– Michelle Arnold, DVM, University of Kentucky Ruminant Veterinarian
Tall fescue (Lolium arundinaceum (Schreb.) Darbysh.) is a cool season, perennial grass. The KY-31 variety is usually infected with the fungal endophyte Neotyphodium coenophialum (also called Epichloë coenophiala) which grows within the intercellular spaces of the leaf sheaths, stems, and seeds. An “endophyte” is a fungus or bacteria that lives entirely within the tissue spaces of plants and is only visible microscopically. The plant and fungus enjoy a symbiotic relationship; mutually beneficial to both organisms. The fungus has free access to the plant’s nutrients and the plant provides a means for the endophyte to reproduce through infected seeds. The fungus, in turn, produces chemicals (ergot alkaloids) that function as chemical defenses, making the plant more vigorous, pest-resistant, drought-resistant, and tolerant of many adverse soil and environmental conditions. Often KY 31 tall fescue is the only grass that can survive and thrive in poor conditions. The endophyte produces a variety of ergot alkaloids, of which ergovaline is the primary concern and accounts for approximately 80-97% of the alkaloids in tall fescue. The highest ergovaline concentrations are found in the stem and seed head. Hay containing infected seed heads can remain high in ergovaline even when stored over several years.
“Fescue toxicosis” is the general term used for the clinical diseases that can affect cattle consuming endophyte-infected tall fescue. The most important in KY is a syndrome frequently referred to as “summer slump”. Affected cattle appear hot with labored respiration (open mouth and/or rapid breathing) and excessive salivation. They avoid grazing during the day, and seek shade or mud wallows to find relief from heat. External signs in cattle include poor growth or weight loss due to decreased feed intake, a dull rough hair coat caused by failure to shed the winter coat and excessive growth of hair, and decreased milk production. Internally, the alkaloids cause vasoconstriction or narrowing of the arteries which leads to poor blood supply to many body systems, causing increased body temperature (hyperthermia), decreased prolactin levels and decreased conception rates. Cattle ultimately spend less time grazing and more time hiding in the shade. Economically, producers can expect reduced pregnancy rates, longer breed back intervals and lighter calves at weaning. Clinical signs vary depending on the genetic makeup of the cattle, the environmental conditions, and the level and duration of the exposure. Cattle may show a vasoconstrictive response to the alkaloids in as little as two days after initial exposure and may stay constricted for up to 6-7 weeks after removal of the animal from infected pasture. The clinical signs may be more severe if tall fescue or other grasses have developed black ergot bodies in place of the seed caused by the fungus Claviceps purpurea which also causes vasoconstriction.
“Fescue foot” is dry gangrene of the extremities (usually hooves) that occurs due to narrowing of the blood vessels supplying blood to these distant areas. This syndrome is much more prevalent in areas other than KY. It generally occurs in late fall or winter when environmental temperatures are cooler. Vasoconstriction affects the hind limbs first. Signs include shifting hind limb lameness, unthriftiness, and finally necrosis (dry gangrene) of the hooves. The affected portions of the hoof will fall off or “slough”. Tips of the ears and tail may also be affected.
A third syndrome, known as “Fat necrosis” or “lipomatosis”, has been observed with chronic fescue toxicosis and is associated with masses of necrotic fat in the abdominal and/or pelvic cavities. These hard areas of fat can obstruct the birth canal and contribute to calving difficulties. Hard fat masses in the abdomen may lead to intestinal blockage. These conditions may be diagnosed with rectal palpation or may be found at necropsy.
Several management practices can be used to improve cattle production on toxic endophyte-infected tall fescue. The highly toxic seed heads are selectively grazed when they are immature and moderately digestible. These seed heads can be regularly mowed or chemically suppressed with Chaparral® herbicide (active ingredient: metsulfuran-methyl). Reductions in seed presence can easily reduce toxicity of the overall forage since ergovaline concentrations are 3-10 times greater in the seed heads than the leaf blades or sheaths. Removing seed heads is an effective tool to manage cattle on pasture without having to get rid of an existing stand of tall fescue. Steers grazing endophyte-infected tall fescue treated with Chaparral to suppress the seed head development had a 39% greater average daily gain (ADG), much higher serum prolactin levels and much lower rectal temperatures than steers on untreated fescue. Rotational grazing will be necessary to accommodate the reduction in forage production when seed heads are suppressed. Additionally, stocking rates may need to be re-evaluated since cattle increase forage consumption when fescue toxicosis is mitigated. Careful attention to soil test recommendations for fertilizer and good weed control are necessary to sustain the stand when tall fescue is grazed hard or it will not persist. When application of Chaparral® is timed for seed head suppression (late April to early May), it will also control winter annual weeds and other early season broadleaves.
Providing pasture with a diverse mixture of forage species will reduce the impact of fescue as animals will selectively graze other forages. Interseeding legumes into endophyte-infected tall fescue pasture can benefit animal performance mainly through better diet quality and dilution of ergot alkaloids. Legumes can improve grass-based forage programs by increasing yield, improving quality, improving summer production, and converting atmospheric nitrogen into a form the plant can use, reducing the need for applied nitrogen fertilizer. Secondary plant metabolites, such as isoflavones produced by red clover, have been found to help alleviate the constriction of blood vessels. February is the month to frost seed clovers in KY or use a no-till drill when seeding between early March and early April. Supplementation with grain or coproduct feeds has also been found effective in reducing the observed effects of toxic fescue. Five pounds per head per day of soy hulls has been found quite effective, especially when calves have an estradiol implant placed in the ear.
