If death has been recent, a post-mortem examination may reveal clues to the cause. For example, threadleaf groundsel poisoning produces a hard, yellow liver, while nitrate poisoning is often identified by chocolate-brown colored blood present for 2 to 4 hours after death. If a layman attempts to perform the autopsy, he should take adequate sanitary precautions to prevent self-infection in the event the animal died from infectious disease.
Correct diagnosis of a poisonous plant death is often a difficult, time-consuming process. Assistance is available from local veterinarians, Texas Agricultural Extension Service, Soil Conservation Service or the Texas Veterinary Diagnostic Laboratory.
Animals grazing rangelands are often exposed to a wide variety of potentially poisonous plants. Because of this, researchers and ranchers have long recognized that grazing management, “the manipulation of livestock to obtain a desired result,” can be important in reducing animal losses to toxic plants. This manipulation of livestock involves decisions about the stocking rate and grazing system used and the species of livestock grazed.
A rancher’s most important management decision is the maintenance of appropriate livestock stocking rates, which involves both the initial setting of rates and subsequent seasonal adjustments. Stocking rate decisions affect not only individual animal performance (i.e., weaning weights and conception rates) but also livestock consumption of toxic plants.
When stocking rates are excessive, range condition declines. Lower range condition results in decreased production and diversity of desirable plants while toxic plants become more numerous. As a result, the probability that livestock will consume toxic plants increases.
Because livestock are selective grazers, they often excessively graze the plants they prefer. Over time these preferred plants may decline or disappear from the range. Grazing systems which allow rangeland to rest periodically, promote the regrowth of preferred plants and reduce poisonous plant problems. Research conducted at the Sonora Experiment Station compared toxic plant deaths in the Merrill three-herd, four-pasture deferred rotation grazing system with those under continuous grazing. The Merrill system reduced livestock losses to bitterweed, oaks and sacahuista over a 20-year period (Taylor, 1990)
Reducing Livestock Losses to Toxic Plants
Allan McGinty and Rick Machen
More than 100 species of toxic plants infest Texas rangelands. The impact of these plants on the ranch business is not restricted to direct loss in terms of livestock death. Depressed conception rates, lower weaning weights, increased supplemental feeding costs and reduced production of more desirable range forage are examples of indirect losses associated with toxic plants. Total direct and indirect losses attributed to perennial broomweed, is estimated to exceed $30 million per year in Texas (McGinty and Welch, 1987).
Some toxic plants produce easily identifiable symptoms. For example, sacahuista causes photosensitization (hyper-reaction to sunlight), which results in obvious inflammation, swelling and sloughing of the skin. But accurate diagnosis of poisonous plant problems can be difficult. Chronic poisoning from perennial broomweed usually causes abortion, which may be mistakenly blamed on improper management practices or disease. Many cases of livestock poisoning are misdiagnosed and millions of dollars wasted with improper treatment.
The time frame over which symptoms occur adds to the difficulty of diagnosis. Plants which produce toxic amounts of prussic acid (i.e., sorghum species) can kill animals within 15 minutes. On the other hand, threadleaf groundsel may not produce clinical signs or death for 9 to 12 months after consumption.
When a sick or dead animal is found, the first step in diagnosis is to determine whether the cause is plant poisoning or infectious disease. To do so it is necessary to carefully observe symptoms exhibited by affected animals and to have a thorough knowledge of plants native to the area.
Assuming a poisonous plant is responsible, the next step is to identify the plant. Again, symptoms are important when narrowing the suspected plant to one or a few species. It is also helpful to observe if any toxic plant species within the pasture exhibit evidence of grazing use. Finally, the rumen of a dead animal can be opened and a sample obtained of the ingested forage. Careful examination of the rumen contents can assist in identification of the toxic plant responsible.
Intensive grazing systems (i.e., short-duration and high-intensity, low frequency) provide flexibility in livestock movement that can reduce toxic plant problems. For example, it may be possible to increase livestock numbers within a particular problem pasture to a point where individual animals are unable to obtain a toxic quantity of a poisonous plant (Ralphs and Sharps, 1988). Also, intensive grazing systems utilizing a large number of pastures can more easily avoid pastures with known histories of poisonous plant problems during critical times of the year.
