Effects of timing of anabolic implant insertion on growth and immunity of recently weaned beef steers.
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We evaluated the effects of timing of estrogenic implant insertion, relative to weaning, on growth performance and measurements of innate and humoral immunity of beef calves. On d -14, Angus Simmental crossbred steers ( = 48; BW = 217 5 kg; age = 191 3 d) were stratified by BW, age, and cow parity and randomly assigned to receive no implant (NOIP) or 36 mg of zeranol on d -14, 0, or 14, relative to weaning (IP-14, IP0, and IP+14, respectively; 12 steers/treatment). From d -14 to 0, cow-calf pairs remained on a single, tall-fescue pasture with no access to concentrate supplementation. Steers were weaned on d 0, stratified by treatment and BW, and then allocated into 1 of 16 drylot pens to receive daily free-choice access to a corn silage-based diet during the preconditioning phase (d 0 to 56). Steers were vaccinated against infectious bovine rhinotracheitis (IBRV), bovine viral diarrhea virus (BVDV), and on d -27 and 0. From d 56 to 252 (postpreconditioning phase), steers remained in their respective feedlot pens and were provided free-choice access to corn silage-based growing (d 56 to 167) and finishing total mixed rations (d 168 to 252). Body weight on d 0 did not differ among treatments ( 0.29) but was greater for IP-14 and IP0 than NOIP and IP+14 steers on d 14, 42, and 56 ( 0.05). Treatment effects were not detected for G:F and DMI from d 0 to 56 ( 0.34), but ADG from d -14 to 56 was greater for IP-14 compared to NOIP ( 0.05) and intermediate for IP0 and IP+14 steers. Plasma IGF-1 concentrations were greater for IP-14 than NOIP ( 0.05) and intermediate for IP0 and IP+14 steers on d -7, 0, 14, and 21. Plasma concentrations of cortisol and haptoglobin and serum titers against BVDV types 1a and 2 did not differ among treatments from d 0 to 56 ( 0.37). However, serum IBRV titers were greater for IP+14 than NOIP, IP-14, and IP0 steers ( 0.02). On d 252, BW was greater for IP-14 and IP0 than NOIP steers ( 0.05) and intermediate for IP+14 steers, but ADG and G:F from d 57 to 252 and carcass characteristics at slaughter did not differ among treatments ( 0.16). Thus, the 36-mg zeranol implant did not elicit an inflammatory response or affect the overall vaccine response of steers (except for IBRV titers). However, growth of steers during a 56-d preconditioning period was enhanced by administering 36-mg zeranol implant 14 d before weaning, without affecting subsequent postpreconditioning growth and carcass characteristics at slaughter.