The signs and cost of fescue toxicosis in cattle

The pathology of cattle consuming endophyte-infected tall fescue varies greatly based on the weather and the alkaloid concentration. The most readily apparent signs of fescue toxicosis include reduced feed intake, weight gain, milk production and reproductive efficiency, as well as tissue necrosis and a rough hair coat. Decreases in productivity caused by fescue toxicosis are estimated to cost U.S. beef producers more than $2 billion annually (Kallenbach, 2015).

Absorption of alkaloids

Calculating the animal’s retention of ergot alkaloids is difficult due to biotransformation. Generally, it is estimated that 76–92% of consumed ergot alkaloids are absorbed, with the other 8–24% excreted in the feces. The math on alkaloid absorption and excretion doesn’t always add up, as alkaloids are broken down and/or biotransformed into numerous metabolites. Most alkaloids are ultimately excreted in the urine as lysergic acid.

Fescue alkaloids and microbiome shifts

An emerging area of research is the interaction between fescue alkaloids and the microbiome. Decreases in the Erysipelotrichaceae family and increases of Ruminococcaceae, Lachnospiraceae and Clostridiaceae, as well as abundances of Planctomycetes, Chloroflexi and Proteobacteria phyla have been reported for cattle grazing infected fescue. Fescue seed extract, when added to in vitro fermentations, led to increased populations of tryptophan-utilizing bacteria. Considering the tryptophan base of ergot alkaloids, this increase likely indicates an up-regulation in detoxification capacity. The characterization and identification of the three isolates with the highest conversion abilities found that all three were gram-positive, spore-forming rods that produced ammonia from tryptophan, classified as Clostridium sporogenes.

Receptor-binding of fescue ergot alkaloids

Due to the structural similarity of ergot alkaloids and serotonin, dopamine, norepinephrine and epinephrine, several receptor types in numerous tissues are affected during fescue toxicosis in cattle. This results in a wide range of effects on physiology and metabolism. As more research examining the underlying mechanisms is completed, the connections between ergot alkaloid receptor-binding and animal performance grow more complex.

Individual animal sensitivity to infected fescue is affected by:

• Environmental conditions.
• The density of the receptors.
• The capacity for liver and ruminal degradation.
• Other genetic factors.

Fescue's effect on weight gains

Ergot alkaloid-induced vasoconstriction reduces heat dissipation, resulting in a variety of physiological fescue toxicosis symptoms in cattle, including an increased respiration rate and elevated core body temperature. Ultimately, this leads to lower weight gains — which is generally known as the summer slump, as animals spend less time grazing as a result of standing in the shade or water to cool off. In colder months, fescue-associated vasoconstriction combines with thermoregulatory vasoconstriction, resulting in tissue death in the extremities, which is commonly known as fescue foot.

Ergot alkaloid consumption also leads to:

• The thickening of the medial layer of blood vessels
• Endothelial cell damage
• Vascular stasis
• Thrombosis
• Ischemia
• Changes in blood pressure, among other cardiovascular effects

Fescue's effect on rumen fill

The frequency and amplitude of the ruminal contractions, as well as changes in eating patterns due to fescue toxicosis in cattle, combine to affect rumen fill, passage rates and intake.

• Vasoconstriction also reduces blood flow to the rumen, decreasing VFA absorption.
• Increased rumen fill provides a negative feedback loop, exacerbating reduced intakes.
• While the total tract digestibility of the feeds is generally unchanged, these alterations work in concert to reduce nutrient availability, contributing to the reduced growth rate frequently observed in cattle grazing fescue.
• Added to this are the effects of alkaloids on circulating serotonin levels, the hypothalamic center and tryptophan-related satiety.

Fescue's effect on energy metabolism

Ergot alkaloids affect energy metabolism primarily when alkaloid intakes are high and during heat stress. Growth differences in cattle during fescue toxicosis are most likely the result of reduced intake, as no differences in retained energy or energy partitioning were caused by alkaloid ingestion when feed intake was equal.

• When fed near maintenance, cattle had lower basal metabolic rates.
• At higher feeding rates, maintenance energy requirements increased.
• When combined with observed increases in fat loss and a higher capacity for gluconeogenesis in the liver, this indicates the prioritization of energy (when available) toward elevated respiration, thermoregulation and alkaloid detoxification.

Economic losses due to fescue toxicosis

As much as 75% of the economic losses attributed to infected tall fescue pastures are related to decreased calving rates.

For cows:

• Alkaloid consumption reduces the circulating levels of several hormones important for reproductive efficiency, including progesterone and estradiol.
• Changes in ovarian follicle development, oocyte quality and luteal function have been reported.
• When combined with vasoconstriction to the uterus and ovaries, it is no surprise that reductions in reproductive efficiency are seen in cows grazing infected fescue.
• There is speculation that vasoconstriction in the umbilical cord contributes to low birthweights and diminished fetal development.

On the bull side of the equation, research indicates that ergot alkaloids may:

• Affect sperm count.
• Increase the occurrence of abnormal sperm.
• Alter motility, especially during the summer months.

Fescue's effect on milk production and calving

The structural similarity of ergot alkaloids to dopamine results in reduced prolactin secretion from the anterior pituitary gland. Further, changes in gene expression in the mammary glands of cattle consuming fescue indicate alterations in their lipid metabolism and small molecule transport. Altogether, these changes translate to reduced mammary development and lower milk production. Similar changes in lipid metabolism can be seen in reduced levels of circulating cholesterol and the occurrence of fat necrosis (lipomatosis), and in some cases of long-term alkaloid exposure, these fatty masses can cause digestive issues and dystocia. 

Fescue's effect on hair coat and immune function

Reduced prolactin also leads to the rough hair coat that is often observed in cattle grazing infected fescue. Originally thought to be a retained winter hair coat, it is now known that low prolactin levels increase hair growth rates. As prolactin is a co-factor in humoral immune function, the long-term grazing of infected fescue can lead to depressed levels of immunoglobulins. However, as nutritional levels also affect immune function, more work needs to be done to determine whether altered immunocompetence is a direct result of ergot alkaloid consumption or a secondary effect of a diminished nutritional status. While there is no true fescue toxicosis treatment for cattle, there are ways to mitigate the challenges that fescue presents through management, nutrition and feed additives.

References and Additional Reading

Fribourg, H. A., D. B. Hannaway, and C. P. West (ed.) 2009. Tall Fescue for the Twenty-First Century. Agron. Monog. 53. ASA, CSSA, SSSA. Madison, WI. 540 pp. Also (http://forages.oregonstate.edu/tallfescuemonograph).

Kallenbach, R. L. (2015). BILL E. KUNKLE INTERDISCIPLINARY BEEF SYMPOSIUM: Coping with tall fescue toxicosis: Solutions and realities. Journal of Animal Science, 93(12), 5487-5495.

Mayberry, K. J. (2018). Evaluation of Genetic Resistance to Fescue Toxicosis in Purebred Angus Cattle Utilizing Phenotypic Variables, Calf Performance and Cytokine Response. Thesis, North Carolina State University.

Melchior, E. A., & Myer, P. R. (2018). Fescue toxicosis and its influence on the rumen microbiome: mitigation of production losses through clover isoflavones. Journal of Applied Animal Research, 46(1), 1280-1288.

Poole, R. K., & Poole, D. H. (2019). Impact of ergot alkaloids on female reproduction in domestic livestock species. Toxins, 11(6), 364