Influence of monensin on some metabolic responses of cattle fed a restricted intake of lucerne hay during heat exposure

Abstract

The effects of monensin supplement (33 mg/kg feed) on ruminal fermentation and body metabolism were studied in Brahman steers fed 5 kg lucerne hay daily at thermoneutral temperature (24¦C) and after heat exposure [I day at 31¦C (acute) or 5 days at 37¦C (chronic)]. At 24¦C, monensin lowered the concentrations of ammonia and protozoa in ruminal fluid and increased the plasma concentration of cholesterol but did not affect the circulating levels of urea, glucose, insulin, thyroxine (T4), triiodothyronine (T3), free T4 and free T3. Acute heat exposure lowered the plasma concentrations of thyroid hormones. Chronic heat exposure increased body temperature, the rate of dry matter digestion in the rumen and urinary nitrogen and allantoin excretion and lowered the plasma concentrations of cholesterol and differentially lowered the concentrations of the thyroid hormones. An increase in the concentration of ammonia in ruminal fluid was the only response to monensin at elevated body temperature that was uncharacteristic of its effects at normal body temperature. The results indicate that a monensin treatment of 33 mg/kg feed does not have any adverse effects on steers suffering metabolic derangement during hyperthermia.