Abstract # W244

Hydroxy trace minerals (HTM) may influence ruminal fiber utilization compared with sulfate trace minerals (STM). This study evaluated Nutrilock^® HTM (Chemlock Nutrition), STM, and Nutrilock^® HTM plus Na₂SO₄ on 48-h in vitro digestibility of a lactating-cow TMR. Diet samples were ground to 1 mm, weighed into acetone-rinsed filter bags (25-µm porosity), and incubated for 48 h in vitro using a composite ruminal inoculum collected from 6 cows. Treatments consisted of HTM incorporated into the total mixed ration (TMR; Zn 61.5 ppm, Mn 63.4 ppm, Cu 21.6 ppm), STM diluted in water and added at the jar level to a base TMR without Nutrilock^® HTM to achieve equivalent concentrations of Zn, Mn, and Cu, and Nutrilock^® HTM-treated TMR with Na₂SO₄ added at the jar level to match the sulfur concentration of STM. Incubations were conducted in 15 jars (5 per treatment), with jar as the experimental unit. Apparent DM digestibility at 48 h (ADMD₄₈) was determined from washed residues, in vitro true digestibility (IVTD₄₈) after neutral detergent extraction, and NDF digestibility (NDFD₄₈) on an aNDFom basis. Data were analyzed using the lm function in R, with Type III tests (car::Anova) and least squares means and differences among means evaluated using emmeans. NDFD₄₈ differed among treatments (P = 0.025) and was greater for Nutrilock^® HTM (57.53 ± 0.64%) than STM (55.37 ± 0.29%; P = 0.023) and Nutrilock^® HTM + Na₂SO₄ (55.13 ± 0.73%; P = 0.013), whereas STM did not differ from Nutrilock^® HTM + Na₂SO₄ (P = 0.77). ADMD₄₈ differed among treatments (P = 0.049) and was greater for Nutrilock^® HTM (72.23 ± 0.32%) than Nutrilock^® HTM + Na₂SO₄ (71.02 ± 0.32%; P = 0.017). IVTD₄₈ also differed among treatments (P = 0.049) and was greater for Nutrilock^® HTM (85.67 ± 0.12%) than Nutrilock^® HTM + Na₂SO₄ (85.25 ± 0.13%; P = 0.035). In conclusion, Nutrilock^® HTM improved 48-h in vitro NDF digestibility relative to STM, and addition of Na₂SO₄ attenuated this response.