Interactive effects of supplemental Zn sulfate and ractopamine hydrochloride on growth performance, carcass traits, and plasma urea nitrogen in feedlot heifers.
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Interactive effects of supplemental Zn and ractopamine hydrochloride (RH) were evaluated using 156 crossbred heifers (initial BW = 527 kg 6.61; gross BW 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 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 RH interactions were observed for plasma Zn or PUN ( 0.58); however, there was a tendency for a RH day interaction for PUN ( = 0.08). Supplementing 100Zn resulted in increased plasma Zn ( = 0.02) compared to 30Zn. No RH Zn interactions were observed for feedlot performance ( 0.24). Final BW and ADG increased with RH supplementation ( 0.02), but DMI was not affected ( = 0.63); thus, feed efficiency improved ( < 0.01) when heifers were fed RH. Supplementing 100Zn tended to reduce ADG ( = 0.07) but did not affect other measures of feedlot performance ( 0.12). Zinc RH interactions were observed for LM area and yield grade ( 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 RH interaction was observed for dressed yield ( = 0.08), but no other interactions or effects of Zn were detected for carcass traits ( 0.11). Supplementing RH increased HCW ( = 0.03) but did not affect other carcass traits ( 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.
