Abstract
Two studies separated effects of dietary ergot alkaloids from effects of feed intake or ambient temperature on respiration rate (RR), heart rate (HR), surface temperature (ST), rectal temperature (RT), blood pressure (BP), serum hormone, and plasma metabolite concentrations in beef steers. The balanced, single reversal design for each experiment used 8 beef steers fed tall fescue seed [2.5 g/kg body weight (BW)] with (E+) or without (E−) ergot alkaloids as part of a 60:40 switchgrass hay: supplement diet. Periods were 35 days with 21 days of preliminary phase and 14 days of feeding fescue seed once daily. Measures of dependent variables were collected on d 20, 25, 29, and 35 of each period at 0730 (before feeding), 1230 and 1530. In Experiment 1 steers weighed 286 kg, gained 0.61 kg BW/day, E+ supplied 2.72 mg ergot alkaloids including 1.60 mg ergovaline per steer daily, and mean minimum and maximum daily ambient temperatures were 23.6 and 32.3°C. In Experiment 2 steers weighed 348 kg, gained 1.03 kg BW/day, E+ supplied 3.06 mg ergot alkaloids including 2.00 mg ergovaline daily, and mean minimum and maximum daily ambient temperatures were 11.9 and 17.4°C. Dry matter intake was not affected by fescue seed treatment (P < 0.20) in either experiment. In both experiments, E+ reduced HR (P < 0.01) and increased insulin (P = 0.07). Systolic BP minus diastolic BP decreased (P < 0.05) for E+ in both experiments, due to increased diastolic BP in Experiment 1 (P < 0.03) and decreased systolic BP in Experiment 2 (P < 0.07). In Experiment 1, above the thermoneutral zone, E+ increased (P < 0.05) RR, RT, and left side ST in comparison to E−, but in Experiment 2, within the thermoneutral zone, E+ and E− did not differ (P < 0.18). Ergot alkaloids from fescue seed affect the cardiovascular system of steers separately from effects of feed intake or environmental temperature. Ergot alkaloids interact with ambient temperatures above the steers' thermoneutral zone to exacerbate the symptoms of hyperthermic stress.
Highlights
Consumption of toxic, endophyte-infected, tall fescue results in ingestion and absorption of ergot alkaloids produced by the endophyte, Neotyphodium coenophialum, which causes fescue toxicosis in grazing cattle
Feed intake was not affected by treatment in either experiment (Table 2) and treatment responses for E+ compared to E− are independent of intake effects
Hahn (1999) suggested a thermal stress threshold of 25◦C for growing cattle fed ad libitum which coincided with decreased feed intake and 21◦C as the threshold for increased respiration rate (RR)
Summary
Consumption of toxic, endophyte-infected, tall fescue results in ingestion and absorption of ergot alkaloids produced by the endophyte, Neotyphodium coenophialum, which causes fescue toxicosis in grazing cattle. Studies with steers or heifers consuming different sources of fescue hay (Hemken et al, 1981) or consuming alkaloids from fescue seed (Burke et al, 2001) at differing ambient temperatures in a factorial experimental design indicate a greater response to diets with endophyte-infected vs endophyte-free fescue (increased respiration and rectal temperatures (RTs) and decreased voluntary intake) during hyperthermic heat stress compared to ambient temperatures within the animals’ thermoneutral zone. Adverse responses of cattle consuming endophyte-infected tall fescue, including lower tolerance to ambient temperatures outside the animals’ thermoneutral zone, decreased voluntary intake, weight gain, and milk production have been linked to hemodynamic effects of ergot alkaloids in the tall fescue (Strickland et al, 2011). The responses include decreased skin temperature or increased RT and increased respiration rate (RR) at ambient temperatures greater than the animals’ thermoneutral www.frontiersin.org
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