Replacement with new grass is a longer term solution to the KY 31 problem. Use of tall fescue varieties artificially infected with novel or “friendly” endophyte strains that do not produce toxic ergot alkaloids result in greater average daily gain (ADG), lower body temperatures, and slick hair coats. These friendly endophyte pastures have slightly decreased carrying capacity but overall greater body weight gain per acre than toxic fescue pastures. In Kentucky, novel endophyte varieties have shown 10+ years of stand survival under good management. A new variety called “Lacefield MaxQ II”, named after Dr. Garry Lacefield (Extension Professor Emeritus at UK), was recently developed and will be marketed soon by Pennington Seed. In university trials, steers grazing Lacefield MaxQ II showed no decrease in serum prolactin levels or increased rectal and skin temperatures as compared to steers grazing KY 31. Careful grazing management is required during periods of slow pasture growth because cattle will continue to graze these fescue stands “into the ground” without the alkaloid present. Endophyte-free varieties are less tolerant to stresses such as drought, low soil fertility and overgrazing and rapidly deteriorate without excellent management. Stand life of endophyte-free varieties is similar to orchardgrass. Regardless of variety, good grazing management practices will be necessary for free and novel endophyte tall fescue to persist when grazed in the late spring or summer.
Perhaps the most effective way to avoid fescue toxicosis is to avoid fescue completely during the most harmful times. This is accomplished by moving cattle to warm season grass pastures or alfalfa during the late spring and early summer when seed heads are present in the stand and cattle are the most vulnerable to severe heat stress and depression in performance. Summer annual grasses such as sudangrass, sorghum x sudangrass hybrids, pearl millet and teff are warm season forage crop options. Alfalfa can be used for grazing throughout most of the grazing season including summer. This management technique provides grazing during active growth of the warm-season grasses and/or alfalfa when there is a decline in tall fescue growth.
In summary, fescue toxicosis is due to a fungal endophyte within the tall fescue plant which produces ergovaline, a compound that causes profound constriction of blood vessels in cattle. The hallmark effect of this vasoconstriction is hyperthermia or elevated body temperature which most often results in poor animal growth and weight loss. Accumulation of the alkaloids in the tissues may cause the vessels to stay constricted for up to 6-7 weeks after removal of the animal from infected pasture. Solutions to the problem may include replanting endophyte-infected pastures with cultivars infected with novel or friendly endophyte or endophyte-free seed, diluting infected pastures with other grasses or legumes, suppressing seed head development through mowing or spraying, or using warm season grass pastures or alfalfa in late spring and early summer instead of fescue as the main forage source. Regardless of forage type, it is essential to always provide a complete mineral mix to ensure adequate intake of the trace elements year round.
Guidelines for tall fescue testing
The diagnosis of ergot alkaloid-associated problems is based on clinical signs as well as forage testing. There are two major options for testing of tall fescue: one is to determine the level of fungal toxins in pasture plants (“Ergovaline testing”), and the other is to determine the percentage of plants infected with the toxin-producing endophyte fungus in a particular field (“Endophyte testing”). Each test provides different information. Details on sample collection can be found at: http://www.uky.edu/Ag/Forage/ForagePublications.htm#Tall Fescue. Information may also be found in the University of KY fact sheet PPA-30 entitled “Sampling for the Tall Fescue Endophyte in Pasture or Hay Stands” which is available at the local cooperative extension office or on the web at http://www2.ca.uky.edu/agc/pubs/ppa/ppa30/ppa30.pdf.
1. Ergovaline testing: To determine the levels of endophyte-associated toxins in the fescue forage, ergovaline testing can be performed. Testing for other endophyte-associated toxins can also be done, but ergovaline is the toxin of highest concentration and is thought to be the toxin of most concern. This test can be performed at the University of Kentucky Veterinary Diagnostic Laboratory Toxicology section. Please see the UKVDL website http://vdl.uky.edu/ for submission forms and shipping information.
Sample collection: Each pasture or field should be sampled separately. To collect samples, randomly select 20 to 30 separate sites within a pasture and pull out a handful of grass, including some root material and the entire plant above ground. Walk in a zig-zag pattern through the field to get samples. Some suggest walking the field in a W fashion and collecting a large handful of pasture grass at the five ends of each “W”. Samples for ergovaline testing should be placed on ice immediately after collection and kept on ice until either shipped or placed in a freezer for storage until time of shipment. Samples should be shipped on ice by overnight courier, or else delivered directly to the laboratory by the client. Samples need to be taken when plants have been growing well for at least a month, so early summer is a good time for testing.
Ergovaline concentrations vary among different fields even with the same grass variety. Levels also vary from season to season and from year to year. Increased fertilization can increase ergovaline concentrations, as can stressful growing conditions. Ergovaline concentrations vary by part of plant, with seed heads typically containing the highest concentrations. One batch of samples collected at one time cannot be considered representative of the field at all times over the year.
2. Endophyte testing: To determine the percentage of plants infected with the toxin-producing endophyte fungus in a particular field, endophyte testing can be done. There are several laboratories that do this test, including the University of Kentucky Regulatory Services laboratory. This test indicates how heavily infected pastures are, but does not give information on the levels of the endophyte toxins. Please see the University of Kentucky Regulatory Services website (http://www.rs.uky.edu/) under the “seed” tab for more information on sample collection and submission for endophyte testing. Note: the UK Regulatory Services accepts samples only from Kentucky farms.