Tall Fescue Toxicosis Research Articles

Transferring endophyte technology to North American farmers

Bacon and others discovered the tall fescue endophyte in 1977. Since that time, North American researchers have developed several endophyte-based technologies to reduce fescue toxicosis. The most recent has been the release of MaxQ, a novel endophyte that enhances persistence but does not cause fescue toxicosis. Despite its advantages, MaxQ has been adopted slowly across the fescue belt of the US. This paper presents possible social and managerial reasons for this slow adoption. It closes by offering some possible solutions. Keywords: MaxQ, novel endophyte, tall fescue toxicosis, pasture renovation, educational efforts

Clinical manifestations of tall fescue (Festuca arundinacea) and perennial ryegrass (lolium perenne) toxicosis in Oregon and Japan

There are multiple vehicles for endophyte toxicosis in animals including exposure from pasture, straw residues and seed screenings. This report discusses the clinical cases typically seen with tall fescue and perennial ryegrass toxicosis in Oregon and Japan. Case I involves a herd of 330 Black Angus cattle. Before the March calving season the owner wished to increase the protein content of the feed ration by feeding pellets made of seed screenings and grass hay. Forty two animals were lost to tall fescue toxicosis and dry gangrene of the feet and legs. Case II involves 1300 beef cows in Eastern Oregon fed grass straw; 485 animals were lost due to dry gangrene characteristic of tall fescue toxicosis. Case III describes 4 of 15 cases of both tall fescue and perennial ryegrass toxicosis in Japanese black cattle. Case IV involves llamas and alpacas on pasture and lawn paddocks where some animals were affected by tall fescue and some by perennial ryegrass. Keywords: tall fescue, Festuca arundinacea L., perennial ryegrass, Lolium perenne, endophyte, Neotyphodium coenophialum, Neotyphodium lolii

Electrospray[+] tandem quadrupole mass spectrometry in the elucidation of ergot alkaloids chromatographed by HPLC: screening of grass or forage samples for novel toxic compounds

Ergot alkaloids are mycotoxins generated by grass and grain pathogens such as Claviceps, for example. Ergot alkaloid-poisoning syndromes, such as tall fescue toxicosis from endophyte-infected tall fescue grass, are important veterinary problems for cattle, horses, sheep, pigs and chickens, with consequent impact on food, meat and dairy industries. Damage to livestock is of the order of a billion dollars a year in the United States alone. HPLC with UV and fluorescence detection are the predominant means of ergot alkaloid determination, with focus on quantitation of the marker compound ergovaline, although ELISA methods are undergoing investigation. These techniques are excellent for rapid detection, but of poor specificity in defining new or poorly characterized ergot alkaloids and related compounds. This paper demonstrates the facility of using electrospray(+) mass spectrometry with multiple reaction monitoring (MRM) detection during chromatographic examination of ergot alkaloid standards of lysergic acid, lysergol, ergonovine, ergovaline, ergotamine, ergocornine, ergocryptine and ergocrystine by HPLC. Ergoline-8 position epimers could be separated on the gradient HPLC system for ergocornine, ergocrystine and ergonovine and appeared as shoulders for ergotamine and ergovaline; epimers generally showed different patterns of relative intensity for specific MRM transitions. There was reasonable correspondence between retention of standards on the 2-mm ESI(+)MS phenyl-hexyl-based reverse phase column and those on the 4-mm C18-based column. Since up to 10% of clinical cases involving toxin exposure display unidentified chromatographic peaks, 11 samples of feed components associated with such cases were studied with developed MRM methods to attempt elucidation of crucial components if possible. Ergotamine appeared in all, ergovaline appeared in five and ergocornine appeared in six; ergonovine, ergocryptine, ergocrystine and lysergol also appeared in several. In addition, molecular weights of compounds newly revealed by mass spectrometry suggested ergosine, ergostine and ergoptine in four samples, for which standards were not available. Dehydrated products of ergotamine, ergocrystine and ergocornine were discovered, along with dihydrogenated ergocrystine and ergocryptine in seven of the samples, and the issue was raised as to whether dehydration was strictly an instrument-derived artifact. Finally, five of the samples, along with fescue seed standard, evidenced one or more of 14 new ergot alkaloids ranging in size from 381 to 611 molecular weight and with key mass spectral characteristics of ergot alkaloids, specifically the pair of peaks m/z 223 and 208, corresponding to the ergoline ring system and its demethylated variant, respectively. It is anticipated that findings such as these will provide impetus to future development of analytical methodology for these heretofore relatively rare ergot alkaloid species.

