Plant toxicities may be grouped in two categories:

1. definite poisonous plants
2. 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 Web site, www.noble.org, and its plant gallery, created by Chuck Coffey and Russell Stevens, for pictorial identification of many of these and other plants.

1. Bitterweed (Actinea spp.)-broad-leaved
2. Black locust (Robinia sp.)-woody
3. Bladderpod (Glottidium sp.)-broad-leaved
4. Bracken fern (Pteridium sp.; very toxic to horses)-broad-leaved
5. Chinaberry (Melia sp.)-woody
6. Cocklebur (Xanthium spp.)-broad-leaved
7. Dogbane (Apocynum sp.)-broad-leaved
8. Goathead (Tribulus sp.)-broad-leaved
9. Groundsels (Senecio spp.)-broad-leaved
10. Horsenettle (Solanum spp.)-broad-leaved
11. Horsetail (Equisetum spp.)-broad-leaved (Granslike)
12. Kochia (Kochia sp.)-broad-leaved
13. Milkweed (Asclepias spp.)-broad-leaved
14. Pokeberry (Phytolacca sp.)-broad-leaved
15. Ornamental yew (Taxus spp.)-woody, very toxic to horses
16. Pigweed (Amaranthus spp.)-broad-leaved
17. Rattlebox (Crotalaria sp.)-broad-leaved
18. Scurfy pea (Psoralea spp.)-broad-leaved
19. Sesbania (Sesbania sp.)-broad-leaved
20. Smartweed (Polygonum spp.)-broad-leaved
21. Snakeroot (Eupatorium sp.)-broad-leaved
22. St. Johns wort (Hypericum spp.)-broad-leaved
23. Wild parsley or carrot (Lomatium, Daucus, and Pastinaca spp.)-broad-leaved
24. Yarrow (Achillea sp.)-broad-leaved
25. Landscaping and garden plants: castor bean, euonymus, gladiolus, ivy, pea vines, privet, nandena, boxwood, and tomato

Fescue Toxicity
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:

1. In all fescue-associated pasture situations-full or part time
   1. Follow good pasture management approaches, including rotational grazing, soil fertility, weed control, and clipping to remove stems and even the residue height.
   2. Add acceptable legumes to the fescue pasture.
   3. Conduct good horse nutrition and health programs, regardless of forage uses.
   4. 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.
   5. Be present at births to help colts out of tough birth membranes.
   6. Be prepared to feed supplemental colostrum and milk replacer when mares foal.
   7. Increase grain feeding sharply when mares foal with agalactia.
      
2. If mares can be removed from fescue
   1. Remove them after the first fall freeze and feed them elsewhere during winter until after foaling.
   2. Alternatively, remove mares from fescue 60 to 120 days before foaling and supplement with high-quality, nonfescue hay and feed.
   3. Practice all the items under 1 regardless of whether mares can be removed from fescue.

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.