Interactive effects of supplemental Zn sulfate and ractopamine hydrochloride on growth performance, carcass traits, and plasma urea nitrogen in feedlot heifers

Interactive effects of supplemental Zn and ractopamine hydrochloride (RH) were evaluated using 156 crossbred heifers (initial BW = 527 kg +/- 6.61; gross BW x 0.96) to determine the impact on feedlot performance, plasma urea nitrogen (PUN), and carcass characteristics. The study was conducted as a randomized complete block design with a 2 x 2 factorial arrangement of treatments. Factors consisted of 1) 30 or 100 mg supplemental Zn/kg diet DM (30Zn or 100Zn) as Zn sulfate and 2) 0 or 200 mg RH/heifer daily. Heifers were blocked by BW and assigned randomly within block to treatments for a 43-d trial. Heifers were housed in partially covered feeding pens (3 heifers/pen; 13 pens/treatment) and provided ad libitum access to feed. Ractopamine hydrochloride was fed for 42 d and removed from the diet until cattle were harvested on d 43. Zinc treatments were fed until harvest. Plasma samples were collected on d 0 and 36 to assess changes in plasma Zn and PUN. On d 43, heifers were weighed, then transported to a commercial abattoir where HCW and incidence of liver abscesses were recorded. Carcass data were collected after 32 h of refrigeration. No Zn x RH interactions were observed for plasma Zn or PUN (P >= 0.58); however, there was a tendency for a RH x day interaction for PUN (P = 0.08). Supplementing 100Zn resulted in increased plasma Zn (P = 0.02) compared to 30Zn. No RH x Zn interactions were observed for feedlot performance (P >= 0.24). Final BW and ADG increased with RH supplementation (P <= 0.02), but DMI was not affected (P = 0.63); thus, feed efficiency improved (P < 0.01) when heifers were fed RH. Supplementing 100Zn tended to reduce ADG (P = 0.07) but did not affect other measures of feedlot performance (P >= 0.12). Zinc x RH interactions were observed for LM area and yield grade (P <= 0.01); LM area decreased and yield grade increased when heifers were supplemented 100Zn with no RH compared to other treatments. A tendency for a Zn x RH interaction was observed for dressed yield (P = 0.08), but no other interactions or effects of Zn were detected for carcass traits (P >= 0.11). Supplementing RH increased HCW (P = 0.03) but did not affect other carcass traits (P >= 0.13). In conclusion, supplemental Zn had little impact on feedlot performance or PUN concentration but may alter muscle and fat deposition when fed in conjunction with RH.