Effects of conventional versus rumen protected fat on growth performance and carcass characteristics of feedlot bulls fed Zilpaterol diets

Abstract

The objective of this study was to compare a conventional fat source vs rumen protected fat in typical Zilmax® finisher diets during the finishing phase of the feeding period on growth performance and carcass characteristics of feedlot bulls. Animals were fed for thirty days plus a three-day withdrawal period as per the manufacturers label registration. Zilpaterol hydrochloride has been registered in South Africa since 1995 for use in terminal slaughter cattle. In this study, conventional fat and rumen protected fat were tested as a way of optimising the use of Zilmax® and by making use of these different fat sources, potentially upregulate receptor affinity and unmask spare Beta-adrenergic receptors leading to the more efficient use of BAA in finisher diets under typical South African feedlot conditions. In this study, 120 Bonsmara type composite feedlot bulls were selected at a commercial feedlot in Mpumalanga, South Africa. The feedlot cattle were randomly selected from a larger group of adapted animals in order to obtain a homogenous population of trial animals. The experimental animals were adapted for a period of 7 days to ensure that the herd structure and hierarchy could be re-established within each experimental pen prior to the start of the trial. A randomized block design was used to allocate experimental animals into the various treatment groups. Animals were randomly allocated into 20 pens with 6 animals per pen, and a total of 5 replicates per experimental treatment group tested. The treatments tested the effects of Megalac® supplementation in combination with conventional fat sources in Zilmax® diets used during the finisher phase on the growth performance, average daily gain (ADG), dry matter intake (DMI) and feed efficiency (FCR) as well as carcass characteristics such as hot carcass weight (HCW) and cold carcass weights (CCW), subcutaneous fat thickness (SCF) over the 13th rib, intramuscular fat content and carcass compactness. Within the 30-day experimental period, the trial was split into three phases for the purposes of blood metabolite concentration determination. Phase one was day 0 of the trial, phase two was day 15 and phase three was day 30 of the trial. The low hominy chop diet without Megalac® (ZH-1) and low hominy chop diet with Megalac® (M1) resulted in an increased circulating blood urea nitrogen concentration during the second blood collection period (P<0.001). At the third blood collection point during phase three, only the low hominy chop diet with Megalac® (M1) group showed an increase in blood urea nitrogen concentration (P<0.05). The low hominy chop diet without Megalac® (ZH-1) treatment group (P<0.001) as well as the moderate hominy chop diet with Megalac® (M2) group both showed statistically significant increases (P<0.05) in circulating beta-hydroxybutyrate concentrations at the second blood collection phase of the trial. At the third blood collection point (day 30), three days prior to slaughter, the M1 treatment group had higher circulating levels of beta-hydroxybutyrate (P<0.05). Bulls receiving the ZH-1 treatment had a higher ADG (P<0.05) compared to the control and M2 treatment groups. No other differences or tendencies were found between treatment groups concerning growth performance during the feedlot phase of the trial. No significant differences were observed between the four treatment groups concerning carcass characteristics. The results support the hypothesis that the higher levels of unsaturated fatty acids may lead to the upregulation of receptor affinity and unmask spare beta-adrenergic receptors in terms of a higher ADG(P<0.05). Further studies with a wider ratio of unsaturated: saturated fatty acids are recommended. In conclusion, the ZH-1 treatment group had the highest ADG (P<0.05) and numerically the heaviest carcass weights when compared to the other three treatment groups.