Two feeding trials evaluated several byproducts from commercial amino acid fermentations as N supplements for lactating cows. Trial 1 was a replicated 5 x 5 Latin square that used 2-wk periods and 25 Holstein cows (five with ruminal cannulae) fed diets containing [dry matter (DM) basis] 28% alfalfa silage, 31% corn silage, 28% high moisture ear corn plus 4 percentage units of crude protein (CP) from: soybean meal, urea, commercial fermentation byproduct 1 or 2, or a blend of fermentation byproducts plus wheat middlings. Diets averaged 15.1% CP and 32% neutral detergent fiber. Intake of DM, body weight (BW) gain, and yield of milk and milk components were greatest for cows fed soybean meal; animal performance was similar with urea, byproduct 1 and the byproduct blend. Intake, BW change, and yield of milk and protein when cows were fed byproduct 2 were lower than when fed urea. Urine output (estimated with creatinine in spot urine samples) was greater on fermentation byproduct 1 and the byproduct blend. There were no differences due to N source in microbial synthesis (based on estimated purine derivative excretion), in situ digestion of alfalfa hay DM, or molar proportions of ruminal volatile fatty acids. Trial 2 was a replicated 5 x 5 Latin square using 2-wk periods and 10 Holstein cows fed diets containing (DM basis) 37% alfalfa silage, 28% corn silage, 29% high moisture ear corn plus 2 percentage units of CP from urea, fermentation byproduct 1, or one of three blends of fermentation byproducts plus wheat middlings. Except for greater DM intake in cows fed the byproduct blends, performance and urinary metabolite excretion did not differ because of N supplement. Relative to other fermentation byproducts and urea, byproduct 1 resulted in reduced milk urea N in both trials. Under the conditions of these trials, fermentation byproducts were less effective than soybean meal, and no more effective than urea, as N supplements.