POISONOUS WEEDS: THEIR IMPACT ON LIVESTOCK AND MAN Joseph M. DiTomaso University of California, Davis

The total economic impact of poisonous plants on livestock is impossible to determine with current available information. Even ballpark figures on death losses appear to be purely speculative. Estimated losses from veteri­ narian reports are generally inadequate, as often only severe losses due to plant poisoning are reported. Isolated or incidental poisonings are usually absorbed within the normal cost of doing business and are not given special attention (13). As a result, the exact proportion of deaths caused by poison plants in all classes of livestock is unknown. To complicate matters, very little published data is available concerning economic losses from poisonous plants, even in areas where thorough records are kept.

As difficult as it is to estimate mortality losses from consumption of these plants, it is nearly impossible to determine the sub-lethal effects of consumption of toxic plants (10). Many poisonous plants do not result in the death of the animal, rather they affect the overall performance of the stock through losses in weight gain, reduced reproduction, reduced longevity of the breeding herd, increased birth malformations and abortion, or a decrease in wool or milk quality (2, 6, 16). The National Academy of Science (12) esti­ mated that 8.7% of the nutritionally sick animals in the western United States are sick as a result of eating poisonous plants. Possibly these losses due to reduction in performance of animals caused by poisonous plants may exceed the cost of death losses (10, 13, 16).

In addition to economic costs resulting from animal health, financial losses from poisonous plants also should include the costs associated with livestock and range management, as well as significant losses in range re­ sources. These indirect costs include fencing, weed control costs, more in­ tensive management, decreased forage production and utilization, altered grazing programs, loss of forage, and occasionally supplemental feeding pro­ grams (6).

Clearly, the likelihood of obtaining accurate estimates associated with plant poisons is difficult. The various direct and indirect effects on lives­ tock and range management makes such a task formidable. Despite these diffi­ culties, however, I have attempted to estimate the economic impact of poisonous plants on direct losses and potential reproductive losses in sheep, cattle, and horse in California. Such estimates require a number of assump­ tions, since accurately reported information in riot available.

An initial assumption would require an estimate of adult cattle and sheep losses per year. The Agricultural Research Service (1) reported that lives­ tock losses per year were similar from 1948 through 1968. Indications are that the average death loss per year in adult cattle is somewhere around 3.0% and sheep losses are about 8-10% (13, 17). Losses in horses is more difficult to obtain, but is conservatively estimated to be about 2.0% annually.

29 Figures reported for death losses due to plant poisonings are not consis­ tent, and are often determined as a fraction of the total deaths. Some au­ thors have reported death rates as high as 3 to 5% a year in cattle, sheep, and horses (16). However, I have chosen to use the more conservative esti­ mates which suggest one third of the death loss in cattle and sheep are due to poisonous plants, 1.0% and 3.5% respectively (13).

In a final assumption, poisonous plants could cause reductions in lamb and calf crops by affecting the reproductive capabilities of the breeding herd. Nielsen (13) assumes that the calf and lamb crops could be increased 1% if a poisonous plant problem did not exist.

ECONOMIC IMPACT

With the exception of reproductive potential, no attempt will be made to estimate the indirect cost of poisonous plants to livestock health. In addi­ tion, no figures are presented concerning control of weeds, management of rangeland, or loss of forage resources.

The January 1984, figures for the number of cattle in California is 5,000,000 head (20). Of these, approximately 1.6 to 1.9 million are dairy cattle and 0.6 million are feedlot beef cattle. An estimated 2.5 to 2.8 mil­ lion head are rangeland beef cattle. Only the latter will be considered in estimates of poisonous plant losses.

The average cost per head of cattle is determined to be approximately $500 (14, 18, 21). If 1% of the total adult cattle population were to die of plant poisons each year, an estimated 25,000 beef cattle fatalities would oc­ cur at a financial loss of $12.5 million (Table 1). These losses are the re­ sult of direct death to the stock.

Table 1. Estimates of direct losses in beef cattle and sheep due to poisonous plants per year in California.

Beef cattle Sheep

Number in California 2,500,000 1,115,000 Estimated % death rate per year 3% 8-10% Estimated % death rate due to 1% 3.5% poisonous plants Number killed by 25,000 39,025 poisonous plants Average cost per head $500 $100 Economic loss due to deaths $12,500,000 $3,902,500

30 Similarly, 1,115,000 sheep were reported in California in January 1984 (20). The average cost per head is approximately $100 (14, 19, 21). From this information, and assuming a 3.5% death loss per year due to toxic plants, an estimated 39,025 sheep, worth $3.9 million, are killed per year.

