TOX­I­COL­OGY Is milk­weed a dan­ger?

EQUUS - - Eq Consultant­s - An­thony P. Knight, BVSC, MS, DACVIM Pro­fes­sor Emer­i­tus Colorado State Univer­sity

Q:My horse oc­ca­sion­ally grazes as I ride him on the trails---he knows when he is al­lowed to eat and when he is not. Nor­mally he’ll snack on grasses, dan­de­lions and clovers, but he will also take an oc­ca­sional bite of milk­weed. I have been told that milk­weed is dan­ger­ous and con­tains a neu­ro­toxin for horses. Is this true? How much must he eat be­fore dam­age oc­curs, and what is it about milk­weed that at­tracts a horse to eat it if it’s not good for him? Sharon Bowen Chicago, Illi­nois

A:Whether or not a horse is poi­soned by milk­weed very much de­pends on the species he has eaten and how much of the plant he con­sumes. More than 100 species of milk­weed grow in North Amer­ica, and all con­tain vari­able amounts of dif­fer­ent tox­ins.

In gen­eral, species of milk­weed with nar­row leaves (less than half an inch wide) at­tached to the stems in whorls (ver­ti­cil­late) con­tain tox­ins that af­fect the ner­vous sys­tem. These nar­row-leaved milk­weeds are quite palat­able when green and es­pe­cially when dried into hay. Most live­stock losses are as­so­ci­ated with the nar­rowleaved species such as whorled milk­weed ( As­cle­pias sub­ver­ti­cil­lata and As­cle­pias fas­ci­c­u­laris).

The other group of milk­weeds that can cause poi­son­ing in an­i­mals are the broad-leaved species con­tain­ing car­de­no­lides, which af­fect the heart and can

cause sud­den death. Horses and other an­i­mals rarely eat these milk­weeds be­cause they are far less palat­able. If your horse is rou­tinely nib­bling a par­tic­u­lar type of milk­weed, it would be wise to take a sam­ple to your lo­cal ex­ten­sion of­fice for iden­ti­fi­ca­tion.

The quan­tity of nar­row-leaved milk­weed nec­es­sary to cause se­vere poi­son­ing and death of a 1,000-pound horse is ap­prox­i­mately two to three pounds of the green plant. Oc­ca­sional brows­ing on milk­weed is not likely to cause any sig­nif­i­cant prob­lem.

As for why a horse would eat a toxic plant---an­i­mals do not in­her­ently know what is poi­sonous. When graz­ing freely, horses get by be­cause they tend to eat only small amounts of a wide va­ri­ety of plants, thereby never eat­ing enough of any one toxin to be­come ill. How­ever, a hun­gry horse con­fined in an area with lit­tle else to eat may con­sume large quan­ti­ties of milk­weed and can be poi­soned.

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