One caution when using intensive grazing systems relates to the length of time a pasture is grazed. Livestock density within a particular pasture usually greatly exceeds the annual carrying capacity of that pasture. If livestock are left too long in a pasture, forage quantity can become limiting, promoting livestock consumption of normally unpalatable toxic plants. Depending on the intensity of the system, this series of events may occur in only a matter of days.
The type of grazing system used is not as important as adhering to the principles of good grazing management. Years of research have shown that regardless of the grazing system used, excessive stocking rates cause range condition to deteriorate and toxic plants to proliferate
Livestock Species Grazed
Livestock species differ in both their grazing behavior and susceptibility to toxic plants. These differences can be utilized to reduce losses of livestock to toxic plants on rangeland. For example, researchers at the Sonora Experiment Station have proved that there are fewer livestock losses to western bitterweed when a combination of cattle, sheep and goats are grazed than when a single species is grazed (Merrill and Schuster, 1978). A second example is woolly paperflower, a palatable plant in the Trans-Pecos region of Texas, that is toxic to sheep but not cattle. Due to this difference in susceptibility, cattle can be grazed ahead of sheep to reduce the quantity of woolly paperflower available for sheep consumption.
Deficiencies in protein, energy, minerals or vitamins may increase the probability that livestock will ingest toxic plants. These deficiencies can be corrected with supplemental feeding programs.
When to Supplement?
Generally, supplemental feeding is most important during the winter and early spring, when forage quality is fair to poor, and most livestock experience their highest demand for nutrient requirements (last one-third of gestation and lactation). Livestock body condition and forage testing can be utilized to define nutrient deficient periods.
With What to Supplement?
Minerals must receive high priority in any supplementation program. For example, phosphorous is considered the most limiting nutrient in most Texas forages. In fact, when rainfall is below normal, forage phosphorus content may never meet basic livestock maintenance requirements. Phosphorous deficiency causes abnormal grazing behaviors or cravings, which make livestock much more likely to consume normally unpalatable toxic plants. Other minerals which may be deficient on Texas rangelands include potassium, copper, magnesium, sodium, iron and zinc.
Some forages, particularly in west Texas, have a calcium content that greatly exceeds the phosphorous level within the plant. In these cases, the use of mineral supplements containing no more than one part calcium for each part phosphorous (1:1 ratio) is recommended. Feeding an “inverted” mineral (1:2 calcium:phosphorous ratio) may also be beneficial. Regardless of the calcium:phosphorous ratio or nutrient source, phosphorous supplementation tends to reduce the probability of toxic plant consumption.
The second most limiting nutrient is protein. Forages are usually protein deficient from first frost until spring greenup. Should winter moisture be available, cool season grasses and forbs may alleviate protein deficiencies to some extent. Unfortunately, toxic plants such as woolly loco are also green and relatively high in protein during wet winters and may be preferred by livestock. Therefore, supplementation must correct any protein deficiencies in order to minimize toxic plant consumption. Results of Experiment Station studies indicate that the form of protein supplemented is also important. To reduce toxic plant consumption, natural proteins (such as oilseed meals) should be used. Nonprotein nitrogen supplements (urea) may aggravate rather than alleviate toxic plant problems.
Vitamin A deficiency may also increase consumption of toxic plants. Vitamin A can be stored by the grazing animal for up to 90 days. If no green forage is available for that length of time, it is generally recommended that Vitamin A be supplemented.
How Much to Supplement?
The quantity of supplement to provide is perhaps the most difficult decision. If too much supplement is provided profitability suffers. If not enough is provided individual animal performance suffers. The correct amount depends upon:
- nutrient content of the supplement
- livestock body condition
- physiological status of the livestock
- forage quantity
- forage quality
- weather conditions
Due to the low amounts of mineral supplement eaten by livestock, it is difficult to monitor consumption. Although, consumption fluctuates throughout the year, as a “rule of thumb” a cow should ingest an average of 0.2 pounds of mineral supplement per day, a sheep and goat 0.05 pounds per day.
