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Our objectives were to evaluate technical lignins for their antifungal properties against 3 molds and 1 yeast causing hay spoilage, and their ability to preserve ground high-moisture alfalfa hay nutritive value in vitro. In experiment 1, 8 technical lignins and propionic acid (PRP; positive control) were tested at a dose of 40 mg/mL. The experiment had a randomized complete block design (RCBD, 4 runs) and a factorial arrangement of 3 molds × 10 additives (ADV). The effects of the ADV on yeast were evaluated separately with a RCBD. Sodium lignosulfonate (NaL) and PRP were the only treatments with 100 ± 2.8% inhibition of fungi. In experiment 2, the minimum inhibitory concentration (MIC) for selected lignins and PRP were determined. At pH 4, NaL had the lowest MIC across the molds (20-33.3 mg/mL) and magnesium lignosulfonate (MgL) for the yeast (26.7) among the lignins. However, PRP had MIC values that were several-fold lower across all fungi (1.25-3.33). In experiment 3, a RCBD (5 blocks) with a 3 (ADV; NaL, MgL, and PRP) × 4 (doses: 0, 0.5, 1, and 3% wt/wt fresh basis) factorial arrangement of treatments was used to evaluate the preservative effects of ADV in ground high-moisture alfalfa hay inoculated with a mixture of the fungi previously tested and incubated under aerobic conditions in vitro. After 15 d, relative to untreated hay (14.9), dry matter (DM) losses were lessened by doses as low as 1% for NaL (3.39) and 0.5% for PRP (0.81 ± 0.77%). The mold count was reduced in both NaL at 3% (3.92) and PRP as low as 0.5% (3.94) relative to untreated hay (7.76 ± 0.55 log cfu/fresh g). Consequently, sugars were best preserved by NaL at 3% (10.1) and PRP as low as 0.5% (10.5) versus untreated (7.99 ± 0.283% DM), while keeping neutral detergent fiber values lower in NaL (45.9) and PRP-treated (45.1) hays at the same doses, respectively, relative to untreated (49.7 ± 0.66% DM). Hay DM digestibility was increased by doses as low as 3% for NaL (67.5), 1% MgL (67.0), and 0.5% PRP (68.5) versus untreated hay (61.8 ± 0.77%). The lowest doses increasing neutral detergent fiber digestibility relative to untreated hay (23.3) were 0.5% for MgL and PRP (30.5 and 30.1, respectively) and 1% for NaL (30.7 ± 1.09% DM). Across technical lignins, NaL showed the most promise as a potential hay preservative. However, its effects were limited compared with PRP at equivalent doses. Despite not having an effect on preservation, MgL improved DM digestibility by stimulating neutral detergent fiber digestibility. This study warrants further development of NaL under field conditions., (Copyright © 2020 American Dairy Science Association. Published by Elsevier Inc. All rights reserved.)