NITROGEN-BALANCE, URINARY 3-METHYLHISTIDINE EXCRETION AND DIET DIGESTIBILITY IN FRIESIAN STEERS FED CIMATEROL

The objectives of this study were (i) to determine the effect of the beta-adrenergic agonist cimaterol on nitrogen (N) retention in young growing steers, (ii) to determine if cimaterol decreases muscle protein degradation as reflected in urinary 3-methylhistidine excretion and (iii) to examine the effect of cimaterol on diet digestibility. Twelve Friesian steers (liveweight 317, s.d. 21.8 kg) were assigned at random to receive either 0 (control) or 0.09 mg cimaterol/kg bodyweight per day. The steers were housed in metabolism crates throughout the experiment which consisted of an 8-day pre-treatment period (data used as covariate), two consecutive 8-day treatment periods and an 8-day post-treatment period. Nitrogen was measured in faeces and urine which was collected on a 2-day basis. Three-methylhistidine was measured in samples from each animal pooled within period. Plasma concentrations of glucose, urea-N and creatinine were determined in blood samples collected from all animals at 09.00 h on days 1, 3, 5 and 9 and daily from day 10 to 33 inclusive. Cimaterol treatment tended to increase N retention with the effect being similar in both treatment periods (mean + 5.5 g/day). Excretion of 3-methylhistidine and dry matter digestibility (DMD) were similarly decreased in both treatment periods (-21 mmol/day and -24 g/kg, respectively) by cimaterol. Plasma concentrations of creatinine were increased and glucose decreased by cimaterol treatment, the effect being greater in the second period of treatment for the latter variable. The plasma concentration of urea was increased by cimaterol in the first but not in the second period of treatment. A continued reduction in DMD and a compensatory increase in the plasma concentration of urea in steers previously treated with cimaterol were the only effects observed upon withdrawal of cimaterol from the diet. It is concluded that cimaterol (i) increased N retention, (ii) reduced protein degradation as reflected in reduced urinary 3-methylhistidine excretion and (iii) had a negative effect on diet digestibility.