Supplementation cannot completely prevent toxic plant consumption. However, ranchers can reduce their losses by providing proper quantities of needed nutrients at critical times.
It is often necessary to control poisonous plants on rangeland. Control may take many forms and be practiced to various degrees. An effective program will reduce or prevent livestock losses and be cost effective.
What Toxic Plants Should be Controlled?
Determining which plant species to target requires considerable thought and planning. Species such as lechuguilla and sacahuista are resistant to most control practices, while others such as whitebrush may not be worth controlling because they pose only a slight threat to livestock. These plants may be more effectively managed by selecting appropriate livestock species, grazing systems and supplemental feeding programs.
In other cases, the first presence of a toxic plant should trigger control efforts. This is especially true for plants such as African rue, a highly toxic species which spreads quickly along ranch roads and is resistant to all but the highest, most expensive rates of herbicides (Sperry et al., 1968).
A rancher must know a poisonous plant’s level of toxicity, its ability to spread across pastures and its susceptibility to control methods. With this information, sound decisions can be made concerning which toxic plant species should be controlled.
Where Should Toxic Plants be Controlled?
Toxic plants pose variable levels of risk, depending on where they grow. Livestock handling facilities and bedding/feeding locations are high risk areas. In these sites the soil is continually disturbed, which promotes the invasion of toxic plants. And because livestock spend considerable time in these areas, they are more likely to eat any poisonous species which grow there.
Ranch roads represent another high risk area. Toxic plants such as African rue, western bitterweed and twinleaf senna enter the ranch as seeds on the tires and frames of vehicles. Disturbed roadsides offer opportunistic areas for the germination of these seeds. To prevent movement of toxic plants into adjacent pastures, these plants should be controlled when they first appear.
Although many toxic plant species remain in localized areas, others such as perennial broomweed and woolly locoweed may densely infest entire pastures. In these situations it is necessary to broadcast herbicides, although efforts must be made to keep the applications cost effective. For example, perennial broomweed could be treated only in pastures used by pregnant livestock. This would minimize costs while reducing or eliminating the abortions caused by perennial broomweed.
When Should Toxic Plants be Controlled?
The “when” of toxic plant control may have both long and short-term connotations. Most control practices are more effective when applied at a specific time of the year. Such seasonal timing will vary greatly depending on the plant species involved and control practice or herbicide used. For detailed information on the seasonal timing of herbicide applications refer to Extension bulletin B-1466, “Chemical Weed and Brush Control – Suggestions for Rangeland” (Welch, 1990), available from your county Extension agent.
Over the long term, the best time to control a poisonous plant is before it increases in range and density. High-risk areas should be constantly monitored and toxic plants treated as soon as they appear. The old saying “an ounce of prevention is worth a pound of cure” applies to toxic plant management.
How Should Toxic Plants be Controlled?
A variety of plant control options are available. These include fire as well as mechanical, biological and chemical methods. Fire is a proven tool to manipulate vegetation. Unfortunately its use in toxic plant management is limited. Fire rarely kills, and it’s use is restricted in west Texas due to low fuel loads and poor fuel continuity.
Mechanical methods such as chaining, railing, rootplowing or grubbing will control woody toxic plants such as whitebrush or mesquite, but the soil disturbance these practices cause may promote the establishment of herbaceous toxic plants such as western bitterweed, twinleaf senna or African rue. Shredding or mowing are somewhat useful in the control of herbaceous, annual toxic plants, although terrain and woody plant cover usually prohibit their use. Hand pulling or grubbing, while certainly not the appropriate method once plants are densely established, has kept many ranches free of toxic plants for decades.
Biological control may be the hope of the future. A moth borer has been used successfully to control prickly pear cactus in Australia. St. Johns-wort, a poisonous range weed in the western United States, is controlled by leaf-eating beetles (Klingman and Ashton, 1975). The Agricultural Research Service is presently evaluating a weevil for control of perennial broomweed.
