All 400-plus Prunus species are toxic to livestock. Prunus is a genus comprised of both deciduous and evergreen plants, but the most commonly recognized species are the stone fruits: cherries, peaches, plums, almonds, apricots, and nectarines. All parts of the plant are toxic except the mature fruits. All species are toxic, whether of the fruiting or ornamental/flowering variety. The black cherry is considered the most toxic of the Prunus species.
The toxic compound in Prunus species is a host of cyanogenic glycosides (a cyanide molecule attached to a sugar molecule). These compounds are stored in plant cell vacuoles. If the vacuoles are ruptured, however – such as by chewing or when a branch breaks – the glycosides are released. Once these compounds come in contact enzymes in other parts of the plant cell, they break down into their constituent molecules: sugar and hydrogen cyanide, a poison.
Partially wilted plants are generally more toxic than live, healthy plant tissue because stress has caused vacuoles to rupture and the cyanogenic glycosides to be released. Fully dried plant parts are usually not dangerous because the concentration of cyanogenic glycosides decreases as drying occurs. There are several conditions which may cause plant production of cyanogenic glycosides to increase and thus cause the plant to become more toxic: when growing conditions are cool and wet, after plants are fertilized with nitrogen, in low phosphorus soils, after frost, during drought, and following applications of herbicides containing 2,4-D.
Cattle, sheep, goats, and other ruminants are more likely to be poisoned by Prunus species because bacteria in the rumen speed up the process of releasing hydrogen cyanide from the cyanogenic glycosides. Furthermore, the acidic stomach or non-ruminant animals inactivates some of the enzymes that facilitate the break down process.
Hydrogen cyanide acts as a poison by preventing red blood cells from releasing oxygen, essentially causing an animal to suffocate. Adult cattle can be killed by eating just 2 pounds of leaves. Death typically occurs within minutes; affected animals are usually found dead. If the animal has eaten a lesser amount, clinical signs may include excitement, rapid pulse, muscle tremors, rapid and labored breathing, salivation, and runny eyes. Sometimes veterinary intervention can save an affected animal but only when the signs are detected early and the intervention is initiated almost immediately.
Like with most toxic plants, animals are not likely to eat plant tissues from Prunus species when there is other forage available. Be sure to provide adequate forage at all times, especially when pastures are sparse due to drought or overgrazing.
Check your pastures to determine if any Prunus are growing. It’s generally not realistic to remove them all, but it’s important to be aware if they are present. You may decide to fence the animals away from them, at least during times of the year when you suspect they may be more highly toxic or when animals are more likely to sample them. Monitor fields after storms and after any tree work is performed (such as by the power company) to ensure downed branches haven’t fallen into pastures or hay fields.