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1377
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 × 0.96) to determine 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. 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/animal daily. Heifers were blocked by BW and, assigned randomly within block to treatments. Heifers were housed in partially covered feeding pens (3 heifers/pen; 13 pens/treatment), fed once daily ad libitum, and RH was fed for 42 d and removed from the diet until cattle were harvested on d 43. Plasma samples were collected on d 0 and 36 to assess changes in plasma Zn and PUN. On d 43, heifers were 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 (P ≥ 0.58); however, there was a tendency for RH × d interaction for PUN (P = 0.08). Supplementing 100Zn increased plasma Zn concentration (P = 0.02) compared to 30Zn. No 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 cattle were fed RH. Supplementing 100Zn tended to reduce ADG (P = 0.07), but did not affect other measures of feedlot performance or incidence of liver abscesses (P ≥ 0.12). Zinc × RH interactions were observed for LM area and yield grade (P ≤ 0.01); LM area decreased and yield grade increased when cattle were supplemented 100Zn with no RH compared to other treatments. A tendency for Zn × RH interaction was detected for dressing percentage (P = 0.08), but no other interactions or effects of Zn were observed 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.
Keywords: feedlot cattle, urea nitrogen, ractopamine, zinc