In many cases herbicides are the most economical and efficient method of control. When plants densely infest large acreages, commercial aerial application should be used. If the targeted plant is scattered along the edges of ranch roads or confined to other localized areas, ground applications will be more appropriate.
Ground applications of herbicides can be made with either broadcast or individual plant treatment techniques. As a general rule, if plant density exceeds 100 to 200 plants per acre, broadcast applications will be more cost effective.
Broadcast, ground applications do not require a large investment in equipment. By attaching a clusterjet (boomless) nozzle or spray boom, livestock sprayers can be cheaply converted ($100 or less) to apply liquid, foliar-applied herbicides. When rinsed, the sprayer may still be used to spray livestock, but not to spray desirable shade or fruit trees. There is also spray equipment ($250 to $750) that attaches to 4-wheel motorcycles.
When plant density is low or erratically distributed, individual plant treatment techniques may be required. The same stock sprayer used for broadcast applications can be used for high-volume, foliar spraying. Herbicide is mixed with water on a volume basis and applied at high pressure with a hand gun until the plant is uniformly wet. When terrain limits the mobility of a stock sprayer, a backpack or small 1 to 3-gallon “pump-up” garden sprayer may be used. Spray equipment mounted on 4-wheeled motorcycles or other off-road vehicles may also be fitted with handguns.
The type of herbicide to use and the rate and season of application can be determined by referring to Extension publication B-1466, “Chemical Weed and Brush Control -Suggestions for Rangeland”.
One special precaution concerns the impact herbicide applications have on plant palatability. Once sprayed plants begin to wilt, they are much more likely to be consumed by livestock. Thus, livestock should not be grazed on treated areas until the plants are completely desiccated.
Researchers at the USDA-ARS Poisonous Plant Lab and Utah State University are diligently working on a practical method of teaching livestock not to eat toxic plants. This behavior modification process is known as food aversion learning (Ralphs and Provenza, 1990).
Tall larkspur is the most important poisonous plant on Utah mountain ranges, causing persistent losses in cattle of up to 10 percent annually. The plant is palatable and acutely toxic. Researchers have successfully taught several groups of cattle not to eat larkspur (Lane et al., 1990; Olsen et al., 1989 and Ralphs and Olsen, 1990). Aversion was accomplished by feeding animals a small quantity of larkspur and then administering a gastrointestinal irritant. When pastured as a separate group, cattle maintained this aversion from one year to the next. Unfortunately, they tended to resume consumption of the toxic plant if pastured with untreated cattle.
Behavior modification is not a practical tool for managing toxic plants at this time. However, increasing plant control costs (mechanical and/or chemical), coupled with improved knowledge of livestock behavior may facilitate future use.
- Do not overgraze rangeland. Many poisonous plants are classified as increasers or invaders, thus becoming more common on low condition rangeland.
- Be cautious when introducing livestock from other geographic locations. They are more likely to consume toxic plants than are native cattle.
- Do not turn hungry stock onto toxic plant-infested pastures. Hungry animals are much less selective about the plants they eat.
- Make certain livestock have free access to salt and mineral supplements. Nutrient deficiencies may reduce selective grazing behavior.
- Provide adequate, clean water.
- Do not feed hay that contains poisonous plants.
- Minimize grazing pressure when poisonous plants are the most dangerous. Use flexible grazing systems so that high risk areas can be left ungrazed when the toxicity hazard is greatest.
- Use the proper kind and class of livestock. One class of livestock is often more resistant to a toxic plant than others.
- Be alert when herding livestock through infested areas. Stock should have full stomachs when trailed or penned. Also, avoid crowding animals.
- Be aware of special environmental conditions that may restrict animal movement or change diet selection. Such conditions include drought, snow or extremely wet conditions.
- Be cautious when grazing areas recently burned or sprayed with herbicide. These practices can increase toxic plant palatability.
- Finally, know the toxic plants that might occur on your ranch and watch for evidence of grazing. When problems are detected early, losses can be minimized.
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