The lactation curve and commertial milk production were studied in 250 lactating multiparous zebu and crossbred cows (zebu x Bos taurus milk type) in 14 small dairy farms Grade C under grayzing, mild heat stress, calf stimulation and one milking by day across raining time in lowland of humid tropic. The maximum heat stress occurred at 01 PM (RH 76.71%, AT_db 32.7°C, DSR 676.3 kcal/m²hr, THI 86.59 and THIDSRWS 92.35). Total commertial milk production and lactation length resulted for zebu 692.07 kg in 177 days, ½ Bos taurus milk type x ½ Zebu 1267.35 in 213 days and for ¾ Bos taurus type milk x ¼ Zebu 2012.16 kg in 262 days (P<.01). The tipic profile of the lactation curve in the zebu group showed initial, peak and final daily milk yield of 3.42, 4.62 at 14 days and 2.23 kg/cow (P<.05); in ½ Bos taurus milk type x ½ Zebu 5.63, 8.63 at 26 days (P<.01) and in 3/4Bos taurus milk type x ¼ Zebu 7.50, 10.95 at 34 days and 4.40 kg/cow (P<.01). The lactation curve resulted in Zebu cows Y_(kg/cow-day)=13.09572121X^-0.0733187e^{-0.005064242X}  (R²=0.38);  in ½ Zebu x ½ Bos taurus Y_(kg/cow-day)=8.5935X^0.13048e^{-0.005115067X} (R²=0.68) and in ¾ Bos taurus lechero x ¼ Zebu Y_(kg/cow-day)=12.7632X^0.24051e^{-0.007117078X}  (R²=0.72). Milk production was potentiated by the proportion of Bos taurus milk type in the crossbred; however, the environment and management altered the lactation curve and reduced phenotype milk ability in all genetic groups in the humid tropical climate.