One of the considerations for chemical and other weed control in horse pasture is the control or elimination of poisonous plants. There are few forage areas devoid of all toxic plants. Most toxic plants are broad-leaved. Horses normally don't relish broad-leaved weeds, but they do if grass forage is limited. Horses tend to browse weeds more when on a higher-concentrate, low-fiber ration. Having a few toxic plants available does not mean there is an acute problem.
Plant toxicities may be grouped in two categories:
- definite poisonous plants
- secondary toxicities or ailments associated with forage plants
We cannot discuss in detail horse poisoning symptoms and treatment here, but we mention a few pertinent items. The list includes primarily common potentially toxic plants but not absolute toxicity syndromes of the plants.
Definite Poisonous Plants
These plants have a definite toxicity syndrome. Some harm horses as well as other livestock, while others' action is unknown. Some palatable weeds are nitrate accumulators. The point of the following listing is to increase awareness of the potential problems and stress the need for weed control. Grasses capable of having toxicity syndromes are discussed later. Refer to the Noble Foundation's Website, www.noble.org, and its plant image gallery, created by Chuck Coffey and Russell Stevens, for pictorial identification of many of these and other plants.
- Bitterweed (Actinea spp.) — broad-leaved
- Black locust (Robinia sp.) — woody
- Bladderpod (Glottidium sp.) — broad-leaved
- Bracken fern (Pteridium sp.; very toxic to horses) — broad-leaved
- Chinaberry (Melia sp.) — woody
- Cocklebur (Xanthium spp.) — broad-leaved
- Dogbane (Apocynum sp.) — broad-leaved
- Goathead (Tribulus sp.) — broad-leaved
- Groundsels (Senecio spp.) — broad-leaved
- Horsenettle (Solanum spp.) — broad-leaved
- Horsetail (Equisetum spp.) — broad-leaved (Granslike)
- Kochia (Kochia sp.) — broad-leaved
- Milkweed (Asclepias spp.) — broad-leaved
- Pokeberry (Phytolacca sp.) — broad-leaved
- Ornamental yew (Taxus spp.) — woody, very toxic to horses
- Pigweed (Amaranthus spp.) — broad-leaved
- Rattlebox (Crotalaria sp.) — broad-leaved
- Scurfy pea (Psoralea spp.) — broad-leaved
- Sesbania (Sesbania sp.) — broad-leaved
- Smartweed (Polygonum spp.) — broad-leaved
- Snakeroot (Eupatorium sp.) — broad-leaved
- St. Johns wort (Hypericum spp.) — broad-leaved
- Wild parsley or carrot (Lomatium, Daucus, and Pastinaca spp.) — broad-leaved
- Yarrow (Achillea sp.) — broad-leaved
- Landscaping and garden plants: castor bean, euonymus, gladiolus, ivy, pea vines, privet, nandena, boxwood, and tomato
The subject of potential plant toxicity reaction is a massive one that we cannot cover in total. There is an excellent reference book on plant toxicities to horses that all horse managers should have for more extensive study: Natural Poisons in Horses, available from the National Animal Poison Control Center, College of Veterinary Medicine, University of Illinois, Champaign-Urbana, Illinois (Hall et al., 1995).
Fescue pasture may be the single most-studied forage specifically for horses primarily because it causes reproductive problems in mares. The toxicity syndrome is variable, with problems including poor performance on pasture, abortion, and reproductive tract malfunctions such as an overly thick placenta that colts cannot break out of. Problems also include sick colts, dead colts, and agalactia (mare does not lactate well), a major syndrome.
Summer slump, fescue foot, and fat necrosis problems with cattle grazing fescue have not been directly associated with horses. Fescue is a tremendous forage in acreage and production per acre in the eastern half of the United States. It contributes much to cattle and horse forage programs. Fescue toxicity may affect only 1 percent of horses, but 100 percent of the mares in a given herd may have the problem.
The effect of endophyte-free fescue on horses is not completely understood. However, indications are that endophyte-free fescue and novel endophyte fescue do not cause the toxicity problems.
The problem with horses grazing endophyte-containing fescue is almost entirely associated with foal-producing mares. Apparently other classes of horses can be grazed on well-managed fescue quite successfully when husbandry practices are good.
The following precautions should be applied when grazing horses on endophyte fescue:
- In all fescue-associated pasture situations-full or part time:
- Follow good pasture management approaches, including rotational grazing, soil fertility, weed control, and clipping to remove stems and even the residue height.
- Add acceptable legumes to the fescue pasture.
- Conduct good horse nutrition and health programs, regardless of forage uses.
- Monitor udder development. If udder development is not obvious two weeks before foaling, expect problems. Even if it is obvious, the manager cannot be certain of milk volume available to the foal. Mares may lactate enough to keep the foal alive, but it will be thin and slow growing because of malnutrition. Remember, one problem leads to another.
