Goats, proper irrigation control weeds

Don Morishita, University of Idaho professor of weed science, reminds growers that moisture makes the weed. Some weed, like knapweed, are a problem in dry areas, while others such as foxtail are found in wet areas. Weed control can depend more on how much you irrigate.

“If you allow irrigation water to run too long in one place, foxtail may become established because it can tolerate saturated ground better than most desirable pasture grasses,” he says.

Once established, it’s difficult to eradicate because you can’t selectively control it with herbicides without injuring or killing other grasses, he notes, adding that it is best controlled with management practices like reduced irrigation.

Key Points

• A weed is any plant growing where it’s not desired.

• A herbicide application should be properly timed with a plant’s life cycle.

• Goats or sheep can help control noxious weeds that cattle won’t or shouldn’t eat.


In dry conditions mullein, knapweed, black henbane and certain types of thistles invade pastures; they tolerate drought better than many desirable forage species, he says.

“Mowing biennials [bull thistles, scotch thistles, musk thistles] can be effective, if you can keep them from going to seed for a couple years,” Morishita adds. “It’s more difficult to control perennials like Canada thistle.”

Tough perennials or biennial weeds like burdock can be controlled with a good broadleaf herbicide if applied in the fall, near the end of the plant’s growth, he notes. “This will kill plants that are storing food reserves in their roots for next year. The herbicide will be taken down into the root and kill the plant,” says Morishita.

But don’t overdo it. “If you use too much, it quickly kills the top growth and doesn’t get down into the taproot, and it survives to regrow next spring,” he warns. But there are more ways to fight weeds than using chemicals.

Shannon Williams, Lemhi County Extension agent, conducted a three-year project to see if goats would be feasible to control spotted knapweed. “We have a lot of knapweed and did grazing trials at University of Idaho Cummings Center to discover the best timing for grazing,” says Williams. “We found the best seed reduction is accomplished by grazing knapweed at bud to bloom stage. Goats go through the patch and strip off seed heads, then come back and strip off all the leaves.”

Feasting on leafy spurge

For leafy spurge, goats enjoy eating the whole plant. Grazing can be an important part of an integrated control approach, using timely grazing, spray and biological [insect] controls.

“The goal with grazing is to stop seed set,” she explains. “If you can keep plants from producing seeds, they won’t continue to spread into new areas.”

“We’ve had great success using goats and insects in our target area, and herbicides on the outside. We’ve been able to keep spurge confined, and ranchers tell us grass is coming back into these areas,” says Williams.

Goats and insects eventually kill the plants, at a slow enough rate that grass, instead of other opportunistic species, can move in. If you kill everything at once, you’ll just get annual weeds or other noxious plants growing in vacant areas, she notes. Goats worked well in conjunction with biological control, adds Williams.

The Bureau of Land Management “had several biological control releases and didn’t want goats interrupting the insects’ mating season,” she says. “But in areas they let us graze, the bugs doubled in number the next year. Goats removed old growth and made it easier for insects to get at the new tender growth. Plus, they hitched rides on the goats.”

The insects that eat leafy spurge don’t fly or move very far and used the goats to travel to their next meal, she says.

Contract grazing with goats is gaining popularity around the West to control spurge, knapweed and yellow star thistle. Goats can safely eat the latter, whereas cattle or horses cannot. Sheep are often used to eat and trample tall larkspur before cattle are moved into a problem pasture. Different animals have different tolerance levels for various compounds within these poisonous plants.

Smith Thomas is from Salmon, Idaho.

This article published in the September, 2010 edition of WESTERN FARMER-STOCKMAN.

All rights reserved. Copyright Farm Progress Cos. 2010.