Productivity of cow–calf pairs grazing tall fescue pastures infected with either the wild-type endophyte or a nonergot alkaloid-producing endophyte strain, AR5421,2

The nonergot alkaloid-producing endo-phyte, AR542, has been shown to improve the persistence and yield of tall fescue pastures without causing the animal disorders commonly associated with tall fescue toxicosis. A 3-yr grazing study was conducted to compare effects of AR542-infected tall fescue pastures with wild type endophyte-infected (E+) tall fescue pastures on cow-calf performance. Replicated 7.3-ha pastures of each treatment were grazed by cow-calf pairs (16 pairs per pasture replication) each year from March to weaning in September. The cows were exposed to breeding on their respective pasture treatments from April 1 through June 15. The treatment groups were compared for reproductive performance, ADG, BCS, calf growth rate, and weaning weight. Blood samples were also collected for serum prolactin (PRL) analysis. There were no significant differences in calving rate (P = 0.98) or calving interval (P = 0.62) between pasture treatments. Cows that grazed the AR542 pastures subsequently gave birth to calves that were heavier (P < 0.05) than calves from cows that had grazed the E+ pastures. Cows grazing the AR542 pastures had higher (P < 0.05) BCS at the end of the grazing period, and had higher ADG during the grazing period. Calves raised on the AR542 pasture had higher (P < 0.05) ADG and weaning weights than calves of the same sex raised on the E+ pastures. Serum PRL concentrations were decreased (P < 0.05) in both cows and calves on the E+ pastures compared with serum PRL concentrations in cows and calves grazing the AR542 pastures. The results indicate that grazing tall fescue pastures infected with the AR542 endophyte may give significant advantages in cow-calf growth rates and BCS over grazing E+ pastures. However, there did not seem to be any benefit in reproductive performance in this trial. There was a small, but significant increase in birth weight in cows grazing AR542 pasture.

Fragmentation patterns of selected ergot alkaloids by electrospray ionization tandem quadrupole mass spectrometry.

Tall fescue toxicosis and other maladies in livestock result from the ingestion of vasoconstrictive ergot alkaloids produced by fungal endophytes associated symbiotically with the grass. In order to facilitate future analyses of grass extracts considered responsible for outbreak of related livestock diseases, we examined the electrospray ionization mass spectra of specific ergot alkaloids under conditions that permit protonation. Our purposes were both to record the spectra with interpretation of mechanisms of fragmentation and to derive commonalities that would allow the prediction of mass spectra of related compounds for which standards were not readily available. With [M + H](+) values in parentheses, water-insoluble lysergic acid peptide ergot derivatives ergovaline (m/z 534), ergotamine (m/z 582), ergocornine (m/z 562), ergocryptine (m/z 576) and ergocrystine (m/z 610) exhibited a consistent loss of water (-18 u) from the C-12' alpha-hydroxy functionality. Of this group, ergovaline and ergotamine generated an m/z 320 fragment deriving from cleavage of ring E amide and ether functions with retention of the peptide ring system methyl group. Ergocornine, ergocryptine and ergocrystine similarly formed an m/z 348 fragment with retention of isopropyl. These assignments were supported by the lack of similar fragments from the water-soluble ergot ergonovine, which lacks a peptide ring system. Clavine-type ergot alkaloids lysergic acid and lysergol lack any substituents beyond simple ones directly on the C-8 position and, similarly to ergonovine, lack significant fragments at m/z 268, 251 and 225 shared by the peptide ergot alkaloids.

Tall Fescue Toxicosis and Management

Tall fescue toxicosis is one of the most costly animal disorders facing livestock producers in the eastern U.S. As reported by Georgia researchers in 1977, it is caused by a fungal endophyte. At present, there is no cure for tall fescue toxicosis. There are, however, proven management strategies that can lessen the effect of toxicosis. One strategy is the replacement of toxic tall fescue with cultivars that are endophyte-free or contain beneficial endophytes. Other strategies involve management of toxic tall fescue pastures by interseeding other forages that can dilute the toxins, rotating livestock to non-toxic pastures, supplementing the diet, limiting the rate of nitrogen fertilizer, controlling the seedheads, ammoniating the hay, and in the case of horses, administering medicinal treatments. Nearly all of these management practices are designed to limit the amount of toxin that is ingested in the animal. This guide will discuss the science and application of these management practices.

