The aim of this study was to identify the foremost model that depict the lactation curve of dual-purpose crossbred cows under a humid tropical environment. Six models were compared to describe the lactation curves of three breed groups using 1231 milk records of 160 cows. The breed groups were 0-25%, F1 (50%) and 62.5-75% Holstein x Gyr crosses. Parameters of the models and curves were estimated using non-linear procedures. The best model was determined by the Akaike information criterion. Wood, Sikka and Wilmink models provided a good fit to the lactation curves for the breed groups; however, the Wood model was applied to depict the lactation curves, the day at which lactation peak was reached and milk yield at the peak. The aspect of the curves differed according to the breed group. The low graded Holstein cows had typical standard curves, and the F1 and high graded cows had no lactation curve peak. Milk yield at start of lactation increased as the Holstein blood increased. The rate of milk per day was close to zero (0.0813 for 0-25% Holstein cows; 0.0247 for F1 and 0.0023 kg/day for 62.5-75% Holstein cows). The aspect of the lactation curve varied depending on the non-linear model and the grade of Holstein. The shapes of the lactation curves vary according to the breed group from normal to without peak milk yield.

Objective. To determine the effect of breed group (BG), parity number PN) and season of calving on lactation length (LL), milk production per lactation (MPL) and per day (MPD) of dual-purpose (DP) cows. Material and methods. Data from 160 Holstein x Gir cows (0 to 75% Holstein) managed in a dual purpose production system in the humid tropics of Chiapas, Mexico were analyzed. Information for LL was analyzed using survival analysis, and that for MPL and MPD by general linear models. The model used included the effects of BG (0-25%, 50% and 62.5-75% Holstein), PN and season of calving. Results. The means for LL, MPL, MPD were 219.3 ± 39.6 days; 2125.1 ± 568.7 kg; 9.66 ± 1.96 kg, respectively. BG and PN affected all variables. The hazard ratio (HR) of 1.815 indicates that 0-25% Holstein cows had higher risk of being dry-off earlier than F1 cows, which had similar LL than 62.5 to 75% Holstein group. The HR was higher for cows calving in the windy season (HR=1) than those calving in the dry and rainy seasons (HR=0.448 and 0.446, respectively). The risk that a primiparous cow was dry-off was higher (HR=2.198). The LL of cows with 2 and ≥3 parities was similar. Conclusions. BG, PN and season of calving affected LL and milk yield of dual-purpose cows.

<pre><strong>Background. </strong>The resumption of ovarian activity after parturition is related to reproductive efficiency. <strong>Objective.</strong> To determine the effect of breed group and some non-genetic factor on resumption of ovarian activity and pregnancy rate of dual-purpose cows. <strong>Methodology. </strong>In study 1, 37 cows were sampled biweekly, from the day 15 to 150 postpartum or until a corpus luteum (CL) was detected. Follicular population (FP), maximum follicle diameter (MFD) and maximum CL diameter (MCLD) were also measured. In study 2, 71 cows were pregnancy diagnosed. Chi</pre>²<pre> tests and analyses of variance were performed to determine the effects of breed group, parity number, body condition score (BCS), BCS change (BCSC) and body weight change (BWC) on the dependent variables. <strong>Results. </strong>In study 1, aproximately, 84% of cows resumed ovarian activity at 68.3 days. More CL were found in multiparous cows (92.59%) and those with BCS ≥3.0 (95.83%). No differences were found in the FP and MFD. F1 cows had the largest MCLD (19.51 mm). In study 2, pregnancy rate was higher in cows with 2-3 (47.83%) and ≥4 parities (54.55%), F1 (54.29%) cows, those gaining BCS (64.29%) and high body weight (66.67%).<strong> Implications. </strong>Results suggest that primiparous cows and those with BCS ≤2.5 require better management; therefore, strategies must be established to improve reproductive efficiency. <strong>Conclusion.</strong> Primiparous and cows with BCS ≤2.5 delayed the first ovulation. Multiparous, F1 cows, those gaining BCS and body weight after calving had the highest pregnancy rate.</pre><pre> </pre>