- Be present at births to help colts out of tough birth membranes.
- Be prepared to feed supplemental colostrum and milk replacer when mares foal.
- Increase grain feeding sharply when mares foal with agalactia.
- If mares can be removed from fescue:
- Remove them after the first fall freeze and feed them elsewhere during winter until after foaling.
- Alternatively, remove mares from fescue 60 to 120 days before foaling and supplement with high-quality, nonfescue hay and feed.
- Practice all the items under 1 regardless of whether mares can be removed from fescue.
German Millet and Pearl Millet Toxicities
Foxtail or German millet can be used, along with other roughages, for horse forage. They are an alternative to producing sudangrass or other sorghum forages. Some pearl millet reportedly has an alkaloid buildup that can induce cattle toxicity. Horses may react to these alkaloids because they are susceptible to alkaloid toxicity syndromes.
All millets can accumulate nitrates, which in grazing or haying millets can reach toxic proportions. Nitrate can be controlled somewhat by reducing the amount of nitrogen per application and increasing the number of applications. German millet can cause oral mechanical lesions.
Sorghum Grass Toxicities
Sorghum grasses include sudangrass, johnsongrass, hybrid forage sorghums, and grain sorghums. Here we consider all classes of forage sudangrasses and associated hybrids the same. In reality, there may be some without the toxicity syndrome problems.
Sudangrass in the green growing stages can produce a horse urinary tract disease called cystitis syndrome or cystitis/ataxia (staggering). The disease is irreversible and believed to be associated with low levels of cyanide (prussic acid) in forage. Piper sudangrass is a low-prussic-acid variety and may be a good choice to minimize this problem.
Hay produced from sudangrasses will not likely cause cystitis/ataxia syndrome because prussic acid dissipates as hay cures. Sorghum pasture can also cause a problem for pregnant mares in the first three months of pregnancy, presumably because of prussic acid content. Foals can be born with contracted tendons, or mares can abort.
We must be cautious about high nitrate content in sorghum pasture and hay. The potential for it can be limited somewhat by cutting plants when they are growing under low stress conditions and on a sunny afternoon.
Sweet-stemmed sudangrasses and other sorghums that are relatively high in sugar also cause a laxative reaction in horses. If it is necessary to use sudangrasses, be sure to use a nonsweet starchy type and try to use other roughages as part of the ration.
Johnsongrass, which is a sorghum, and other sorghums can be high in prussic acid (cyanide), which can occur in any green plant and especially stressed ones. Rapid growth after a drought, drought or cold-stressed plants, and plants at and soon after frost are especially hazardous. Prussic acid does not occur in dangerous amounts in properly cured, dry hay. Prussic acid poisoning is not as severe a problem in horses as in cattle, but it can occur. Johnsongrass can also have a high nitrate content.
Secondary Toxicities or Ailments Associated with Pasture Plants
These potential toxicities and other horse reactions are associated with common horse forage. Probably no forage is absolutely safe. These common forages are relatively toxin free, but there are some things to be aware of:
- Alfalfa needs to be used with special caution to avoid overeating syndromes and blister beetle ingestion, which can cause colic and death.
- Bermudagrass fungus can cause some problems in cattle. Its effect on horses is unknown, but they have been known to develop colic on bermudagrass pastures and hay.
- Clovers, particularly red clover, can develop a mold that causes some problems in horses, such as slobbering or diarrhea.
- Ergot is a fungus that occasionally grows in the seed head of dallisgrass, wild rye, Old World bluestems, and other grasses. It can induce blood vessel constriction and other associated problems.
- Fescue toxicity syndromes are detailed elsewhere in this report.
- German millet is a nitrate accumulator and can cause oral lesions.
- Horses sometimes relish johnsongrass rhizomes, which cause possible sand colic from ingestion of soil dug up with the rhizomes. Johnsongrass also contains prussic acid. The rhizomes can be more than 10 percent crude protein.
- Some pearl millets can accumulate nitrates and alkaloids.
Horse Ailments Associated with Pasture
Several ailments are attributed to pasture management, and controlled management can control these ailments:
- founder and other intestinal disorders
- sand colic from eating rhizomes from forage such as bermudagrass and johnsongrass
- excessive slobbering attributed to legume grazing
Potential Fence Toxicities
Treated wood rails and posts create potential toxicity reactions to cribbing horses. The CCA-C treated wood commonly contains residue of chromate copper arsenate (CCA-C). Penta treated wood contains residue of pentachlorophenol. Both of these chemicals can potentially induce colic and other reactions. Caution prevails-if horses crib and then eat the treated wood, measures to prevent that behavior should be in force.