Threshold tolerances for sorghum ergot in cattle feedlot rations

Summary

Sorghum ergot is a fungal disease of sorghum that has been shown to depress performance of cattle fed contaminated grain in feedlots. Earlier studies by the current research team have shown marked reductions in intake and growth rate of cattle consuming sorghum grain containing as low as 1.5 mg/kg (parts per million; ppm) dry matter (DM) of dihydroergosine (DHES), an alkaloid compound which is the toxic component of the ergot sclerotes in grain. The effects of the ergot appeared most severe when fed during the hotter months of the year, due to an apparent impairment of heat dissipation mechanisms in the animals. Nevertheless, even in winter contamination at 3 ppm alkaloid impaired animal performance.

The present experiment was designed to

(i) measure the effect on cattle performance of various concentrations of sorghum ergot alkaloid, at or below the industry standard for ergot contamination in feedlot rations fed during the cooler months of the year; and

(ii) determine the threshold level for ergot alkaloid concentration below which there is no effect on cattle growth rates.

At the DPI's Animal Husbandry Research Farm (AHRF), Rocklea, Hereford steers (307.8 - 12.04 ( s.d.) kg liveweight) were fed concentrate / hay (90:10) rations based on sorghum grain with various concentrations of ergot alkaloid, as achieved by mixing clean and ergot-contaminated grain in varying proportions. The infected grain used in this experiment contained approximately 8% ergot and 23.5 ppm alkaloid (DHES), so that the current industry standard of 0.3% ergot by weight was exceeded in this study when the alkaloid concentration exceeded 0.9 ppm.

The DHES (alkaloid) concentrations in the grain component of the rations were initially 0 (control; clean grain only), 0.3, 0.6 and 1.2 ppm. Several weeks into the experiment, the concentrations of alkaloid in two treatments were increased from 0.3 to 2.3 (week 7) and 0.6 to 4.6 (week 8) ppm. Thus treatment alkaloid concentrations overall were: 0 ppm throughout (Control); 0.3 then 2.3 ppm after week 7 (E 0.3/2.3); 0.6 then 4.6 ppm after week 8 (E 0.6/4.6); and 1.2 ppm throughout (E 1.2).

The experiment continued for 18 weeks between May 8 and September 11, 2000. The steers were weighed and rectal temperatures taken once weekly, prior to feeding in the morning. Conditions remained relatively cool throughout the experiment and the temperature-humidity index (THI) was at all times below 70, the point above which Bos taurus cattle are likely to show signs of heat stress in the presence of ergot. Rectal temperature of the steers was not affected by ergot inclusion in the diet at any level. Control steers had an intake equivalent to 2.8% liveweight (LW; DM basis) and grew at 1.25 kg/day over the total feeding period. The ergot appeared to have little effect on animal performance until the concentrations of alkaloid were increased in weeks 7 and 8. After this change, the trend was for intake to decline with ergot inclusion, especially for the E 0.6/4.6 group relative to the control group (2.27 vs. 2.75% liveweight; P