Effects of dietary Zilpaterol hydrochloride and non-steroidal growth implant on growth and carcass characteristics of feedlot lambs

The purpose of this research was to investigate the effect of zilpaterol hydrochloride (ZH) in combination with a non-steroidal growth implant on growth and carcass characteristics of feedlot lambs. The use of ZH is common in commercial beef production, however is not yet registered for use in lamb production without a veterinary prescription on an extra label basis. Lamb producers are looking to a more intensive production system for sheep, which requires a finishing phase and therefore warrants the research of growth promotants that can be used in this phase to increase efficiency. In this study 180 South African Mutton Merino male-type lambs were selected at a commercial feedlot. After the growing phase 20 lambs were excluded from the group and 160 lambs were used for the finishing phase. Half of the lambs were implanted with zeranol marketed under the name Ralgro® (MSD), a non-steroidal, oestrogen like compound. The lambs were randomly put into 8 groups with different treatments, to test the different combinations of ZH, zeranol and duration of treatment. ZH was either fed for 18 days with a 3 day withdrawal period or for 25 days with a 3 day withdrawal period. The lambs were slaughtered and carcasses were chilled for 3 days before samples were taken and analysed. ZH had an effect on average daily gain (ADG), cold carcass mass and tenderness (P <0.05). ZH tended to have an effect on hide weight and cooking loss (P <0.10). The lambs treated with ZH grew 16.7g/day more than the control group and had 0.51kg heavier carcasses than the control group. Zeranol significantly affected ADG and cold carcass mass (P <0.05). Zeranol tended to influence subcutaneous fat thickness (SCF) of the 10th rib (P <0.10). Lambs implanted with zeranol grew 18.8g/day more than the control and had 0.35kg heavier cold carcasses. Duration of treatment significantly affected cold carcass mass, hide weight, pH at 45 minutes post mortem, pH after chilling, fat thickness, tenderness and SCF of the 8th rib (P <0.05). Duration of treatment had a tendency to influence cooking loss, with a longer time on treatment causing more loss (P <0.10). There were a number of interactions. ZH and zeranol (Z*R) caused significant interactions in ADG, cold carcass mass and hide weight (P <0.05). Z*R also tended to cause interactions in pH after chilling (P <0.10). Zeranol and duration of treatment (R*D) significantly caused an interaction in pH after chilling (P <0.05) and ZH and duration of treatment tended to cause an interaction in pH after chilling (P <0.10). ZH, zeranol and duration of treatment caused a significant interaction in cold carcass mass (P <0.05) and tended to cause an interaction in cooking loss (P <0.10). ZH improves ADG, as well as cold carcass mass and did not influence carcass characteristics, probably due to a low dosage. ZH increases the sheer force values of lamb post slaughter and has no practical influence on cooking loss. Feeding ZH for a longer duration also causes tougher meat. Carcass composition did not differ significantly due to ZH treatment as expected, because ZH is known to be a repartitioning agent. ZH has better results in increasing ADG and cold carcass mass, when lambs were implanted with zeranol during the growing phase. The combination of ZH and zeranol seems to have an additive affect on ADG and cold carcass mass. Using Zeranol in combination with ZH for 25 days proved to yield the best results for both ADG and cold carcass mass. ZH tended to decrease the hide weight and therefore shows the repartitioning effect of ZH. A follow up study on different dosage levels of ZH can be helpful to determine if ZH has an effect on carcass composition in lamb.