Genetic Parameters for First Lactation Milk Yields at Three Levels of Herd Production

Abstract

A multivariate linear model was used to estimate sire variance and covariance components and residual variance components for first lactation milk yield and logarithms of yield at three herd production levels using Restricted Maximum Likelihood with the ExpectationMaximization algorithm. Data for four separate analyses were 305-d, mature equivalent first lactation milk records from cows sired artificially in the northeastern United States that freshened in 1970, 1971, 1976, and 1984. Respective numbers of records for each year were 42,618, 40,207, 33,581, and 34,196. Corresponding numbers of sires were 298, 289, 305, and 313. Herd production level was defined by mean yield of all cows freshening in same herd-year-season.For untransformed records sire and residual components of variance increased as mean increased, both within and between years. Correlations between sire effects at different production levels were all above .85. Heritabilities increased as production level increased. These results indicate that it may be necessary to account for heterogeneous genetic and environmental variance in sire evaluations.For logarithms of yield, sire components of variance were similar for each of the three production levels within a year. Residual components for logarithms decreased as production level increased. Change in variance from one production level to another was considerably more for logarithms than for untransformed yields.

Interação reprodutor x rebanho na produção de leite da raça Holandesa no Brasil

“…Praticamente não houve alterações do componente de variância residual quando no modelo foi ajustado o termo de interação. Aumento nas estimativas dos componentes de variância genética e residual, com aumento do nível de produção dos rebanhos, foi também observado por DE VEER e VAN VLECK (1987), BOLDMAN e FREEMAN (1990), DONG e MAO (1990), SHORT et al (1990), COSTA (1998) e TORRES (1998).…”

“…Esses valores representam o ganho genético esperado nos rebanhos de médio e baixo desvios-padrão, por unidade de ganho genético obtido nos rebanhos de alto desvio-padrão, e maiores ganhos seriam esperados Tabela 1 -Médias e desvios-padrão (D.P.) da produção de leite ajustada para 305 dias de lactação e idade adulta, número de lactações, de classes de rebanho-ano-estação (RAE), de rebanhos (REB) e de lactações por ordem de parto (OP), conforme classes de desvio-padrão VAN VLECK (1987) encontraram correlações genéticas que variaram de 0,86 a 0,97 entre as classes de rebanhos, estratificadas de acordo com a média da produção de leite, para as classes de rebanho-ano-estação, em baixo, médio e alto níveis de produção. BOLDMAN e FREEMAN (1990) detectaram correlações genéticas, para produção de leite, que variaram de 0,90 a 1,02 entre os níveis de produção baixo, médio e alto.…”

The effect of heterogeneity for yearling weights measured in different test phases of the South African National Beef Cattle Improvement Scheme

“…Until recently, in most genetic evaluations using mixed model methodology, variances were assumed to be constant across environmental groups. However, several studies have shown that these assumptions do not hold in many cases (Hill et al, 1983;Hill, 1984;DeVeer & Van Vleck, 1987;Boldman & Freeman, 1990). Variances for production and conformation traits of dairy cattle have been found to be heterogeneous across herds, herdclasses, environments or other types of fixed effect levels (Dodenhoff & Swalve, 1998).…”

Inference about multiplicative heteroskedastic components of variance in a mixed linear Gaussian model with an application to beef cattle breeding

“…First, a transformation of data can be performed in order to match the usual assumption of homogeneity of variance. A log transformation was proposed by several authors in quantitative genetics (see eg Everett and Keown, 1984;De Veer and Van Vleck, 1987;Short et al, 1990, for milk production traits in cattle). However, while genetic variances tend to stabilize, residual variances of log-transformed records are larger in herds with the lowest production level (De Veer and Van Vleck, 1987;Boldman and Freeman, 1990;Visscher et al, 1991).…”

“…A log transformation was proposed by several authors in quantitative genetics (see eg Everett and Keown, 1984;De Veer and Van Vleck, 1987;Short et al, 1990, for milk production traits in cattle). However, while genetic variances tend to stabilize, residual variances of log-transformed records are larger in herds with the lowest production level (De Veer and Van Vleck, 1987;Boldman and Freeman, 1990;Visscher et al, 1991). ' The second alternative is to develop robust methods which are insensitive to moderate heteroskedasticity (Brown, 1982 (1990,1992) where X i and Z; are (n i x p) and (n i x q i ) incidence matrices, corresponding to fixed J3 (p x 1 ) and random u i (q i x 1 ) effects respectively.…”