Tall fescue summer toxicosis
Endophyte Neotyphodium coenophialum in tall fescue (Festuca arundinacea Schreb.) in the USA produces summer toxicosis in beef cattle. This association can cause lower fed intake, lower weight gains, lower milk production, higher respiration rates, higher body temperatures, reduced serum prolactin levels and reduced reproductive performance (Bacon et al. 1 986; Hoveland 1 993; Ball 1 997). Steers grazing tall fescue infected with
endophyte were less active and spent less time grazing than steers on tall fescue not infected with endophyte (Seman et al. 1 990). The ergopeptine alkaloids have been implicated, and of these ergo valine occurs in the greatest concentration (Fletcher 1 993b) and is one of the most toxic (Bacon et al. 1986). Feeding levels of endophyte-infected tall fescue seed with ergovaline levels 500 to 2000 ppb significantly reduced feed intake and average daily gain of lambs as compared to controls (Debassai 1 993). The serum prolactin levels were markedly lowered and lambs on the ergovaline-containing diets were lethargic and depressed (Debassai 1 993).
Symptoms, predisposing factors and causal alkaloids
Ergopeptine alkaloids also occur in perennial ryegrass (Lolium perenne) infected with the endophytic fungus Neotyphodium lolii (Rowan and Shaw 1 987). Ergovaline has been detected in ryegrass infected with wild type endophytes at similar levels to those of tall fescue infected with N. coenophialum (Rowan et al. 1990b). Easton et al. ( 1 995) believes that ergovaline levels in perennial ryegrass pastures are high enough to cause fescue toxicosis symptoms in livestock if ambient weather conditions are suitable. The ergopeptine alkaloids are dopamine agonists which can act as vaso-constrictors and reduce peripheral blood flow and the ability to dissipate excess heat (Rhodes et al. 1 99 1 ). Latch ( 1 994) believes that few farmers are aware of the effects that toxins such as ergovaline may have on animal production.
Sheep grazing endophyte infected ryegrass have reduced serum prolactin levels and liveweight gains (Fletcher and Barrell 1984), an effect possibly due to the presence of ergopeptine alkaloids (Rowan et al. 1 990b). Heat stress problems were first exposed in experiments using endophyte-infected ryegrass pasture which did not produce lolitrem B ( 1 87BB endophyte) and where the usual symptoms of rye grass staggers were not observed (Fletcher 1993b, Fletcher and Sutherland 1993). The highest rectal temperatures and respiration rates were in lambs grazing the ryegrass/endophyte associations producing the highest ergovaline levels (Fletcher 1993a). Respiratory problems such as excessive coughing and slow labored breathing were more evident in
lambs grazing ryegrass infected with 1 87BB endophyte which had the highest ergovaline concentrations.
Conclusive confirmation of ergovaline as the causative agent in this heat stress syndrome is not possible due to confounding with other endophyte/ryegrass alkaloids such as paxilline (Fletcher 1993b). Ergovaline is not believed to be linked to the ryegrass staggers syndrome, since administration of the dopamine antagonist metoclopramide restored prolactin to levels of sheep grazing endophyte free ryegrass but had no effect on ryegrass staggers (Piper and Fletcher 1990). It is now recognised that the dominant and debilitating effects of ryegrass staggers may have been obscuring more subtle symptoms of heat stress (Fletcher 1993b ). Foot ( 1 997) reported catastrophic events of ryegrass staggers in Australia resulting in large numbers of animals dying in creeks or dams, an effect possibly exacerbated by heat stress.
Observations of heat stress have been reported in cattle exhibiting decreased milk production, and depressed appetite. Cattle were visibly stressed, drooling and in some cases panting (Easton et al. 1995). The symptoms were similar to those described for tall fescue toxicosis (East on et al. 1 995). Easton et al. ( 1995) believes that it is not unusual for cattle grazing endophyte-infected ryegrass in northern New Zealand to ingest herbage containing over 1 .0 ppm ergovaline, and sometimes over 1 .5 ppm. Easton et al. ( 1995) believes that the relatively infrequent incidence of clinical heat stress in New Zealand cattle grazing perennial ryegrass is due to relatively short-term exposure to elevated levels of ergovaline, and to the lower ambient conditions compared to those of southeastern United States of America.
