Abstract # T282

Herd-level dyscalcemia, defined as total serum calcium (Ca) <2.2 mmol/L at 4 days in milk (DIM), has been shown to be a key indicator of post-transition performance. The objective of this field study was to compare the prevalence of dyscalcemia in herds fed an acidified pre-fresh diet with the same herds fed a zeolite A (ZA; CalBal®, Chemlock Nutrition) pre-fresh diet. Twelve commercial dairy herds located in CA, NY, WI, TN, MN, and ID were enrolled after being identified by nutritionists as utilizing an acidified pre-fresh diet for 21 d prior to calving. The DCAD levels of the acidified diets ranged from -10 to -37. Urine pH’s were not reported. Following the determination of baseline serum Ca concentrations at 4 DIM, pre-fresh rations were reformulated to include ZA in place of the acidified strategy. The inclusion of ZA was formulated at 7.4 times the amount of phosphorus in the diet (between 2-2.75% of the diet on a dry matter basis). Data were collected from 622 apparently healthy fresh cows between October 2023 and September 2025, representing approximately 85,800 milking cows. Herds consisted of Holstein (n = 9), Jersey (n = 2), and crossbred (n = 1) cattle. Blood samples were collected at 4 ± 1 DIM, serum was harvested by centrifugation, and Ca concentration was analyzed by the Michigan State University Veterinary Diagnostic Laboratory. Cows that spent fewer than 7 d on the close-up diet were excluded. Data were analyzed using PROC MIXED of SAS (v. 2026), with herd included as a random effect. Cows were classified as receiving either an acidified diet (n = 162) or a ZA diet (n = 311). Cows fed the acidified diet exhibited a greater prevalence of dyscalcemia compared with cows fed ZA (43.5% vs. 20.5%, P < 0.0001). Mean serum Ca concentration was also greater for cows fed ZA compared with those fed an acidified diet (2.31 vs. 2.19 mmol/L, P < 0.0001). In summary, inclusion of zeolite A in pre-fresh diets was associated with higher serum Ca concentrations and a reduced prevalence of dyscalcemia at 4 DIM.