HERD HEALTH By ANNE KOONTZ
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 percent of consumed ergot alkaloids are absorbed, with the other 8–24 percent 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 upregulation 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
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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 Carolina Cattle Connection q JULY 2021
Regular copy deadline is JULY 5 for the AUGUST issue Spotlight material is due JULY 1 for the AUGUST issue • 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.
