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Evaluation of the dietary vitamin A requirement of finishing steers via systematic depletion and repletion, and its effects on performance and carcass characteristics.

A randomized complete block design experiment with 30 yearling crossbred steers [initial average body weight (BW) = 297.6 ± 32.8 kg] fed a steam-flaked corn-based diet was used to evaluate finishing performance and carcass characteristics when provided with different concentrations of vitamin A (Rovimix® A 1000; DSM Nutritional Products Ltd., Sisseln, SUI) subsequent to a depletion phase. Steers were blocked by BW (n = 5 blocks; 6 steers/block), assigned to pens (n = 2 steers/pen), and randomly assigned to one of the following dietary treatments: no added vitamin A (0IU; 0.0 IU/kg [DM basis] of additional vitamin A), vitamin A supplemented at the NASEM requirement [2,200IU; 2,200 IU/kg of dietary dry matter (DM) of additional vitamin A), and vitamin A supplemented at 5× the estimated requirement (11,000IU; 11,000 IU/kg of dietary DM of additional vitamin A). The basal diet included minimal vitamin A activity (& 200 IU of vitamin A activity/kg of dietary DM) via the provitamin A, beta-carotene. After all animals underwent a 91- d vitamin A depletion period, additional vitamin A was top-dressed at feeding via a ground corn carrier. Liver biopsy samples, BW, and blood were obtained on d -91, -35, 0, 28, 56, 84, and 112. Final BW were collected prior to shipping on d 112. Carcass data were collected by trained personnel upon harvest. Sera and liver samples were used to monitor circulating vitamin A and evaluate true vitamin A status of the cattle. Vitamin A status did not affect interim average daily gain (ADG) or feed efficiency (G:F; P > 0.05). Throughout the duration of the study, dry matter intake (DMI) for the 0IU cattle was depressed (P = 0.01). Differences were not observed across treatments for hot carcass weight (HCW), rib eye area (REA), back fat thickness (FT), kidney-pelvic-heart fat (KPH) %, marbling score, or dressing percent (P ≥ 0.10). A treatment × day interaction occurred for both (P & 0.01) sera retinol and liver retinol during phase 2 of the trial. The treatments and sera retinol concentrations were incorporated into a repletion model, resulting in an estimation of liver retinol changes (P & 0.01; R 2 = 0.682). However, models used to evaluate depleted animals were less effective. The current NASEM recommended vitamin A requirement of 2200 IU/kg is adequate for repletion of vitamin A status of feedlot steers.