Author
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Aiken, Glen |
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Submitted to: Cow Country News
Publication Type: Popular Publication Publication Acceptance Date: 2/17/2015 Publication Date: 3/1/2015 Citation: Aiken, G.E. 2015. From the Lab Bench: Why the heat and cold stresses on tall fescue pasture. Cow Country News. Pg. 48 http://issuu.com/cowcountrynews/docs/ccnmarch2015/49?e=6680951/11554905. Interpretive Summary: It is important for livestock producers to understand the biological mechanisms involved in causing “fescue toxicosis” and the related malady, “fescue foot”. Cattle producers with pastures of Kentucky 31 tall fescue know that one sign of fescue toxicosis is that their cattle spend too much time standing in ponds and shade during times of day when they should be grazing. Some also have had the misfortune of dealing with their cows suffering from fescue foot, which causes extreme lameness and eventually a gangrenous condition in hoofs and limbs. Although toxicosis-related heat stress is observed in the late spring and summer months and fescue foot is typically, but not always, observed during colder times of the year, the cause of both is very similar. In both cases, ergot alkaloids produced by the fungal endophyte causes a persistent constriction of blood flow to peripheral tissues. During the warm season, cattle grazing toxic endophyte-infected tall fescue cannot effectively dissipate excess body heat and therefore become heat stressed. With colder air temperatures, the constricted blood flows reduce the amount of heat and oxygen received by these tissues to cause lameness and eventually a gangrenous condition in hoofs and limbs. This information can benefit livestock producers that manage tall fescue and should understand the need for special management and handling of animals to mitigate alkaloid-induced heat and cold stress. Technical Abstract: A column is written to provide information on why cattle that graze toxic endophyte infected tall fescue are vulnerable to both heat and cold stress. Peer reviewed research conducted by scientists at the USDA-ARS Forage-Animal Production Research Unit demonstrated that ergot alkaloids produced by the fungal endophyte that infects tall fescue can negatively affect blood flow and that ergovaline is the most potent ergot alkaloid in disrupting blood flow. Mammals can maintain a fairly constant body temperature through internal regulation of the flow of blood that carries heat to the skin where heat is dissipated. Ergot alkaloids restrict the vessels, hampering this body heat reduction process and consequently cause cattle exposed to ergot alkaloids to be vulnerable to severe heat stress with elevated air temperatures. Under cold conditions, the vessels normally constrict to conserve heat to the internal organs and reduce heat loss via the skin, but the vessels will periodically relax to transfer needed heat to the limbs. During cold temperatures, the persistent vessel constriction caused by the ergot alkaloids restricts flow of warm blood to the limbs, tails and ears. A lack of heat and oxygen delivered to these tissues will eventually lead to hoofs, tails and ear tips developing a gangrenous condition commonly referred to as “fescue foot”. Therefore, cattle grazed on toxic endophyte-infected tall fescue are vulnerable to heat stress, but they are also vulnerable to frost bite during the cold months. |
