What’s good for the butterfly may be toxic to livestock

Friday, Sept. 7, 2018 2:43 PM
Milkweed, the food source for monarch butterflies, may be poisonous to livestock, but officials say issues can be easily avoided with proper management.

The milkweed plant may be a crucial food source for the embattled monarch butterfly, but it is not so friendly to livestock.

With proper management, however, officials say issues can be easily avoided.

For years, monarch butterfly populations have declined at an alarming rate, with many researchers attributing the insect’s shrinking numbers to a loss of milkweed habitat (its only food source) and loss of winter habitat.

Efforts to restore monarch butterfly populations have included restoring milkweed plants to help feed the insect during its winter migration from the northern United States and Canada to Mexico and Southern California.

Caution must be taken, however, for landowners who have property with both grazing livestock and milkweed on it, said La Plata County weed control coordinator Ben Bain.

Milkweed is a native plant, so it is not regulated, Bain said. But it can be toxic if ingested by livestock, such as horses, cattle, sheep and goats.

For landowners with grazing animals, Bain said it is recommended property owners either remove milkweed or make sure it is blocked off from livestock.

Bain said it is rare to hear a report of livestock dying from ingesting milkweed, especially when compared with the number of animals that die from eating poisonous hemlock.

For the most part, animals know to avoid milkweed. With a healthy pasture, Bain said livestock will have plenty to eat and not come into contact with milkweed.

“Animals eat all the desirable plants and leave the milkweed,” he said. “And so it’s kind of ironic … that over-grazing can lead to more milkweed growing.”

But sometimes, landowners aren’t aware milkweed is growing on their property, so when they cut hay for the season, milkweed can become interspersed with the hay.

Then livestock, especially younger animals such as calves who haven’t fully developed the senses to avoid poisonous plants, can ingest it.

Most people, when informed of the potential risks of milkweed to livestock, will spray it with an herbicide, Bain said. But despite the removal, Bain said there’s plenty of milkweed in the county for monarchs to feed on.

“If someone sprays it on their parcel, it is a drop in the bucket compared to the overall population,” Bain said.

Gail Morris, coordinator of the Southwest Monarch Study, a citizen science research project based in Arizona, said it is a balancing act to restore milkweed habitat and protect livestock.

But in her research, she has found most livestock easily avoid milkweed, as well as other poisonous plants abundant in pastures. It is in severe conditions, when sufficient forage is not available or in drought years, it can be consumed by livestock.

“We’re always marveling … how the cattle and horses eat around it,” Morris said. “It has a bitter taste, so they taste it and turn away.”

Bain said milkweed does not typically pose a risk to pets or children.