The loss in reproduction through abortions and potential loss by death of the adult animals must be considered (Table 2). The percentage of adult ani­ mals which reproduce per year is 40% in cattle and 66% in sheep (20). Assum­ ing a 1% drop in reproduction due to poisonous plants, as suggested by Nielsen (13), a loss of 25,000 calves and 11,150 lambs occurs annually in the total population. With the average cost of a calf at about $300 and a lamb at $65 (14), the economic loss is estimated to be $7.5 million a year for cattle and $.7 million for sheep.

Table 2. Estimates of reproductive losses in beef cattle and sheep due to poisonous plants per year in California.

Calves Sheep

Percent breeding stock 40% 66% Estimated decrease in newborns 1% 1% Reduced number of newborns 25,000 11,150 due to poisonous plants Average cost per newborn $300 $65 Loss due to decrease in $7,500,000 $724,750 newborns

The losses in sheep and cattle from both direct death and reproductive losses total $20 million for beef cattle and approximately $4.6 million for sheep (Table 3).

Table 3. Total losses in beef cattle and sheep as a result of poisonous plants per year in California.

Beef cattle Sheep

Loss per year due to $12,500,000 $3,902,500 death of adult animals Loss per year due to $7,500,000 $724,750 decrease in reproduction Total economic loss per year $20,000,000 $4,627,250

31 The economic impact of poisonous plants is also important in horses. The U.S.D.A. Crop Reporting Service (20) estimates the number of horses in Cali­ fornia at approximately 570,000. Though no accurate figures have been re­ ported regarding percentage of deaths caused by plant poisons, I will assume about 1%. This figure is considerably more conservative than the 3% suggested by Schuster (16). If the average value of a horse is about $700 (3) and ap­ proximately 5,700 horses are killed by plants a year in California, the esti­ mated financial loss is nearly $4 million (Table 4).

Table 4. Estimates of direct loss of horses to poisonous plants per year in California.

Estimated number of horses 570,000 Estimated % deaths due to poisonous plants 1% Estimated number killed by poisonous plants 5,700 Average value per horse $700 Economic loss per year due to $3,990,000 poisonous plants

From these figures the total direct losses in horses, sheep, and beef cattle per year in California is estimated to be $28.6 million.

Horses $4.0 million Sheep $4.6 million Beef cattle $20.0 million

Total $28.6 million

These estimates, however, are considered conservative. Similar estimates with cattle and sheep in Texas suggest losses between $50 and $100 million per year (16). It may be realistic to credit poisonous plants with up to $50 mil­ lion in direct losses in California, and if the indirect losses discussed ear­ lier are considered, a range of $50 to $100 million annually is likely. Dr. Lynn James (7) at the Poisonous Plant Research Laboratory in Logan, Utah, sug­ gests that losses in the 17 western states may exceed $350 million per year. Similarly, he indicates that Nielsen's (13) estimates of direct losses total­ ling $107 million a year in the 17 western states is extremely conservative and probably should be doubled.

These figures for economic losses do not include dairy or feedlot cattle, swine, goats, poultry, and other minor domesticated animals. It is likely that the total financial impact of poisonous plants on all aspects of the livestock industry is considerably higher than previously thought.

32 POISONOUS WEEDS

Cattle

Locoweeds (Astragalus spp.) have long been thought to be the most impor­ tant plants in livestock poisoning. In addition to high fatality rates after ingestion of relatively small quantities, locoweeds are also known to cause significant losses in calves and lambs from abortion (8). Serious cases of locoweed poisonings are not unique to cattle, since all animals are known to be susceptible.

Sampson and Malmsten (15) noted that larkspurs or staggerweeds (Del­ phinium spp.) "probably cause more losses among cattle in [California] as a whole than any other group of plants." Today larkspurs are still considered the major plant poison to cattle in California (5). Larkspurs grow in a var­ iety of plant communities in mountain and foothill ranges throughout the state (5). Most species of tall and low larkspurs contain complex alkaloids highly toxic to cattle, but for some unknown reason sheep are much less susceptible (9).

Liver poisoning by pyrrolizidine alkaloids is one of the major contribu­ tors to cattle fatalities in California (4). Most cases involve coast fiddle­ neck (Amsinckia intermedia Fisch. and Mey) or common groundsel (Senecio vulgaris L.). Though they occasionally cause problems in rangelands, the ma­ jority of cases involve contaminated hay, especially that made after the first cutting of grain and alfalfa (5). In contrast to cattle, sheep are relatively resistant to pyrrolizidine alkaloid toxicity (9).

There are several additional serious, though lesser important, plant poi­ sons which contribute to cattle mortalities. The saponins of threadleaf sna­ keweed or broomweed (Gutierrezia microcephala (DC.) Gray [=Xanthocephalum sp.]) have been known to poison large numbers of cattle, and occasionally sheep, under range conditions (9). Death can occur directly, though the prin­ ciple economic loss is from abortion (8).

