Abstract:
In this study, 14 typical South African feedlot bulls received no beta-adrenergic
agonist for the last 30 days of the finishing period (C), 14 received 120 mg R-salbutamol per
animal per day for the last 30 days of the finishing period (S30), 13 received 120 mg Rsalbutamol
per animal per day for the last 40 days of the finishing period (S40) and the last
group of 13 bulls received 60 mg zilpaterol hydrochloride per animal per day for the last 30
days of the finishing period (Zh). All animals were slaughtered after a 3-day withdrawal
period. Parameters included weight gain, feed intake, feed conversion rate, warm and cold
carcass mass, dressing %, subcutaneous fat thickness, hide yield %, internal carcass fat
distribution, % bone, % fat and % muscle of the prime rib-cut, carcass classification code,
conformation, compactness, post-mortem carcass pH profiles, cooking loss, shear force,
blood urea nitrogen, creatinine and residue levels of the beta-adrenergic agonists.
No differences were observed between any of the four treatment groups concerning
live feedlot performance. Bulls receiving the S30 and Zh treatments had lower internal
carcass fat distribution compared to C bulls (P <0.05). Bulls receiving the S40 treatment had
a lower % fat in the prime rib-cut compared to Zh bulls (P <0.05). Carcasses from S30 bulls
had higher pH values 24 hours post mortem compared to carcasses from Zh bulls (P <0.01).
Meat samples from Zh bulls had higher shear force, which indicates less tender meat,
compared to samples from S40 bulls (P <0.05). Change in serum creatinine levels increased
only in Zh treated bulls from the start to the end of treatment and may reflect a higher protein
turnover in Zh bulls. The results of this study indicate that R-salbutamol has a more
pronounced effect on fat metabolism in feedlot bulls compared to zilpaterol hydrochloride,
while zilpaterol hydrochloride has a more pronounced effect on protein metabolism. The
residue levels in samples of the liver, kidney, muscle and feaces from zilpaterol
hydrochloride and R-salbutamol treated bulls were well below acceptable limits.
