The in sacco rumen degradability and intestinal true digestibility values are used to characterize the nutritional values of feedstuffs in the INRA feeding system for Ruminants (2018) and its rationing software (INRAtionV5). Nevertheless, as INRAE has decided not to retain fistulated ruminants from 2025, alternative in vitro methods are being developed. That raised the need to perform a sensitivity analysis (SA) for evaluating the effect of variations in nutritional input variables on the output variables of INRAtionV5. Five key variables were selected for the SA: organic matter digestibility (OMd), gross energy (GE) and crude protein (CP) contents, and variables obtained through in sacco experiments, i.e., effective ruminal degradability (ED6_N) and true intestinal digestibility (dr_N) of nitrogen. A one-at-a-time SA was performed on outputs of the INRAtionV5 module for dairy cows with 6 diets varying by the type of forages (corn silage, grass fresh/hay/silage) and concentrates (cereals and oil seed meals). These 6 pivot diets were formulated using INRAtionV5 to meet 95% of the potential milk production for a multiparous cow (wk 14 of lactation). Then, for each of the 6 diets considered as total mixed rations, the values of the 5 key input variables of each feedstuff were randomly sampled under the normal distribution (n = 50 test diets/ input variable). The Normalized sensitivity coefficient (NSC, %), ratio of each input to output variables, was used as the SA index. Predicted dry matter intake was mainly affected by GE (NSC = −44 ± 34.0%). In addition, predicted milk protein yield (MPY), milk yield (MY), and milk protein contents were mostly affected by OMd (NSC = 98 ± 39.0%, 71 ± 29.0%, and 27 ± 10.0%, respectively). However, MPY and MY were mainly affected by ED6_N, especially in grass silage-based diet (NSC = −56% and −46%, respectively). The fecal N was most affected by ED6_N (NSC = −53 ± 14.7%) and urinary N was most affected by CP (NSC = 283 ± 34.5%). Results of SA showed that amplitude of responses depends on the diet, and that productive response with grass silage diet is particularly sensitive to in sacco ED6_N measurements.