Extensive losses of cattle, sheep, and goats occasionally occur following ingestion of chokecherry (Prunus virginiana L.) on mountain and foothill slopes (15). The cyanogenetic glycoside amygdalin is broken down to release hydrocyanic acid, resulting in respiratory failure.

Cockleburs (Xanthium spp.) are toxic to all classes of livestock, partic­ ularly swine (5). Poisonings are nearly always associated with consumption of seedlings in the cotyledonary stage. As the plant matures, the toxicity de­ creases (9).

Nightshades (Solanum spp.) have long been known to contain the glycoalka­ loid solanine which is toxic to all classes of livestock (9). Pine needles, in particular pondersosa pine (Pinus ponderosa Doug. ex Laws), can cause high rates of abortion when grazed by cattle (8). The problem, however, is more

33 severe in Arizona, Colorado, Utah, New Mexico, and especially Texas, than in California (7). Sorghum, johnsongrass, or sudan grass (Sorghum spp.) toxicity is connnon to cattle in California (4). In most instances, loss is the result of cyanide poisoning, though sorghum can also accumulate toxic levels of ni­ trates (9).

Sheep

Death rates in sheep are considerably higher than cattle presumably be­ cause sheep usually spend more time on the rangeland (13). As a result, a wide variety of plants have been reported in sheep poisonings. Among the more important poisonous plants, death camas (Zygadenus spp.) is considered by Kingsbury (9) to be responsible for the "greatest loss of life in sheep on the spring range." In California, death camas is quite common in both the coast ranges and the Sierra.

Among the more dramatic plant poisons, halogeton (Halogeton glomeratus (Bieb.) C.A. Mey) would certainly rank near the top. Introduced from Russia probably in the 1920's, halogeton spread over 10 million acres of the western states by 1960. The high concentration of soluble oxalates have been respon­ sible for tremendous sheep losses in Nevada, Utah, Idaho, and northeastern California (9). Sheep herds grazing in halogeton infested regions have re­ ported losses as high as 1,200 to 1,300 head at one time. In one case, over 2,000 head of sheep were lost over a period of a couple of years (13).

Littleleaf and spineless horsebrush (Tetradymia glabrata Gray and T. canescens DC) have produced losses from liver damage of more than 1,000 head of sheep at a single time (9, 15). Pregnant ewes which survive often abort soon after (8). Additional symptoms include photosensitization, which often lowers the quality of the wool.

Sampson and Malmsten (15) states that among plants responsible for sheep poisoning, lupines (Lupinus spp.) rank a close second to death camas in Cali­ fornia. Whether this is still true is debatable. Nevertheless, lupine poi­ soning in sheep is unquestionably of economic importance. Several cases involving the loss of more that 1,000 head of sheep are -0n record (9). In Montana, Idaho, and Utah, lupines are responsible for greater losses among sheep than any other plant. Several toxic quinolizidine alkaloids have been isolated from many lupine species. Highest concentrations appear to be found in the seeds (8). The alkaloids may also contribute to birth deformities in lambs.

Milkweeds (Asclepias spp.) most commonly poison sheep, although the plant is toxic to all classes of livestock. Poisonings usually occur on rangelands but may also occur if animals are fed hay containing large amounts of milkweed (8).

34 Snakeweed, cocklebur, locoweeds, and nightshade are also important in sheep toxicity, as well as cattle poisonings.

Horses

The most important plant poisons in horses are toxic weeds which contami­ nate hay. In particular, the pyrrolizidine alkaloids of coast fiddleneck and common groundsel account for a large proportion of horse fatalities in Cali­ fornia. Yellow starthistle (Centaurea solstitialis L.) has recently become a major cause for concern. Though initially a weed of hayfields, yellow starthistle has spread throughout the state to become a serious problem in rangelands as well. Ingestion of the weed produces a nervous syndrome called "chewing disease" (9), which appears to affect only horses (5).

Among the minor plant poisons affecting horses, oleander (Nerium oleander L.) is regarded as perhaps the oddest. Cases of horses poisoned by oleander usually occur following pruning of the ornamental shrub in areas where the an­ imals are free to browse. Very little of the plant is required to kill an adult horse, perhaps only a few leaves. In such cases, cardiac glycosides within the plant act to weaken the heart (9).

Tree tobacco (Nicotiana glauca Graham) is common in the foothills of cen­ tral and southern California. Like all members of the genus, it contains highly toxic alkaloids, including nicotine (9). Horses are occasionally poi­ soned by browsing on the leaves.

Other toxic plants which are known to result in horse fatalities include cockleburs, locoweeds, and nightshades.

