Case Study: Recovery from ergot alkaloid-induced vasoconstriction for steers conditioned to grazing seedhead suppressed and unsuppressed pastures of toxic endophyte-infected tall fescue

Authors: WILLIAMSON, JESSICA - PENNSYLVANIA STATE UNIVERSITY; AIKEN, GLEN

Submitted to: Professional Animal Scientist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/26/2017
Publication Date: N/A
Citation: N/A

Interpretive Summary: A fungal endophyte that infects most plants tall fescue produces ergot alkaloids that adversely affect cattle performance and well-being. Seedheads of tall fescue are the most toxic part of the plant and, unfortunately, cattle readily graze the highly toxic seedheads. Previous research demonstrated that seedheads can be chemically suppressed and this suppression of seedhead emergence can enhance steer performance and relieve the adverse effects of ergot alkaloids on cattle physiology; however, it is not known if seedhead suppression can improve post-graze performance or relieve alkaloid-induced constriction of blood flow that can cause severe heat stress. A pen experiment with non-toxic feeds was conducted with steers after they had been conditioned to grazing either toxic endophyte-infected tall fescue with or without chemical seedhead suppression cattle, or a non-toxic forage. The caudal artery, which supplies blood to the tail, was relaxed in less than 20 days for steers grazed on suppressed pastures. It took greater than 34 days for their caudal arteries to relax in steers grazed in unsuppressed pastures. Body weight gains with the suppressed treatment was similar to the nontoxic treatment within two weeks, and the body weights with the unsuppressed treatment never reached those of the nontoxic treatment. Chemical seedhead suppression of toxic endophyte-infected fescue can relieve alkaloid-induced vasoconstriction and improve post-graze performance, which will be of interest to cattle producers who grain finish beef calves that have grazed toxic endophyte-infected tall fescue.

Technical Abstract: Chemical seedhead suppression of toxic endophyte-infected (E+) tall fescue can enhance steer performance and mitigate the adverse effects of ergot alkaloids on cattle physiology; however, it is not known if seedhead suppression can mitigate alkaloid-induced vasoconstriction and improve post-graze performance. A 2year experiment was conducted with Angus crossbred steers using a pasture phase to precondition the steers to grazing seedhead suppressed E+ tall fescue, unsuppressed E+ tall fescue, or a bermudagrass (Cynodon dactylon), white clover (Trifolium repens), and Kentucky bluegrass (Poa pretensis) nontoxic control, followed by a pasture phase to monitor luminal areas of the caudal artery for assessment of alkaloid-induced vasoconstriction and body weight (BW) to compare the E+ treatments with the nontoxic treatment. Following grazing, caudal arteries with the suppressed treatment in both years were initially constricted, but relaxed to those similar to the steers on the nontoxic treatment in less than 20 days. Those with the unsuppressed treatment were less than with the nontoxic treatment for the full duration in the first year (42 days), and until day 34 in the second year, with a duration of 41 days. Body weights with the E+ treatments were less than with the nontoxic treatment, but compensatory BW gains with the suppressed treatment resulted in their being similar to the nontoxic treatment by days 8 and 13 in the first and second years, respectively, but BW with the unsuppressed treatment never reached those of the nontoxic treatment. Results indicated that chemical seedhead suppression of E+ fescue can relieve alkaloid-induced vasoconstriction and improve post-graze performance.