Consumption of tall fescue forage infested with the endophytic fungus Acremonium coenophialum can result in a condition termed "fescue toxicosis," which is characterized by decreased weight gains, milk production, conception, and serum prolactin and an inability to dissipate body heat by beef cattle. These decreases in productivity have been estimated to cost beef producers more than $600 million annually. The unthrifty appearance of cattle consuming endophyte-infected tall fescue (E+) is most evident during periods of environmental heat or cold stress, suggesting an interaction with environmental conditions. Without temperature stress, cows consumed similar amounts of E+ and endophyte-free tall fescue (E-). But, when temperatures exceeded 32 degrees C, cows that grazed E+ consumed less forage than cows that grazed E-. After removal of E+ from the diet of steers, compensatory growth was observed, indicating no long-term negative effects of E+. A decrease in serum prolactin but apparently little change in other hormones has been measured after consumption of E+. Under periods of heat stress, animals had reduced ability to dissipate body heat, and blood flow to peripheral (rib skin), core-body (duodenum colon), and brain (cerebellum) tissues was decreased. Due to the depressive effects of E+ on prolactin and heat dissipation, dopamine antagonist therapy has been used in an attempt to rectify these changes. Dopamine antagonists have increased serum prolactin but there is limited evidence to suggest an improvement in heat dissipation. Experiments to elucidate the effects of E+ on alpha-1 and -2 adrenergic receptors may also offer insights into developing strategies to overcome the negative effects of E+ consumption. Preliminary results suggest that injections of an alpha-1 adrenergic antagonist (prazosin) in rats fed E+ under 32 degrees C conditions increased DM intake and reduced rectal temperature.

The effects of fescue toxicosis on beef cattle productivity

Effects of endophyte-infected tall fescue on animal performance

Previous research has implicated an endophytic fungus as being associated with fescue toxicity (summer syndrome) in cattle grazing Kentucky 31 tall fescue (Festuca arundinacea Schreb.) pastures. Hay and seed were harvested from Kentucky 31 pastures known to be either fungus-free or heavily infested with an endophytic fungus identified as Acremonium coenophialum Morgan-Jones and Gams. Four diets containing either 60% fungus-free seed, 60% fungus-infested seed, 85% fungus-free hay or 85% fungus-infested hay were group-fed to three steers each (avg wt 239 kg) in a 53-d feeding trial. Presence of the fungus reduced (P less than .05) daily gains (kg/d) in steers fed either the seed diets (.96 vs .20) or the hay diets (.66 vs .28). Feed intake was depressed 36% for the seed diets and 8% for the hay diets when the fungus was present. Rectal temperatures were elevated .6 C (P less than .05) for both groups receiving diets containing the fungus, but respiration rate was elevated only in the fungus-infested seed group. In vitro dry matter disappearance was not decreased by presence of the fungus. Steers receiving fungus from either seed or hay were nervous and highly excitable, which resulted in large variations in plasma epinephrine and norepinephrine concentrations. The results of this experiment further implicate an endophytic fungus as being associated with fescue toxicity.

Association of an endophytic fungus with fescue toxicity in steers fed Kentucky 31 tall fescue seed or hay.

Foamy bloat of cattle. A review.

Two experiments were conducted to determine the effects of endophyte fungus (Acremo- nium coenophialum) ingestion in tall fescue diets and environmental temperature on heat dissipation and diet utilization by cattle. In Exp. 1, 12 Angus heifers (average weight 244 kg) were allotted by weight to either an endophyte-free (E -1 or endophyte-infected (E+; 381 ppb of ergovaline) diet. Environmental temperature varied between 22 and 32°C. Voluntary DM and water intakes were similar (P > .lo) among treatments. Rectal temperatures and concentrations of prolactin in plasma were lower (P < .05) after ingestion of the E+ diet. Plasma triiodothyronine, thyroxine, and cortisol concentrations were not af- fected by diet. In Exp. 2, 24 Holstein steers (average weight 114 kg) were allotted by weight to either E- or E+ (285 ppb of ergovaline) and one of two environ- mental temperatures (22 or 32°C). At 32"C, feed intake was reduced by 22%, and water consumption was increased by 62% compared with steers housed at 22°C. Consumption of E+ reduced feed intake by 10% but did not influence water consumption. Plasma concentration of prolactin was decreased (P < .05) within 48 h after consumption of E+. Rectal tempera- tures increased in response to both environmental temperature (P < .05) and E+ consumption (P = .06). Digestibilities of DM and OM for E+ were 9% lower (P < .05) than for E-. An interaction between diet and temperature (P < .05) revealed that steers fed E- at 32°C had greater skin vaporization, but steers that consumed E+ did not increase vaporization; values were similar to steers housed at 22°C. These results suggest that consumption of E+ at elevated tempera- tures decreased the ability of cattle to remove excess body heat through surface vaporization.

Don R. Jacobson

Papers

Growth, Physiological Responses, and Evidence of Toxicity in Yearling Dairy Cattle Grazing Different Grasses

Feedlot Bloat Studies. II. Physical Factors Involved in the Etiology of Frothy Bloat

Skin temperature and blood flow in the tail of dairy heifers administered extracts of toxic tall fescue.

Feedlot Bloat Studies. I. Animal and Dietary Factors

A Simplified Method of Obtaining Differential and Total Viable Anaerobic Counts of Rumen Bacteria Employing Sealed Small-Diameter Glass Tubing1