ABSTRACT. Chilean milk processing industry is providing clear signs to milk farmers regarding the significant importance of milk solids within raw milk payment schemes. A litre of milk containing 3% of fat and 3% of protein is defined as a basic litre and, each additional kilo of fat and/or protein receives an extra payment. These are new production traits which must be researched on. In this study, 64,029 lactations, on 24 Región de Los Ríos at Southern of Chile herds, were used. Genetic parameters for milk yield and fat and protein above 3% yield were estimated. A multiple trait linear model, solved by BLUP methodology, was used. Variance components were estimated using AIREMLF90 and VCE software. Estimated heritability for milk yield, and fat and protein above 3% yield were 0.16±0.004, 0.44±0.007 and 0.42±0.006, respectively. Estimated genetic correlations were -0.285 and -0.331 between milk yield and fat and protein above 3% yield, respectively. It is concluded that there exist genetic variation for the two new traits proposed by the Chilean milk processing industry and, genetic selection for these traits should be done based on their estimated breeding values. However these two traits, plus milk yield, should be included in a selection index to account for the negative genetic correlations among them and minimise selection against milk yield.Key words: genetic improvement, milk solids. RESUMEN.En Chile la industria está dando señales claras al productor lechero al otorgar una importante ponderación al contenido de sólidos lácteos en la determinación del precio de la leche cruda. Para este efecto se definió como "litro base" a un litro de leche con contenidos de 3,0% de grasa y 3,0% de proteína. Sobre dicha base, cada kilogramo de ambos sólidos recibe una valoración adicional, constituyéndose así nuevas características composicionales que deben ser estudiadas. En este estudio se estimaron parámetros genéticos para producción de leche y producción de grasa y proteína sobre 3% de concentración en leche, a partir de 64.029 lactancias provenientes de 24 rebaños de la Región de Los Ríos en el sur de Chile. Se utilizó un modelo mixto multivariado resuelto con metodología BLUP. Los componentes de varianza fueron estimados usando los programas computacionales AIREMLF90 y VCE. Las heredabilidades estimadas para volumen de leche y producción de grasa y proteína sobre el 3,0% fueron 0,16±0,004, 0,44±0,007 y 0,42±0,006, respectivamente. Las correlaciones genéticas entre producción de leche y producción de grasa y proteína sobre 3% fueron -0,285, -0,331, respectivamente. Se concluye que existe variabilidad genética para las dos nuevas características creadas por la industria compradora de leche y que la selección genética debería hacerse basada en valores genéticos estimados para estas.Palabras clave: mejoramiento genético, sólidos, leche.

Milk is a fundamental feed of all newborn mammals and a good source of proteins, fatty acids, micro elements, calcium, phosphorus, among others, for non-lactating humans. As world demand for foods increases, so does the concern and requirements of consumers for food quality and safety. In view of evidences of possible pathologies associated to milk consumption, detractors of the dairy industry have appeared, making use of these arguments. Research on the risks associated to milk ingestion has been conducted on the last twenty years and the existence of specific milk protein variants able to cause disorders such as milk intolerance, diabetes mellitus type 1, ischemic heart disease and food allergies, has been established. The determination of casein genetic variants and the association of the A1 beta-casein with human health disorders represent an opportunity, for the farmers to select replacements, retaining those cows with the desirable alleles, and also for the dairy industry to obtain safer dairy products. As a result, all the dairy supply chain, from producers to consumers, will be benefited.