Summary
Sorghum ergot is a fungal disease, which has the potential to have a major impact on the economic viability of Queensland�s sorghum crop and also on livestock production systems reliant on sorghum as a feed grain. It was first detected in sorghum in Australia in early 1996 but was initially considered harmless to livestock. However, evidence of depressed milk production in sows and dairy cattle in 1997 prompted an urgent re-evaluation of its effect on cattle in feedlots, a major user of grain sorghum. An initial study carried out in the 1998 summer / autumn period at the DPI's Animal Husbandry Research Farm (AHRF), Rocklea, evaluated the effect of 4 levels of ergot contamination of sorghum on the performance of Hereford steers in the feedlot, viz., 0, 0.5, 1 and 2% ergot, representing 0, 1.5, 3 and 6 parts per million (ppm) ergot alkaloid in the grain. The alkaloids are the toxic compounds in the ergot.
The results of this study indicated that: Ergot at any concentration in the grain depressed growth rates of cattle in the feedlot (by between 26 and 45%), and thereby increased the period taken to finish steers for slaughter; Growth rate depression tended to increase with alkaloid concentration, but differences in growth rate between different concentrations were small; Depressed growth rates were associated with reduced intake of feed with only small effects on food conversion ratios; The effect of ergot appeared to be more severe in the hotter, more humid months (ie., high temperature/humidity index; THI) and it appeared that presence of ergot in the diet reduced the tolerance of the cattle to heat stress; and Rectal temperatures of the steers were increased when ergot was included in the diet. In view of the apparent association between alkaloid concentration, heat tolerance and animal performance, the current study was designed to measure the effects of ergot in sorghum when fed predominantly during the cooler months of the year (June to November).
A feedlot study, similar to that described above, was carried out in which 35 Hereford steers (271 kg, initial liveweight) were individually fed conventional feedlot rations (90% rolled grain/concentrate: 10% hay) with various concentrations of ergot alkaloid in the grain, achieved by mixing different proportions of clean (no ergot) and contaminated (ca. 24 ppm alkaloid) sorghum grain. Concentrations of alkaloid used were control, 0; E 3, 3; E 6, 6; E 9, 9; and E 12, 12 (ppm in grain). Steers were fed ad libitum, once daily in the morning, and weighed with rectal temperatures taken once weekly. Despite the experiment continuing into the spring months, ambient temperatures remained relatively low and THI remained below 70 throughout the experiment.
Nevertheless, some steers receiving ergot in their ration showed signs of mild heat stress, especially after exercise. Steers receiving ergot also had longer, rougher coats than those on clean grain, which is characteristic of ergot feeding. Control steers grew at a relatively slow rate (1.26 kg/d) throughout the 20 week (139 d) feeding period, which is thought to be related to their previous lack of experience with feed troughs and also possibly to growth retardation at the source property, as indicated by most steers having two or more permanent teeth despite their low weights. Nevertheless, this growth rate was on average 49% greater than that for the ergot treatments (0.83 kg/d; P0.05) and E 9 (2.2% LW; P