MANAGEMENT CONSIDERATIONS

It is widely believed that grazing abuse on rangelands is the most signi­ ficant cause of death from poisonous plants (7, 11). On rangelands in good condition, poisonous plants are usually only minor constituents (10), and do not pose a serious threat to the health of livestock. Since poisonous plants are usually unpalatable, they do not become a serious problem unless other forage is scarce.

Under heavy grazing conditions, a specific management program is neces­ sary to control the rapid invasion of toxic species. It is critical that the range manager be able to recognize which poisonous plants are present, how they affect livestock, and under what conditions are they poisonous (10). Even where losses are not heavy, there are many cases in which proper manage­ ment and control can prove profitable.

35 ACKNOWLEDGEMENTS

I wish to thank Drs. Gary Carlson, Arthur Craigmill, Murray Fowler, Lynn James, Darwin Nielsen, and Kent Olson for their advice in the preparation of this manuscript. I especially thank Dr. Larry Mitich for his continuous sup­ port and assistance.

LITERATURE CITED

1. Agriculture Research Service. 1968. Twenty-two Plants Poisonous to Livestock in the Western States. U.S. Department of Agriculture. Bulletin No. 327, Washington, D.C. 2. Dwyer, D.D. 1978. Impact of poisonous plants on western U.S. grazing systems and livestock operations. In, Effects of Poisonous Plants on Livestock. R.F. Keeler, K.R. Van Kampen, ad L.F. James (edi­ tors). Academic Press, New York. 3. Carlson, G., D.V.M. Department of Medicine, School of Veterinary Medi­ cine. University of California, Davis. Personal communication. 7 January 1985. 4. Fowler, M.E., D.V.M. Department of Medicine, School of Veterinary Medi­ cine. University of California, Davis. Personal communication. 7 January 1985. 5. Fowler, M., A.L. Craigmill, B.B. Norman, and P. Michelsen. 1982. Livestock-Poisoning Plants of California. Division of Agricultural Sciences, University of California, Leaflet no. 21268. 6. James, L.F. 1978. Overview of poisonous plant problems in the United States. In, Effects of Poisonous Plants on Livestock. R.F. Keeler, K.R. Van Kampen, and L.F. James (editors). Academic Press, New York. 7. James, L.F., PhD. Poisonous Plant Research Laboratory, ARS-USDA, Logan, Utah. Personal communication. 8 January 1985. 8. James, L.F., R.F. Keeler, A.E. Johnson, M.C. Williams, E.H. Cronin, and J.D. Olson. 1980. Plants Poisonous to Livestock in the Western States. U.S. Department of Agriculture. Agriculture In­ formation Bulletin No. 415. 9. Kingsbury, J.M. 1964. Poisonous Plants of the United States and Canada. Prentice-Hall, Inc., Englewood Cliffs, New Jersey. 10. Krueger, W.C. and L.A. Sharp. 1978. Management approaches to reduce livestock losses from poisonous plants on rangeland. Journal of Range Management 31:347-350. 11. Merrill, L.B. and J.L. Schuster. 1978. Grazing management practices affect livestock losses from poisonous plants. Journal of Range Management 31:351-354. 12. National Academy of Science. 1968. Prenatal and postnatal mortality in cattle. Publication No. 1685. Washington, D.C.

36 13. Nielsen, D.B. 1978. The economic impact of poisonous plants on the range livestock industry in the 17 western states. Journal of Range ManagementA 31:325-328. 14. Nielsen, D.B., PhD. Department of Economics, Utah State University, Lo­ gan, Utah. Personal communication. 7 January 1985. 15. Sampson, A.W. and H.E. Malmsten. 1942. Stock-Poisoning Plants of Cal­ ifornia. University of California, College of Agriculure, Bulletin No. 593, Berkeley. 16. Schuster, J.L. 1978. Poisonous plant management problems and control measures on U.S. rangelands. In, Effects of Poisonous Plants on Livestock. R.F. Keeler, K.R. Van Kampen, and L.F. James (edi­ tors). Academic Press, New York. 17. U.S. Department of Agriculture. 1975-1976. Meat animals, production, disposition, income. Crop Reporting Board, Statistical Reporting Service. 18. U.S. Department of Agriculture. 1977a. Beef cattle numbers, January 1, 1977. Crop Reporting Board, Statistical Reporting Service. 19. U.S. Department of Agriculture. 1977b. Breeding sheep, 1 year and older, January 1, 1977. Crop Reporting Board, Statistical Reporting Service. 20. U.S. Department of Agriculture. 1984a. January 1, 1984. Crop Report­ ing Board, Statistical Reporting Service. 21. U.S. Department of Agriculture. 1984b. Annual Price Summary, June 1984. Crop Reporting Board, Statistical Reporting Service.

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