Use of nonergot alkaloid-producing endophytes for alleviating tall fescue toxicosis in stocker cattle1,2

Grazing studies were conducted to determine cattle growth performance, evaluate toxicosis, and compare grazing behavior in stocker cattle grazing nonergot alkaloid-producing endophyte-infected (AR542 or AR502), endophyte-free (E-), or wild-type toxic endophyte-infected (E+) Jesup, Georgia-5, and Kentucky-31 tall fescue. Replicated 0.81-ha tall fescue paddocks were established at the Central Georgia Branch Station at Eatonton and the Northwest Georgia Branch Station at Calhoun during October 1998 and were stocked with beef cattle for autumn and spring periods from fall 1999 through spring 2002. Mean ergot alkaloid concentrations were higher (P < 0.01) on E+ pastures than the other treatments at both locations. At Calhoun and Eatonton, post-treatment serum prolactin concentrations were decreased (P < 0.01) on E+ compared with AR542, AR502, and E- tall fescue. Cattle on AR542, AR502, and E- pastures had lower (P < 0.05) post-treatment rectal temperatures than cattle grazing E+ tall fescue during spring at Eatonton and Calhoun. Calf ADG was higher (P < 0.05) on AR542, AR502, and E- as compared with E+ tall fescue during autumn and spring grazing at Eatonton, and at Calhoun, cattle on E+ pastures had lower (P < 0.05) ADG in both autumn and spring. Gain/hectare was higher (P < 0.05) on AR542, AR502, and E- than on E+ during autumn at Eatonton and during spring at both locations. In autumn at Calhoun, gain/hectare was greater (P < 0.05) on AR502 and E- compared with E+ tall fescue. During April, May, and June, cattle grazing E+ pastures at Eatonton spent more (P < 0.01) time idling, more (P < 0.01) time standing, and used more (P < 0.01) water than cattle on AR542 and E- tall fescue. Daily prehensions and biting rate were each higher (P < 0.01) on AR542 and E- tall fescue than E+ tall fescue in both grazing seasons. There were no differences among pasture treatments for bite size in either spring (P = 0.50) or autumn (P = 0.34). Steers grazing E+ pastures had lower DMI than steers grazing AR542 and E- pastures during spring (P < 0.10) and lower DMI than steers grazing E- pastures during autumn (P < 0.05). Daily steer water usage was decreased (P < 0.10) in E+ pastures compared with AR542 and E- pastures during late fall. These results indicate that nonergot alkaloid-producing endophyte technology is a promising option for alleviating tall fescue toxicosis in stocker cattle.

Use of nonergot alkaloid-producing endophytes for alleviating tall fescue toxicosis in sheep.

Nonergot alkaloid-producing endophytes from New Zealand were inserted into tall fescue (Festuca arundinacea) cultivars in an attempt to address the problem of fescue toxicosis in grazing sheep. A 3-yr grazing study was conducted to determine lamb performance and to evaluate toxicosis in lambs grazing nonergot alkaloid-producing endophyte-infected (AR542 or AR502), endophyte-free (E-), or wild-type toxic endophyte-infected (E+) Jesup tall fescue or nonergot alkaloid-producing endophyte-infected (AR542) Georgia-5 tall fescue. Replicated 0.11-ha tall fescue paddocks were established at the central Georgia Branch Station during September 1997 and stocked with lambs from spring 1998 through autumn 2000. Mean ergot alkaloid concentrations were higher (P < 0.01) in E+ forage than in AR542, AR502, and E- tall fescue, and ergot alkaloid concentrations in E- plants and plants infected with AR542 and AR502 were low. Forage availability did not differ (P = 0.92) across treatments during autumn and was higher (P < 0.05) in Georgia-5 AR542 than in Jesup AR502 and E+ pastures. Initial serum prolactin (PRL) concentrations did not differ (P = 0.58) across treatments during autumn, but were higher on Jesup AR542 than E+ during spring. Post-treatment serum PRL concentrations were depressed (P < 0.01) on E+ compared with AR542, AR502, and E- in both spring and autumn. Signs of heat stress were observed in E+ lambs during periods of high ambient temperatures. Mean post-treatment rectal temperature and mean stocking rate exhibited treatment x year interactions (P < 0.05). Lamb ADG was higher (P < 0.05) on AR542, AR502, and E- than on E+ tall fescue. Similarly, gain/hectare was higher (P < 0.015) on AR542, AR502, and E- than on E+. Tall fescue pastures containing AR542 and AR502 endophytes yielded lamb performance that did not differ from that on E- tall fescue and which was superior to performance on E+ tall fescue. Depressed PRL concentrations and elevated rectal temperatures as indicators of toxicosis were evident only in lambs grazing E+ tall fescue, suggesting that nonergot alkaloid-producing endophyte-infected tall fescue is a viable alternative for alleviating tall fescue toxicosis.

