Maize straw is the main roughage for dairy herds in campesino farms in central Mexico. The objective was to evaluate feeding milking cows on maize straw treated with 40 g/kg DM of urea (A) or untreated straw (B), and 3.0 kg/d of 18% CP concentrate. Twenty-four Holsteins in late lactation from 8 farmers were sorted in two groups: sequence A-B-A or B-A-B; periods were 28 days. Mean daily milk yield for the last two weeks per period, and live-weight and body-condition score every 14 days were used for analysis. Maize straw was ad libitum. Chemical analysis and in vitro digestibility were analysed by Student's t test, animal variables by a switch-back design. 'A' had 44.3 g/kg DM more CP and 106.5 g/kg DM higher in vitro digestibility than 'B' (710 g/kg DM +/- 0.75 'A' vs. 603.5 g/kg DM +/- 1.44 'B'). Despite higher digestibility and intake, there were no differences (P > 0.05) for milk yield, live-weight or body-condition score, although there were in straw intake (P < 0.05). Cows on 'A' ate 1.7 kg/cow/day more straw DM than on 'B'. Lack of response did not offset higher feeding costs although margins were high. Lack of response is attributed to short length of periods and late lactation of cows.