for control be taken quickly and aggressively. Without adequate control the carrying capacity of the SNG will be reduced resulting in greater grazing pressure and a loss of wildlife habitat. It would be difficult to improve nesting and brood habitat for prairie chickens with decreasing quality of grassland habitat. The wholesale use of herbicides over large areas is not desirable from the standpoint of losing biodiversity. A reduction in forbs may cause a reduction in insects which could impact prairie chickens and other wildlife species. Such a change was noted to affect partridge chicks (Southwood and Cross 1969).

Summary

  1. Leafy spurge infestations occur on more that 50% of the SNG and has a very important negative economic and ecological impact by reducing grazing capacity and biodiversity.

  2. Herbicides will control (not eradicate) leafy spurge but pose significant problems in sensitive areas. Herbicide use will also decrease biodiversity.

  3. Sheep and goat grazing provides an effective means of controlling leafy spurge with less loss of biodiversity than using herbicides.

  4. It is imperitive that leafy spurge be controlled on the SNG or the goal of maintaining or restoring wildlife populations cannot be attained.

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