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CORVALLIS, Ore. - Cows that eat bark, berries or branches from western juniper trees late in pregnancy are more likely to abort their calves or give birth early, Oregon State University researchers have discovered.

The tree's harmful effect on pregnant cattle was unknown until an eastern Oregon veterinarian noticed a pattern of lost calves and asked the OSU Extension Service about it.

"People had always wondered what happened to the five to 10 percent of cows with lost pregnancies," said Tim Deboodt, a range management specialist with Extension in Crook County. "So we started our research from scratch on a tip."

OSU researchers pinpointed that western juniper contains toxins known as labdane acids. These chemical compounds constrict the flow of oxygen to a fetus. In the early 1970s, labdane acids, specifically isocupressic acid, were identified in ponderosa pine needles, which trigger premature birth in cows in a condition called pine needle abortion.

Two of the six heifers OSU researchers monitored lost calves after eating western juniper during the last 30 days of pregnancy. Only a small percentage of calves born early because of juniper or pine needles are likely to survive without intensive care, said Cory Parsons, a livestock specialist with Extension in Baker County.

OSU researchers are now conducting a larger study to examine juniper's effect on more than 20 cows in late pregnancy. Results are expected by summer 2013.

Oregon State researchers will also analyze if juniper consumption inhibits conception or bears any consequences early in pregnancy. Based on prior knowledge about pine needles, OSU researchers suspect that juniper is most likely to cause cows to abort during the last trimester of gestation - when fetuses need the most oxygen.

OSU's juniper research has been supported through a number of grants from the Oregon Beef Council and published in a handful of academic journals, including Rangelands and the International Journal of Poisonous Plant Research.

Western juniper abortions have not registered a large economic impact so far, Parsons said. Although some grazing cattle are in contact with juniper on a daily basis, cattle do not naturally seek it as food.

In recent decades, juniper trees have been piled up as riprap to stabilize the banks of creeks and streams being restored in western states. Cows may come in contact with these trees when they use these watering holes.

"If cattle have plenty to eat, they have no desire to chew on juniper," Parsons said, but then cautioned, "When cows are hungry and bored, they're going to eat to fill their bellies up, especially during times of drought and heavy snow."

To reduce the risk of exposure to juniper during the last trimester of pregnancy, Parsons recommended slowly introducing cattle to areas where juniper exists if they have not already been acclimated to the site. He also suggested cutting lower branches off trees if possible and providing adequate feed daily to reduce the animals' desire to graze juniper.