Grazing endophyte-infected ryegrass at ambient temperatures below 25 °C, may not create a heat stress problem but temperatures above this may result in a significant effect on animal production (Fletcher 1 993a; Familton et al. 1995). The critical ambient temperature above which hyperthermia was evoked in rats by ergotamine (structurally similar to ergo valine) dosing was 30 °C and an interaction between dose rate and environmental temperature has been observed (Smith et al. 1 993; Smith et al. 1 995). Reducing intake is a normal thermo-regulatory response to hyperthermia. These effects
are likely to be exaggerated in large animals such as dairy cows with a high metabolic rate and a higher rate of excess heat production (Fletcher and Easton 1997). It is thought that sheep are more tolerant of high ambient temperatures (Fletcher 1993a). Comprehensive studies in several trials, covering a range of conditions, have revealed that reduced prolactin levels are one of the most consistent responses to grazing ryegrass endophyte associations and associated ergovaline levels (Fletcher et al. 1 996). Prolactin may also be an important link in mediating, at least in part, some of the effects of ergopeptine alkaloids or ergovaline on animal responses (Fletcher et al. 1996). Prolactin levels in healthy sheep tend to increase as photoperiod, ambient temperature and consequent heat load on the animal increase (Fletcher et al. 1997). As ambient temperature increases, prolactin levels in sheep grazing endophyte-free grasses increases rapidly, while those grazing endophyte associations with ergovaline fail to respond (Fletcher and Easton 1997). The prolactin response is characterised by a low threshold to ergovaline, with maximum depression occurring at or below 0.5 ppm ergovaline in pastures (Cheeke, et al. 1 993). Significantly reduced serum prolactin levels in sheep grazing endophyte have been reported (Fletcher and Barell 1984; Fletcher 1993b). Fletcher et al. (1996) reported prolactin levels in ewes of 1 75 ng/ml for endophyte-free ryegrass, and 74 ng/ml for wild-type endophyte-infected ryegrass, taken during February.
Significant increases in body temperature and respiration rate of lambs grazing endophyte-infected pastures have been reported (Fletcher et al. 1 994; Fletcher and Easton 1997). Body temperatures of grazing lambs, measured on six dates during summer, were 40.4, 40.9 and 40.85 °C respectively for lambs grazing endophyte-free ryegrass, ryegrass with its wild-type endophyte and ryegrass infected with 187BB endophyte (Fletcher and Easton 1997). In a number of field trials at Lincoln, maximum body temperature was reached at an ergovaline concentration of 1 .0 ppm ergovaline under warm field conditions (Fletcher and Easton 1997).
Ergovaline concentrations are highest in the tissues most important for the survival and dissemination of the endophyte, namely the crown, the source of new emerging tillers,
and the developing reproductive organs (Davies et al. 1 993; Easton et al. 1 993;Lane et al. 1 997a). Ergovaline levels vary through the season, between districts and according to management (Easton et al. 1993). Measured ergovaline concentrations are evidently highly sensitive to experimental conditions (Davies et al. 1 993). There is a wide variation in ergovaline concentration between individual plants (Rowan et al. 1990b; Lane et al. 1 997a). Concentrations have been found to increase when plants are under water deficit conditions (Barker et al. 1993; Lane et al. 1997c). Typically in pastures, concentrations are lowest in winter (Rowan et al. 1990b) with an initial peak in December with reproductive development, and another peak in late summer associated with a low pasture profile and heat stress (Easton et al. 1 993). Ergovaline levels of 0.5-
1 .0 ppm are commonplace on leafy pasture of commonly sown cultivars (Easton et al. 1 993; Davies et al. 1993; Woodburn 1 993;). These values are comparable to concentrations reported for tall fescue herbage in the United States (Rowan et al. 1 990b). Lane et al. ( 1 997b) has found extreme ergovaline concentrations exceeding 1 5 ppm in perennial ryegrass with natural endophyte and unidentified strains from greenhouse-grown pseudostem fractions.
Management
Ryegrass hay or straw should be analysed for ergovaline concentration to determine the risk of feeding it (Welty 1993). Grazing management systems to minimise grazing basal parts of the ryegrass plant where ergovaline is concentrated has yet to be experimentally tested as a means of minimising heat stress in ryegrass/endophyte associations (Familton et al. 1 995).