Poison hemlock (Conium maculatum) grows throughout the United States in areas with abundant moisture (i.e., creeks, ditches etc.). Animals eating poison hemlock die from acute respiratory failure or have deformed offspring. The most critical season to avoid poison hemlock is spring because the plant often appears before other forage has emerged. Green seed pods may be eaten in mid-to-late summer. Furthermore, poison hemlock may regrow in fall after seeds shatter. Ingestion during fall may coincide with birth defects in pregnant cattle, if they are i n the firs t trimester of gestation (days 30-75). If poison hemlock invades hay fields, the contaminated hay can poison livestock. Even though toxicity decreases upon drying, sufficient toxins may be r etained to poison livestock. Cattle appear to be particularly susceptible because of their acceptance of the plant and their sensitivity to the alkaloids t hat c ause birth defects. Poison hemlock can be easily controlled with phenoxy herbicides.

38of rotational grazing scheme. Herbicidal control of lupines is feasible, but is usually more expensive than altering a grazing management program.

37These herbicides do not reduce toxic alkaloid concentrations in treated larkspur plants, and metsulfuron may increase toxicity. Therefore, sprayed areas should not be grazed until larkspur has withered and decomposed.

Senecio or groundsel species (Senecio spp.) and houndstongue (Cynoglossum officinale) contain highly toxic pyrrolizidine alkaloids. These alkaloids are potent liver toxins that cause wasting and photosensitization. Senecios and houndstongue occur on many western U.S. rangelands. Only seven of more than 112 senecio species are known to be toxic so correct identification is essential. Managing rangelands so that plant communities are in good condi- tion and adequate forage is available is crucial to reduce losses to senecio. Generally, senecios are not very palatable, and are avoided by grazing livestock if other forage is available. Drought stress and overgrazing can increase populations of threadlleaf groundsel, as the plant is an aggressive invader. Drought is an especially dangerous time because other forage may be lacking and the toxic alkaloid concentration in senecio plants increases during drought, so grazing animals may ingest higher quantities of more toxic forage. Senecio species are also most toxic when plants are reproducing, thus avoiding pastures when these plants are in bud, flower, or seed is prudent. Proper grazing management must consider stocking rates, as excessive stocking may increase the amount of toxic plant consumed when alternative forages become limited. Excessive stocking may lead to degradation of the desirable plant community allowing senecio species to increase. Herbicidal control may alleviate some problems if incorporated into an overall management program