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This study investigated the fermentation of liquid feed for pigs and the effect of lactic acid bacteria (LAB) supplementation on fermentation rate, dry matter losses (DML), formation of biogenic amines, and degradation of phytate-P. The basal substrate in all three in vitro batch experiments consisted of 50% canola meal, 25% wheat, and 25% barley. The mixed substrates were adjusted to a dry matter (DM) content of 28.4% and fermented in 1-liter vessels at 37 °C for 24 h. Experiment 1 focused on changes in pH profiles over time. Treatments were as follows: 1) liquid feed without additive (control) and 2) liquid feed supplemented with a mixture of Lactobacillus plantarum , Pediococcus pentosaceus , and Lactobacillus lactis (adLAB) at 2.0 × 10 5 CFU/g liquid feed (wet wt.; n = 8). Substrate pH was measured every 2 h. Experiment 2 focused on DML and the impact of fermentation on phytate-P. Treatments were identical to experiment 1 (control and adLAB; n = 8). Measured parameters included concentration of lactic acid, acetic acid, ethanol, and phytate-P, and DML after 24 h of fermentation. Counts of molds, Enterobobacteriaceae, yeasts, and LAB were determined in one combined sample of all replicates. Dry matter losses were lower in LAB-supplemented fermentations (5.89%) compared to the control (11.8%; P < 0.001). Supplementation with LAB reduced the phytate-P content (2.66 g/kg DM) compared to the control (3.07 g/kg DM; P = 0.002). Experiment 3 evaluated DML and the impact of fermentation on formation of biogenic amines. Treatments were as follows: 1) control, 2) adLAB (2.0 × 10 5 CFU LAB/g liquid feed), 3) adLys (0.60% DM supplemented lysine), and 4) adLAB+Lys (combination of adLAB and adLys; n = 8). The fermentation of adLys resulted in a nearly complete breakdown of supplemented lysine, whereas only 10% of supplemented lysine was lost in adLAB+Lys. Furthermore, all adLys samples tested positive for cadaverine (mean concentration 0.89% DM), whereas no adLAB samples contained cadaverine above the detection limit ( P < 0.001). Results indicate that DML is reduced in fermentations supplemented with homofermentative LAB. Fermentation of liquid feed with homofermentative LAB can effectively reduce the degradation of supplemental lysine and has the potential to further improve P availability., (© The Author(s) 2022. Published by Oxford University Press on behalf of the American Society of Animal Science.)