The purpose of the current study was to determine the best method of delivery for glucocorticoid receptor agonist (GRA) treatment. A total of 167 Pig Improvement Company (PIC) piglets (body weight (BW) 7.35 ± 1.24 kg) were weaned at 25.0 ± 0.81 days of age and randomly assigned to 14 treatment groups based on a 2 × 7 factorial arrangement with sex (gilts vs. barrows), in-feed antibiotic (ANT; 110 mg/kg in-feed tylosin), repeated intramuscular (I.M.) injection of GRA (two injections, 0.2 mg/kg BW dexamethasone (DEX)), low dose in-feed GRA (LF, 2.5 mg/kg diet DEX ), high dose in-feed GRA (HF, 5 mg/kg diet DEX), low dose in-water GRA (LW, 0.8 mg/L DEX ), high dose in-water GRA (HW, 1.6 mg/L DEX ), and no treatment control (CON) as the main factors. Body weight and feed intake were measured daily from days 0 to 7 and weekly from days 7 to 28 post-weaning. The interaction effect for average daily gain (ADG) was significant with gilts performing better in the I.M., ANT, and LF groups (p = 0.05). All treatment groups, with the exception of the HW group, had a higher ADG than the CON group. Gilts in the I.M., LF, and HF groups had the highest ADG compared to other treatment groups (p ≤ 0.05). Sex and the interaction between sex and treatments had no effect on the gain-to-feed ratio (G:F; p ≥ 0.21). All treatment groups had a higher G:F than the CON group (p ≥ 0.04). These results suggest that the low-dose, in-feed GRA treatment is the best GRA delivery method and is a suitable alternative to in-feed sub-therapeutic antibiotics.
Keywords: antibiotics; delivery method; glucocorticoid receptor agonist; growth performance; piglet.