Two experiments were conducted to evaluate the effect of addition of hydrated milk whey (HMW) or a combination in equal parts of HMW previously fermented or not fermented (H/FMW) on tropical grass (TG) silage. The first experiment evaluated the fermentation characteristics and aerobic stability of silage from a mixture of two naturalized TG, Buffel (Cenchrus ciliaris) and railroad-track grass (Dichanthium annulatum) when HMW was applied at the rate of 0,5, and 10% (w/w) to green forage. Results indicated that HMW addition at both rates reduced the pH to levels below 4.5 during the fermentation versus values greater than five in the control treatment. Upon exposure to air the pH remained low in the treatments with additive, especially 10% HMW, relative to that of the control. The second experiment evaluated Pangóla grass (Digitaria decumbens) silage with respect to microbial populations, fermentation characteristics, and aerobic stability. Additives were 10% HMW and 10% H/FMW. Although both additives reduced the pH, increased the population of lactic acid-producing bacteria, decreased the coliform population, and did not affect the populations of molds and yeasts, the addition of H/FMW was more effective than that of HMW. Upon exposure to air, the silages with additive were better preserved in terms of pH, temperature, content of soluble carbohydrate, and in vitro dry matter degradability. It is concluded that both HMW and H/FMW improve the quality of silage made fromTG because of their contribution of fermentable carbohydrate.