Dry matter (DM) content of corn and alfalfa silages was measured by a handheld near infrared light (NIR) device and forced air-drying oven (FAO) on 13 d of a 17-d sample period on four farms. The effect of unaccounted DM content fluctuations on nutritional value of the mixed diet, its cost and associated nitrogen excretion were estimated using NRC (2001) calculations. Suitability of a handheld NIR device was assessed by comparison of DM measurements with FAO. The handheld NIR was found to have good accuracy (r = 0.848) and precision (Cb = 0.972) when measuring DM content of alfalfa silage, and had an even greater degree of accuracy (r = 0.917) and precision (Cb = 0.994) when measuring DM content of corn silage. Unaccounted DM content changes were identified in both feed ingredients but did not have a large impact on CP, NDF, ADF, N excretions or cost factors in any of the four cases.