Objective: We hypothesized that silage additives may alter the undigested neutral detergent fiber -uNDF- content through ensiling. Therefore, urea and formic acid were applied to corn, whole barley crop -WBC- and alfalfa to change uNDF content of the ensiled forages. Methods: Six experimental diets at two groups of high uNDF -untreated corn and alfalfa silages [CSAS] and untreated whole barley and alfalfa silages [BSAS]- and low uNDF -urea-treated corn silage+untreated alfalfa silage [CSUAS], urea-treated whole barley silage+untreated alfalfa silage [BSUAS], untreated corn silage+formic acid-treated alfalfa silage [CSASF], and untreated whole barley silage+formic acid-treated alfalfa silage [BSASF]-, were allocated to thirty-six multiparous lactating Holstein dairy cows. Results: The untreated silages were higher in uNDF than additive treated silages, but the uNDF concentrations among silages were variable -corn silage0.05-. Milk yield tended to increase in the cows fed high uNDF diets than those fed low uNDF -p = 0.10-. The cows fed diet based on urea-treated corn silage had higher milk yield than those fed other silages -p = 0.05-. The substitution of corn silage with the WBC silage tended to decrease milk production -p = 0.07-. Changing the physical source of NDF supply and the uNDF content from the corn silage to the WBC silage caused a significant increase in ruminal NH3-N concentration, milk urea-N and fat yield -p< 0.05-. The cows fed diets based on WBC silage experienced greater rumination time than the cows fed corn silage -p<0.05-. Conclusion: Administering additives to silages to reduce uNDF may improve the performance of Holstein dairy cows. Silages have become the necessary forage component in the ration of dairy cows over the last few decades [1]. They are often a wise choice when selecting forages for providing fiber and energy needed to optimize rumen function [2]. A critical component for assessing of silage quality is fiber digestibility that impact on feed intake and milk yield [2]. Silage digestibility, which is a complex function, is related to the dynamic processes of degradation and passage rate from the rumen [3]. A 0.01 unit increase in neutral detergent fiber -NDF- degradability cause an increase in daily intake of 0.17 kg and fat-corrected milk -FCM- with 0.25 kg/d [4]. Raffrenato and Van Amburgh [5] noted that fiber digestibility relates to two digestible pools, followed the first order kinetic, but each has a different rate of digestion; fast digestion

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