Toxic forbs: Not all that is green is good for livestock


Cow-Calf Commentary

As grass and sedges green up across range and pastureland in the High Plains, so do forbs and shrubs. After a long, dreary winter, livestock browse the tender greens with relish as producers look on with springtime grins.

As most producers know, however, not all the springtime green is livestock food. Some of the greening forbs are toxic, and the potential for poisoning is most acute early in the year.

Livestock will generally avoid many of the toxic native forbs so long as there is plenty of grass, sedge and non-toxic forbs for them to browse. Most of the vetch and loco weed species are bitter and unpalatable to livestock and they rarely consume enough to
cause illness.

Two prominent exceptions to this rule are deathcamas and larkspur, and both plants are widespread across tri-state range and pasturelands.

Both deathcamas and larkspur green up early and out-produce grass and sedge in biomass production. Sheep, which prefer broadleaf browse, frequently sample deathcamas as they graze, and only a few ounces can be lethal. Grass-loving cattle are prone to sampling larkspur as they graze, and again, only a small quantity can be lethal.

As grass production catches up later in the season, making livestock-preferred browse far more abundant, the threat of poisoning from larkspur and deathcamas falls off sharply.

As producers scout range and pastureland prior to turnout, they should watch for and plan to manage toxic forbs.

Larkspur

Of the two larkspur species native to the region, low larkspur, Delphinium bicolor, is the most prevalent, with plains (often prairie) larkspur, Delphinium virescens, generally found east of the Nebraska Panhandle though it does appear in scattered areas of the Panhandle, southeast Wyoming and northeast Colorado.

Low larkspur is a perennial forb 6-24 inches in height with deep blue and white cylindrical flowers featuring a characteristic spur near the stem, blooming in May and June.

Larkspur contains several alkaloids and is highly poisonous to cattle. Early green-up and palatability to cattle make larkspur a serious threat for producers early in the spring. Poisoning symptoms in cattle are primarily weakness and falling, with animals frequently collapsing to their knees in front while remaining standing in the rear. Bloat is common. Death is usually due to respiratory failure. Death has been observed in 1,000 lb. animals within an hour of consuming as little as five pounds of young larkspur. Toxicity falls off sharply after seed dispersal but the seeds remain toxic.

Many producers choose to treat limited larkspur infestations as a management problem, fencing off troublesome areas until later in the season when plant toxicity falls off and grass is abundant. Likewise, infested pastures are generally grazed later in the season in rotational systems. Heavy infestations can be sprayed with Grazon P+D or Cimarron. Be sure to follow label directions and rates. Cost ranges from $6-$26 per acre. Cattle in particular should not be turned onto sprayed rangeland until larkspur plants are completely dead as the stressed plants tend to concentrate alkaloids and become more toxic.

Deathcamas

Deathcamas (meadow dathcamas), Zigadenus venenosus, is found across the region on dry rangeland to wet prairies and open ponderosa pine woodlands.

Deathcamas is a perennial forb, 8-30 inches in height, with grass-like leaves and clustered yellow-white to greenish-white flowers blooming from May through June.

Deathcamas is highly poisonous to livestock, containing the alkaloid zigadenine which is more toxic than strychnine. Deathcamas greens up even earlier in the spring than larkspur, and while it is generally quite unpalatable, sheep especially (sometimes cattle and horses) may graze it when there is little or no other desirable green forage available. Poisoning symptoms include rapid breathing, excessive salivation and swallowing, nausea and weakness, staggering, convulsions and coma. Symptoms appear within a few hours of ingestion with death or recovery within 12-48 hours. Some cases of human poisoning and death have been reported when foragers thought they were eating wild onions. Wild onions have tube-like leaves and a characteristic onion odor. Deathcamas has flat, grass-like leaves and is odorless.

As there appear to be no herbicides specifically labeled for death camas control in the U.S., producers are probably limited to employing management techniques similar to those for larkspur. In general, death camas is rarely a problem for livestock producers when grass is plentiful, probably due to its
unpalatability.

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