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Casein --- Casein --- Milk protein --- Milk protein --- Polymorphism --- Polymorphism --- genetic parameters --- genetic parameters --- Productivity --- Productivity --- Dairy cows --- Dairy cows --- Selection. --- Selection

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The test day model suggested nowadays for the genetic evaluation of dairy bulls and cows has several advantages over the traditional 305-day lactation model. The primary challenge in the development of test day model in a large scale is the estimation of genetic parameters of test day yields that are required for the implementation of an evaluation system. For this purpose, estimations of (co) variance components for test day milk, fat and protein yields for Holstein cows are being realized with a multitrait approach associated to covariance fonctions and a random regression approach. Heritability estimates with the first approach were lower at the start of lactation and usually increased with lactation stage. Heritability estimates were higher for milk (0.16 to 0.22) than for fat (0.10 to 0.18) and protein yields (0.15 to 0.19). Heritability estimates tended to be slightly lower across lactation. Genetic and phenotypic correlation were higher between milk and protein than between milk and fat traits. Within a yield trait, genetic correlations declined from superior or equal 0.93 for adjacent lactation stages to 0.52 for milk, 0.58 for fat, and 0.60 for protein between initial and final lactation stages. Within lactation stages, mean genetic correlations were 0.78 between milk and protein yields, 0.40 between milk and fat yields, and 0.56 between fat and protein yields ; corresponding mean phenotypic correlations were 0.92, 0.64 and 0.66. Variance components estimated with random regression model were generally higher than those estimated via multitrait and covariance functions models, especially at the beginning and the end of lactation. Heritabilities were generally around 20 % higher than those obtained with previous approach.

Dairy cows --- Dairy cows --- Milk production --- Milk production --- genetic parameters --- genetic parameters --- evaluation. --- evaluation --- Breeds (animals) --- Breeds (animals) --- USA --- USA

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Dairy cows --- Dairy cows --- Digestive absorption --- Digestive absorption --- Rumen digestion --- Rumen digestion --- Grain legumes --- Grain legumes --- Feed legumes --- Feed legumes --- feeds --- feeds --- processing --- processing

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Domestic animals --- Domestication --- Human-animal relationships --- Horses --- Cows --- Animaux domestiques --- Domestication --- Relations homme-animal --- Chevaux --- Vaches

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Vache laitière --- Dairy cows --- Production laitière --- Milk production --- Économie de production --- Production economics --- Alimentation des animaux --- Animal feeding --- Lactation --- Reproduction --- reproduction --- Logement des animaux --- Animal housing --- Santé animale --- Animal health --- Gestion de l'exploitation agricole --- Farm management --- Zone tropicale --- Tropical zones --- reproduction. --- Filiere