Data from 289 milk yield records of purebred Holstein, ¾ Holstein ¼ Gyr and ½ Holstein ½ Gyr were used to compare milk production traits of these genotypes in an intensive operation in a subtropical region of northwestern Mexico. The performance traits studied were: 305-d milk yield days (P305), total milk yield per lactation (TMY), average daily milk production (DMP); lactation length (LL), peak milk yield (PM) and day of peak milk. Higher (P<0.05) P305 averages were observed for purebred

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... eins (5417 ± 96 kg) than ¾H ¼G (4807 ± 131kg) and ½H ½G (4541 ± 92 kg). The shortest lactations were observed for ½H ¼G animals (288 ± 2.0 d) whereas the longest lactations were observed for purebred Holstein (303 ± 2.1d). The highest 305-d milk yield was observed in cows calving in January (5283 ± 1090 kg), whereas the lowest milk yield occurred with calvings in June (3989 ± 896 kg). These results indicate that, under intensive management in a subtropical setting, purebred Holstein performed better than crossbred animals, in terms of milk yield. However, in this subtropical environment reproductive performance of purebred Holstein cows is very poor; thus, this study emphasizes the importance of crossbreeding Holstein x Gyr cattle to produce cows that are more adapted to the hot-humid environmental conditions of the region. These results also showed the drastic impact of heat stress during the summer months on the milk yield of these cows.