Technical note: epimerization of ergopeptine alkaloids in organic and aqueous solvents.

Purified ergopeptine alkaloids are often used in studies related to tall fescue toxicosis without regard to epimerization that occurs when ergopeptines are solvated. The objectives of this study were to measure the rates of alpha-ergocryptine epimerization to alpha-ergocryptinine at room temperature and at -40 degrees C, and to measure the rate of ergovaline epimerization to ergovalinine at 37 degrees C. Alpha-ergocryptine tartrate was stable (< 0.5% epimerization) in protic or aprotic solvents when stored at -40 degrees C for 20 to 52 d. At room temperature, alpha-ergocryptine epimerization in chloroform did not occur; epimerization was modest in acetone and acetonitrile (< 5%) but was substantial in methanol (78% by 38 d) and in a 70:30 water methanol mix (47% by 42 d). Ergovaline epimerization to ergovalinine occurred at 37 degrees C in 0.1 M phosphate buffers (pH 3, 7.5, and 9) in 9% aqueous solutions of fetal bovine serum (FBS), and in water, methanol, and acetonitrile. The degree of epimerization at 37 degrees C was solvent-dependent. Epimerization rates with respect to time were roughly linear in phosphate buffer (pH 3 only), water, methanol, and acetonitrile; epimerization rates resembled first-order kinetics in phosphate buffers (pH 7.5 and 9) and in the presence of FBS (pH 3, 7.5 and in Dulbecco's culture media). Epimerization equilibria (48 to 63% ergovaline) were reached within approximately 1 to 19 h. Results from this study indicate that researchers conducting studies with purified ergopeptines should carefully control the storage conditions of solvated ergopeptines and measure isomeric composition under the actual experimental conditions used in experiments.

Alterations in hemograms and serum biochemical analytes of steers after prolonged consumption of endophyte-infected tall fescue.

The objective of the current study was to delineate changes that occur in serum analytes and blood cellular elements in cattle that graze endophyte-infested (Neotyphodium coenophialum) tall fescue. Tall fescue is grown on more than 35 million acres (14.2 million ha) of pasture in the United States, and three-fourths of the pastures are infected with the endophyte at a 60% or greater level. Tall fescue toxicosis caused by endophyte-produced ergot alkaloids continues to be the most important grass-related disease in the United States, in terms of economic loss to animal producers. However, the agronomic attributes of tall fescue make it an attractive forage species because of its ability to withstand cool temperatures, drought, poor soil conditions, and intensive defoliation from herbivore species, including insects. Tall fescue toxicosis is a complex disease and the need exists to understand the mechanisms of the toxic effects in order to institute effective, prophylactic control measures. Our group previously reported changes that occur in serum biochemical analytes of cattle that graze endophyte-infected tall fescue. An additional year's worth of data have been added, strengthening and corroborating these data. Consistent and significant changes associated with tall fescue toxicosis during the 3-yr study included decreased serum concentrations of cholesterol, globulin (increased albumin/globulin ratio), prolactin, total protein, and copper. The activity of alanine aminotransferase was decreased in serum, whereas an increase in serum concentrations of creatinine and total bilirubin occurred. The present report also documents comparative hemograms of cattle that grazed endophyte-infected or endophyte-free tall fescue over a prolonged period. The mean erythrocyte counts were increased in cattle that grazed endophyte-infected tall fescue, whereas mean corpuscular hemoglobin and mean corpuscular volume were decreased, as were mean eosinophil counts. Thus, repeatable changes have been identified that occur in serum biochemical and blood cellular values of cattle grazing endophyte-infected tall fescue that will aid in understanding the pathogenesis of the disease. In addition, these consistently altered parameters can be used to assess the effectiveness of potential prophylactic treatments.

