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Estimate of genetic change in milk yield in a Gyr herd in Brazil

Cláudio de Ulhôa Magnabosco^I; Raysildo Barbosa Lôbo^II; Luiz Antonio Framartino Bezerra^II; Mario Luiz Martinez^III

^IEMBRAPA-CENARGEN/Brazilian Zebu Breeders Association - ABCZ. Caixa Postal 02372, 70770 Brasilia, DF, Brasil. Send correspondence to C.U.M.
^IIDepartamento de Genética, Faculdade de Medicina de Ribeirão Preto, Universidade de São Paulo, 14049-900 Ribeirão Preto, SP, Brasil
^IIIEMBRAPA/CNPGL, 36155 Coronel Pacheco, MG, Brasil

ABSTRACT

An evaluation was made of genetic change in 305 clay milk yield, lactation length and daily milk yield in Gyr cattle selected for milk production. The data were obtained from the records of a herd raised in the municipality of Cajuru, State of São Paulo. A total of 3,987 lactations corresponding to the period from 1962 to 1987 were analyzed. Several statistical models were utilized to estimate repeatability, and phenotypic, genotypic and environmental changes. Month and year of calving, bull, cow within-bull, cow's age at calving, and lactation length effects were significant. Repeatability estimates calculated by intraclass correlation for milk yield, lactation length and daily milk yield were 0.46, 0.34 and 0.43, respectively. The method used to estimate genetic change was based on repeated observations of the same animal in different years. The estimative of genetic change for milk yield during the period was 18.5 kg/year. With adjustments for length, the estimated change was 12.9 kg/year. The estimated genetic changes were 0.05 day/year and 0.015 kg/day per year for lactation length and daily milk yield, respectively.

Keywords: genetic change; milk yield; Gyr herd.

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