Many of the tropical lowland areas of warm climate in México have a livestock tradition, small dairies and cheese industries, but any breed of European origin has been able to produce and reproduce successfully in these environments. The tropical milking criollo (TMC) cattle is a Bos taurus adapted that can produce high quality milk feeding exclusively on pasture. In this study, the lactation of TMC was studied and a genetic evaluation was carried out using a random regression model for 119 sires and 602 cows, with 15 377 test-day yield records from herds of México, (10 475) and Nicaragua (4902), from 1974 to 2006. Heritability and repeatability indexes for milk yield accumulated for 305 d were 0.24 (±0.036) and 0.43 (±0.068). In the June-August season daily milk yield was higher by 0.60 kg over the December-February season, which had the lowest production of the four quarterly periods. The average daily yield at parities 2, 3, and 4 or more, was higher at first parity by 0.39, 0.61 and 0.70 kg. Lactation did not show the characteristic initial rise in production in temperate climates with European breeds. The annual genetic progress detected in bulls (7.7 kg per lactation) and cows (11.04 kg per lactation) shows an advance of the breeding program used with the TMC cattle of México and Nicaragua since 1974.

Muchas de las áreas tropicales bajas de clima cálido en México tienen vocación y tradición ganadera, pequeñas industrias lecheras y queseras, pero ninguna raza de origen europeo ha podido producir y reproducirse satisfactoriamente en estos ambientes. El ganado Criollo Lechero Tropical (CLT) es un Bos taurus adaptado que puede producir leche de alta calidad alimentándose exclusivamente en pastoreo. En el presente estudio se evaluó la lactancia del CLT y se realizó una evaluación genética utilizando un modelo de regresión aleatoria para 119 sementales y 602 vacas, con 15 377 registros de producción en días de prueba, provenientes de hatos de México (10 475) y Nicaragua (4902), de 1974 a 2006. Los índices de herencia y de constancia para producción de leche acumulada a 305 d fueron 0.24 (±0.036) y 0.43 (±0.068). En la estación de junio a agosto la producción diaria de leche fue mayor en 0.60 kg respecto a la de diciembre-febrero, la cual registró la menor producción de las cuatro épocas trimestrales. La producción media diaria en los partos 2, 3 y 4 o más, fue superior al primer parto en 0.39, 0.61 y 0.70 kg. La lactancia no mostró el ascenso inicial característico de la producción en climas templados con razas europeas. El progreso genético anual detectado en toros (7.7 kg por lactancia) y vacas (11.04 kg por lactancia) muestra el avance del programa de mejoramiento usado en el ganado CLT de México y Nicaragua desde 1974.

Finite mixture models are helpful for uncovering heterogeneity due to hidden structure; for example, unknown major genes. The first part of this article gives examples and reviews quantitative genetics issues of continuous characters having a finite mixture of Gaussian components. The partition of variance in a mixture, the covariance between relatives under the supposition of an additive genetic model and the offspring-parent regression are derived. Formulae for assessing the effect of mass selection operating on a mixture are given. Expressions for the genetic correlation between a mixture and a Gaussian trait are presented. If there is heterogeneity in a population at the genetic or environmental levels, then genetic parameters based on theory treating distributions as homogeneous can lead to misleading interpretations. Subsequently, methods for parameter estimation (e.g., maximum likelihood) are reviewed, and the Bayesian approach is illustrated via an application to somatic cell scores in dairy cattle.