Determination of a sub-chronic threshold for lolitrem B and perennial ryegrass toxicosis in Angus cattle consuming endophyte-infected perennial ryegrass (Lolium perenne) straw over 64 days

Abstract

Lolitrem B is a potent tremorgen responsible for the syndrome known as “ryegrass staggers” in livestock consuming perennial ryegrass (Lolium perenne) infected with the endophyte fungus Epichloë festucae var. lolii. We evaluated clinical signs in cattle continuously fed endophyte-infected perennial ryegrass straw at four concentrations of lolitrem B (LB)/ergovaline (EV) (groups: I-2256/638, II-1554/373, III-1012/259 or IV-<300/0 µg/kg LB/EV, n=5-6 steers/group) over 64 days to establish a sub-chronic threshold of toxicity for this mycotoxin; animals were supplemented with 1 kg grain mix/day. Most animals in groups I and II developed clinical signs consistent with ryegrass staggers by day 64; thus, it is recommended that a threshold level of 1554 µg/kg LB should not be exceeded for extended feeding regimens using endophyte-infected perennial ryegrass feed material alone. This toxicity threshold is lower than that observed with shorter feeding periods, i.e. 1800–2000 µg/kg LB when fed for less than 30 days. No clinical signs associated with ergovaline toxicosis (i.e. decreased weekly weight gain) were observed. As adipose tissue appears to be the main compartment where LB is deposited, the concentration of LB and its metabolic derivatives in fat samples was determined at 37 and 64 days. Lolitrem B and three previously defined metabolites bioaccumulated in a dose- and time-dependent manner throughout the feeding experiment in fat tissue, at levels similar to those found in past research. Ratios of metabolites to LB were consistent across time points and dose groups, indicating that the enzyme(s) responsible for LB metabolism were not saturated. In summary, LB toxicity thresholds are dependent upon specificities of the dietary program including the duration feed material is fed, and should be considered when formulating safe livestock feeding regimens.