Low moisture, cooked molasses blocks: A limited intake method for supplementing trace minerals to pre-weaned calves

•Molasses blocks are an effective strategy for delivering minerals to pre-weaned calves.•Molasses blocks require less frequent delivery of supplement (labor reduction).•Mineral-fortified molasses blocks improved mineral status of calves. Two studies were conducted at the Range Cattle Research Education Center (Ona, FL) to evaluate the preference of beef calves for different sources of trace minerals in low moisture cooked molasses blocks (MB; Exp.1) and to evaluate the effects of optimal trace mineral status at weaning while investigating the use of free-choice, MB in a limited intake creep-feeding system (Exp. 2). In Exp. 1, eighteen early-weaned calves (120 d and 185 kg BW) were randomly assigned to 6 bahiagrass pastures (Paspalum notatum; n = 3/pasture). Treatments consisted of 3 MB formulations containing 550, 1650 and 2200 mg/kg of Cu, Zn and Mn from 3 different sources (hydroxychloride, organic and sulfate). Calves had continuous and simultaneous access to each of the 3 MB treatments. Intake was greatest (P = 0.04) for MB fortified with hydroxychloride and was similar between organic and sulfate (0.35, 0.27, and 0.28 kg/d, respectively). In Exp. 2, forty-eight cows with heifer calves (n = 24 pairs/year) were randomly assigned to 12 bahiagrass pastures (n = 2 cow-calf pairs / pasture). Treatments consisted of 2 MB formulations; (1) Control = no added minerals; or (2) Fortified = 550, 1650, and 2200 mg/kg of Cu, Zn, and Mn from hydroxychloride sources. Sodium, Ca, Se, Co, and I were formulated into the Fortified MB treatment at levels to meet NASEM recommendations. Following weaning, calves were transferred to fully covered individual pens for a 30-d evaluation. Molasses block intake was greater (P = 0.01) among calves assigned to Control vs. Fortified treatment (0.35 vs 0.21 kg/d). A treatment × year effect was detected (P ≤ 0.01) for liver trace mineral concentrations, where calves assigned to Fortified vs. Control treatment had greater concentrations of Co, Cu, Mn, and Se at weaning in year 1, but not in year 2. For the Fortified treatment, MB intake was 38 % less in year 1 vs. 2, which correlated with less liver Cu (r = 0.52; P < 0.01) and Se (r = 0.79; P < 0.01). In summary, MB supplements, fortified with hydroxychloride sources of Cu, Zn, and Mn, are preferred by calves when compared to other mineral sources. The supplementation of pre-weaned calves with mineral-fortified low moisture, cooked MB was effective as a limit-feeding vehicle for trace