Hot, dry weather can increase the chances of lethal weeds invading your pastures.

According to Penn State Extension, droughty conditions can result in poisonous weeds in pastures and hay that can be toxic to livestock if consumed in large enough quantities.

Check out this slideshow of the most notorious problem weeds that could threaten your animals.

Thistles, dogbane, milkweed, horse nettle, poison hemlock and jimsonweed are among the biggest problem weeds Dwight Lingenfelter, Extension weed specialist at Penn State, sees in pastures across the region.

“During drought periods, perennials tend to thrive since they have established and deep root systems to help survive long periods of drought,” he says. “Annuals weeds tend to slow their growth during dry periods, and if it persists too long, they could eventually succumb due to lesser and shallower root systems that might not have access to moisture deeper in the soil profile.”

Tools to fight back

Spot controls like 2,4-D + dicamba; Crossbow (2,4-D+triclopyr); or glyphosate can be effective, but it’s important to remember the following:

“Certainly, annual plants become much more ‘tolerant’ with dry weather, maturity and persistent warm temperatures. Large, drought-stricken annuals are harder to kill. With perennial weeds, the effect of drought is less clear,” Lingenfelter says. “Cool-season perennials including Canada thistle, quackgrass and dandelion will certainly go into a summer dormancy period when dry, warm weather persists. 

“If possible, they should not be treated with an herbicide until actively growing. Cool-season perennials mimic the same growth cycle as your lawn — active in the late spring and early summer followed by a slow period and then a rebound in later summer and early fall. Once the ‘heat’ of summer has passed and assuming they have relatively healthy green leaves, then an effective systemic herbicide should work well.”

Here are other considerations: