The influence of dietary energy and zinc source and concentration on performance, trace mineral status, and gene expression of beef steers

The objective of this study was to determine the effects of increased supplemental Zn from differing sources on growth performance of steers fed diets differing in net energy. Angus steers (n = 72, 324 ± 2.1 kg) with Genemax gain scores of 3, 4, or 5 were blocked by BW and stratified by Genemax gain score into 12 pens of 6 steers each for 158 d. Pens were randomly assigned to 1 of 3 Zn treatments (ZNTRT): 1) control (no supplemental Zn, analyzed 33 mg Zn/kg DM; CON); 2) inorganic Zn (CON + 120 mg supplemental Zn/kg DM as ZnSO4 for entire trial; INZN); or 3) 120 mg supplemental Zn/kg DM as Zn-amino acid complex (Availa-Zn; Zinpro, Eden Prairie, MN) for first 60 d, then a blend of ZnSO4 and Zn-AA complex (CON + 60 mg supplemental Zn/kg DM as ZnSO4 + 60 mg supplemental Zn/kg DM as Zn-amino acid complex) for the remainder of the trial (ZNBLD). Two dietary energy strategies (ENERGY) were formulated to reach ADG rates of 1) 1.6 kg/d (LE) or 2) 2.0 kg/d (HE) utilizing a 3 × 2 factorial arrangement (12 steers/treatment). All steers were fed LE for a 60 d growing period, then pens were randomly assigned to ENERGY treatments fed the remaining 91 d. Day 60 BW tended to be greater (P = 0.07) in steers receiving supplemental Zn vs. CON. Liver Cu was decreased in Zn supplemented steers vs. CON (P = 0.02). Liver Zn concentrations on d 56 did not differ for Zn vs. CON (P = 0.22) nor were there differences due to Zn source (P = 0.98). There were or tended to be ZNTRT × ENERGY effects for d 67-90 ADG and G:F (P ≤ 0.01), and d 122 BW and d 90-122 G:F (P ≤ 0.10) driven by improved performance for ZNBLD-HE over ZNBLD-LE, while ENERGY within CON and INZN did not differ. Day 90-122 ADG, overall ADG and overall G:F was greater (P ≤ 0.02) and d 67-90 G:F tended to be greater (P = 0.10) for HE vs. LE. No ZNTRT × ENERGY or ZNTRT effects were detected for HCW, REA, BF, KPH, MS, or YG (P ≥ 0.37) while HE increased HCW, BF, MS, and YG compared with LE (P ≤ 0.05). In the liver, ZNTRT affected d 97 MT1A expression (P = 0.03) where INZN was greater than ZNBLD or CON (P ≤ 0.02), while ZIP14 was unaffected due to ZNTRT, ENERGY, or the interaction (P ≥ 0.39). Supplying supplemental Zn as ZNBLD during the transition period appeared to improve performance measures, but no final performance advantages were noted due to increased supplemental Zn, regardless of source. Additionally, differences in liver MT1A expression may indicate differing post-absorptive metabolism between Zn sources.