ABSTRACT. Data on productive and reproductive performance of cows in dual-purpose herds were analyzed to determine the effect of some environmental and genetic factors on saleable milk yield (SMY), lactation length (LL), daily saleable milk per lactation (DMYL), calving interval (CI), and daily saleable milk per calving interval (MYCI) in dual-purpose herds in Yucatan, Mexico. Repeatabilities (r_e) for these traits were also estimated. Data were obtained from monthly visits to 162 herds from 1996 to 2000. The fixed factors studied were: region (central, eastern and southern), parity number (1 to 6), calving year (1996 to 2000) and calving season (dry, rainy and windy and rainy), genetic group (low- (<0.50%), medium- (50%) and high- (>50%) grade cows with European genes). About 2700 to 7700 cows were evaluated for each trait. All factors had significant effects (P < 0.05) on the traits except for region on CI and calving season on DMYL. The overall means for SMY, LL, DMYL, CI, and MYCI were: 1322.3 ± 80.5 kg, 224.8 ± 1.3 days, 5.8 ± 0.1 kg, 555.1 ± 16.5 days, and 3.0 ± 0.1 kg, respectively. The r_e values for SMY, LL, DMYL, CI, and MYCI traits were: 0.19 ± 0.03, 0.08 ± 0.04, 0.16 ± 0.04, 0.00 ± 0.08, and 0.08 ± 0.07, respectively. First parity cows had lower SMY, shorter LL, longer CI, and lower MYCI means than cows with more than one parity. Medium grade cows produced more SMY, DMYL and MYCI and had shorter CI than low- and high-grade cows. Therefore, under Yucatan conditions medium-grade cows should be exploited, and more attention should be given to first parity cows in order to improve the productivity in the herd. The relatively high r_e estimates for SMY and DMYL can be used to calculate most probable producing abilities, in order to identify which cows should be culled.

Key words: Dual-purpose cattle, Reproductive performance, Tropics, Genetic parameters