Trace mineral metabolism and nutrient digestibility in lambs supplemented with zinc sulfate during an adrenocorticotropic hormone challenge

Abstract

The objective was to evaluate the effects of Zn supplementation and adrenocorticotropic hormone (ACTH) injections on nutrient digestibility, trace mineral metabolism and status. It was hypothesized that ACTH would decrease nutrient digestibility and Zn retention, but Zn supplementation would improve Zn retention, regardless of ACTH administration. Twenty-two Polypay cross weathers previously enrolled in a trial, consumed a moderately Zn deplete diet (22 mg Zn/kg DM) for 52 d, followed by being housed in metabolism crates for 15 d while receiving one of two diets: CON (Zn deplete diet) or ZINC (CON plus 40 mg Zn/d from ZnSO4). Lambs rested for one day outside of crates then returned to crates for this 5 d study. Lambs continued to receive dietary Zn treatments and were randomly assigned to receive injectable treatments, once daily on d 1–4 at feeding: SALINE (2 mL saline I.M.), STRESS (80 I.U. ACTH I.M.); lamb was experimental unit (n = 5–6/treatment) in a 2 × 2 factorial design. Total fecal and urine collection was conducted on d 3–5, and blood samples were collected on d 0, 1, and 4, 3 h post feeding. Day 4 plasma Zn concentrations, covariate adjusted with d 0 concentrations, displayed a diet × injection tendency (P = 0.06) where ACTH given to ZINC lambs depressed plasma Zn to values similar to CON lambs. Day 1 and 4 cortisol concentrations determined 3 h after injection administration were increased in STRESS vs. SALINE (P < 0.01). Day 4 plasma Cu concentrations, covariate adjusted with d 0 concentrations were lesser in STRESS vs. SALINE (P = 0.04). Day 4 plasma Fe concentrations were unaffected by diet or injection treatment (P ≥ 0.18). Apparent absorption of Zn was increased (P < 0.01) in CON vs. ZINC, but were similar in STRESS vs. SALINE (P = 0.13). Zinc retention, and apparent absorption of Mn and Cu was similar across treatments (P ≥ 0.19). Organic matter (OM) digestibility tended to be decreased in ZINC vs. SALINE (P = 0.07); neutral detergent fiber (NDF) and acid detergent fiber (ADF) digestibility were decreased (P ≤ 0.04) in ZINC vs. SALINE. Injection treatment had no effect on macronutrient digestibility (P ≥ 0.37). The CON lambs were marginally Zn deficient based on plasma Zn concentrations and rate of apparent absorption, while fiber digestion was impaired by Zn supplementation. Plasma cortisol concentrations indicate lambs receiving ACTH experienced physiological stress, however this had limited effects on macro or micronutrient metabolism.