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, Switzerland) subsequent to a depletion phase. Steers were blocked by BW (n = 5 blocks; 6 steers per block), assigned to pens (n = 2 steers per pen), and randomly assigned to one of the following dietary treatments: no added vitamin A (0IU; 0.0 IU/kg dry matter [DM] basis of additional vitamin A), vitamin A supplemented at the estimated National Academies of Sciences, Engineering, and Medicine (NASEM) requirement (2,200IU; 2,200 IU/kg of dietary 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 days -91, -35, 0, 28, 56, 84, and 112. Final BW was collected prior to shipping on day 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 or feed efficiency (G:F; P > 0.05). Throughout the duration of the study, dry matter intake for the 0IU cattle was depressed (P = 0.01). Differences were not observed across treatments for hot carcass weight, rib eye area, back fat thickness, kidney-pelvic-heart fat %, 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; R2 = 0.682). However, models used to evaluate depleted animals were less effective. The current NASEM recommended that vitamin A requirement of 2,200 IU/kg is adequate for repletion of vitamin A status of feedlot steers.