Abdominal lipomatosis attributed to tall fescue toxicosis in deer

Abdominal lipomatosis attributed to tall fescue toxicosis in deer

Genetics of disease resistance inBos tauruscattle

SummaryThis review summarises evidence for genetic variation ofBos tauruscattle to diseases encountered under temperate conditions, including internal and external parasitism, susceptibility to mycotoxic diseases (tall fescue toxicosis, facial eczema, ryegrass staggers), mastitis, ketosis, pasture bloat, leukosis, tuberculosis, foot and mouth, brucellosis and BSE. Averaging mean heritability estimates reviewed from 8 diseases (weighted equally) gave a value of 0.21, indicating that measurable genetic variation for disease traits inBos tauruscattle is somewhat less than that for production traits, such as milk yield or body weight. Many estimates, however, have high standard errors, and there could be an upward bias resulting from non-reporting of zero or non-significant estimates.Few single-trait selection experiments have been conducted to study the genetics of disease resistance traits in cattle. For the disease traits where selection is being applied extensively, index selection for improved disease resistance and increased production is more common than single-trait selection. Results from a long-term (25 year) divergent selection experiment with resistance/susceptibility to pasture bloat in cattle in New Zealand are reviewed. Four single-year experiments comparing progeny of ‘high’ versus ‘low’ sires for resistance to disease are also reviewed, one in Australia studying faecal nematode egg counts, one in the USA involving the mycotoxic disease, tall fescue toxicosis, a third in New Zealand involving the mycotoxic disease, facial eczema, and a fourth in the USA involvingBrucella abortus.

Toxic endophyte-infected tall fescue and range grasses: historic perspectives.

A historic profile of endophyte-induced tall fescue toxicosis is presented. A chronology of events is presented, beginning with the importance of finding Balansia-infected grasses in a tall fescue pasture in north central Georgia. This initial finding was followed by the discovery that another related endophyte was present in tall fescue and other major forage grasses. This species of endophyte was identified as Acremonium coenophialum. After this report was the important discovery that cattle performance was poor on Acremonium-infected tall fescue. Thus, this endophyte and its presence in tall fescue was implicated as the cause of tall fescue toxicosis and fescue foot. It was later established that this endophyte also produced ergot alkaloids. The related endophyte of perennial ryegrass, A. lolii, was subsequently shown to be responsible for the ryegrass staggers syndrome. Several other species of Acremonium have been associated with other important forage and turf grasses. Finally, important papers leading to the revelations that endophytic fungi and their grass hosts are ecologically significant and that most should be considered mutualistic symbioses are reviewed. Symbiotic grasses have enhanced physiological and morphological characteristics that offer biotechnological exploitations on one hand, but on the other solutions to the toxicity of tall fescue are difficult because grasses free of their fungal partner are generally ecological failures.

Equine fescue toxicosis: signs and solutions.

Gravid mares grazing endophyte-infested (E+) tall fescue exhibit increased gestation lengths, agalactia, foal and mare mortality, tough and thickened placentas, weak and dysmature foals, increased sweating during warm weather, reduced serum prolactin and progesterone, and increased serum estradiol-17 beta levels. Also, E+ tall fescue hay is less digestible than endophyte-free (E-) hay. Unlike many other species, horses consuming E+ tall fescue do not exhibit increased body temperature. Young horses consuming only E+ pasture do not gain as well as those consuming E- pasture. There is little difference in gain when the pasture is supplemented with enough concentrate to meet NRC requirements for growth. Neither selenium injections nor supplementing with corn at 50% of the NRC requirements for energy reduces the effects of toxic tall fescue on reproduction and lactation in gravid mares. It seems that the alkaloids of E+ tall fescue are serving as D2 dopamine receptor agonists. This activity would explain their prolactin-lowering effect. Domperidone, a dopamine receptor antagonist, is effective in preventing the signs of tall fescue toxicosis in horses without neuroleptic side effects.

