An existing fermented ammoniated condensed whey product (LactoWhey) has been in use in dairy rations for more than 30 years; however the last published studies were almost 30 years ago. It was of interest to test the efficacy of using this product in more modern dairy rations typical of the northwestern US dairy region which contain a wide variety of protein sources and by-product feeds. Therefore, a field trial was conducted at 3 commercial dairies and one university dairy. Lactowhey is liquid fermented ammoniated condensed whey containing 44% CP equivalent and 38% lactic acid equivalent from ammonium lactate. Lactowhey was added to the ration to replace other protein sources (primarily SBM and Canola meal) on a CP equivalent basis. Rations were balanced and fed to meet requirements to multiple pens on each commercial dairy (or through individual feeding gates on the university dairy) starting from an average of 66 DIM for 90 d. On the commercial dairies, milk tank weights and composition were recorded as well as DHIA test day milk and composition. A subset of 18 to 20 cows in each pen was followed for individual milk and composition, and rumen fluid and blood was sampled at 0, 3 and 13 wk of treatment (at the university dairy all cows (n = 20 per treatment group) were also sampled at 6 wk of treatment. Milk production ranged from 35.9 to 45.5 kg/d at start of trial and 41.7 to 35.2 kg/d during treatment period, with no effect of treatment. Milk composition averaged 3.80% and 3.21% and 12.3 mg/dl for fat and protein and MUN across the period and farms, with no effect of treatment. There were no differences due to treatment in BW or BCS change, or rumen pH or in blood NEFA, glucose or BUN. The proportion of rumen bacterial DNA as measured by microbiome analysis was primarily Prevotella species, with a variety of other species represented. Species proportions varied among the 4 dairies and among individual cows, with a generally consistent pattern within dairies and individual cows. There was no consistent effect of the fermented whey on bacterial proportions. The work confirms the utility of this fermented whey product to support high milk production in commercial herd when used to replace other protein sources.