Drought stress induces physiological processes in plants which have evolved to ensure plant survival.
For example, the production of the natural plant hormone abscissic acid in plant roots during times of environmental stress such as drought signals stomata in plant leaves to close, reducing transpiration and decreasing water loss in the plant.
In addition to this natural physiological mechanism, some plants possess mechanisms that will reduce herbivory by grazing animals. While these physiological mechanisms are beneficial to the plant, they can have negative, often detrimental effects on livestock consuming the forage.
The recent event of 15 head of cattle on a Texas ranch dying as a result of prussic acid poisoning from Tifton 85 bermudagrass has brought the negative implications of forages from drought stress to the forefront in the media.
Initial media reports mistakenly identified the grass as a genetically-modified organism, claiming the grass emitted cyanide gas which caused the cattle deaths. Shortly after however, these false claims were laid to rest by various blog posts and articles released by those in the agricultural community.
While the deaths of these cattle are unfortunate, this incident brings to light the need for cattle producers to be vigilant for potentially unfavorable forage conditions that could be detrimental to livestock health.
Besides prussic acid poisoning problems such as nitrate toxicity can also occur. Under stressful conditions forages can accumulate nitrates. This is especially common with forages such as corn, sorghum and Johnsongrass but occurs in others, too. Nitrate poisoning has similar effects on livestock as prussic acid. It reduces oxygen carrying capacity of blood, essentially suffocating the animals affected.
Drought conditions decrease forage supply, sometimes resulting in livestock consuming weeds that they may not have initially found desirable. Another recent report from Kansas confirmed deaths of several calves from consumption of a weed in the Senecio family causing acute liver toxicity. The cattle lacked adequate forage and had begun to graze on weeds. While many of the weeds they consumed were non-toxic it only took one toxic weed to cause death.
In these difficult times of drought, it is more important than ever for livestock producers to be aware of their forage conditions. If there is any shadow of a doubt that consumption of forage could have negative implications, forages should be tested.
If adequate forage is not available, provide supplemental forage to deter livestock from consuming potentially toxic weeds and forages. If possible remove cattle from affected pastures. To learn more about what forages may pose an issue on your farm or ranch I recommend contacting your state’s forage specialist.