Ryegrass endophyte-related heat stress in cattle

Heat stress in Northland cattle has been shown to be similar to tall fescue toxicosis as described in south-east USA, but incidence has not been correlated with the presence of tall fescue on farms. Tall fescue toxicosis results from grazing endophyte-infected tall fescue, and is caused by the alkaloid ergovaline. Cases are described of cattle suffering typical symptoms of ergovaline poisoning, though they had negligible access to tall fescue. Pasture surveys have shown ergovaline levels in ryegrass pastures to often be sufficient to cause toxicosis. Ambient temperatures interact with ingestion of toxin to cause heat stress. It is suggested that usual weather conditions in New Zealand temper the negative effects of the toxin. Higher temperatures increase the levels of ergovaline in pasture, and increase the sensitivity of livestock to it. Heat stress in North Island cattle probably usually results from the interaction of particular environmental conditions with the grazing of perennial ryegrass. Keywords: endophyte, ergovaline, fescue toxicosis, heat stress, Lolium perenne

Improved extraction and HPLC methods for ergovaline from plant material and rumen fluid.

Ergovaline, the main ergopeptine alkaloid produced in tall fescue infected with Acremonium coenophialum, is known to cause tall fescue toxicosis. Current methods in use for quantifying ergovaline in plant material have several disadvantages, including large solvent volumes and long analysis time. We report here improvements in extraction and cleanup and the high-pressure liquid chromatographic methods. Our improvements include a 24-hour extraction time, the use of smaller solvent volumes during sample preparation, and fast analysis on the polymeric reverse-phase column. In addition to allowing the analysis of large batches to assist practitioners in the accurate diagnosis of fescue toxicosis, our method is also easily modified for other matrices, such as rumen fluid.

Induction of tall fescue toxicosis in heat-stressed cattle and its alleviation with thiamin.

Livestock grazing endophyte (Acremonium coenophialum)-infected tall fescue (Festuca arundinacea Schreb. cv. Kentucky 31) at high ambient temperatures may suffer from fescue toxicosis. Adult Angus cows (Bos taurus) were fed 0 to 1 kg/d of 70% infected tall fescue seed containing about 4.4 g of loline alkaloids in factorial combination with thiamin at 0 or 1 g/d. Cows assigned to the zero level of tall fescue seed received a supplement of equivalent energy and protein. Ingestive behavior was measured at 1330 to 1430 EDT during two 4-d periods in two consecutive weeks in August on alfalfa (Medicago sativa L.) pastures using a tethered grazing system. Alfalfa intake per measured grazing session of cows given tall fescue seed declined linearly (P less than .01) as air temperatures during grazing increased above 25 degrees C, largely because of shorter grazing meals. Thiamin increased alfalfa intake per measured grazing session by extending grazing time. Alkaloids in ingested endophyte-infected tall fescue induce thiamin deficiencies in cattle that result in symptoms of tall fescue toxicosis.

Thiamin supplementation and the ingestive behavior of beef cattle grazing endophyte-infected tall fescue.

Livestock grazing endophyte (Acremonium coenophialum Morgan-Jones and Gams)-infected tall fescue (Festuca arundinacea Schreb.) perform poorly due to tall fescue toxicosis, especially when animals are under heat stress. In order to determine whether thiamin promotes recovery from tall fescue toxicosis, 1 or 0 g of thiamin per day, as mononitrate, was fed orally to adult Angus (Bos taurus) cows (380 +/- 8 kg) grazing either tall fescue pasture with and without endophyte or alfalfa (Medicago sativa L.). A tethered grazing system employing a split-plot design was used to estimate intake and components of ingestive behavior. No significant differences attributable to thiamin supplements were seen in rates of intake and biting, grazing time and intake per bite when cows grazed endophyte-infected tall fescue during the first 4 d of exposure. When cows grazed endophyte-infected (greater than 95%) tall fescue with 2,091 micrograms/g loline alkaloids after 4 d of exposure, the untreated animals ingested herbage dry matter (DM) at 1.19 kg/h, whereas the cows receiving thiamin ate 1.57 kg/h (P less than .05). Cattle achieved these rates of DM intake by forming bites of 1.0 and 1.2 g DM at 24 and 26 bites/min when treated with 0 and 1 g of thiamin per day, respectively. Thiamin supplements had no effect on ingestive behavior of cows grazing endophyte-free tall fescue or alfalfa after exposure to these forages for 4 d. Responses to thiamin generally were greater when cattle grazing endophyte-infected tall fescue were exposed to heat stress. Oral thiamin supplementation may alleviate tall fescue toxicosis of beef cattle during warm weather.