POISONOUS WEEDS: THEIR IMPACT on LIVESTOCK and MAN Joseph M

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).

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