Comparison of genetic evaluation models for lactation persistency

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  • 乳用牛の泌乳持続性に対する遺伝評価モデルの比較
  • ニュウヨウギュウ ノ ヒツニュウ ジゾクセイ ニ タイスル イデン ヒョウカ モデル ノ ヒカク

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Abstract

The objective of this study was to compare the genetic evaluation models for lactation persistency. Persistency was defined as the difference in production between 60-day and 240-day on a phenotypic or genetic lactation curve. Data were test-day milk records from first lactation cows calving from 1974 to 2007 in Hokkaido, Japan. A random regression model (NoHYC) was applied to test-day milk yields for the estimation of genetic parameters and breeding values. An alternative model (HYC) was identical to NoHYC except that HYC contained fixed linear regressions as herd-specific lactation curve. For both models, estimated breeding values (EBV) for persistency were calculated from individual genetic curves. A bivariate animal model (LM) with observed persistency and 305-day milk yield was also considered. EBV from cows with 8 or more test-day records and sires with 20 or more such daughters were selected for the comparisons. Higher heritability for persistency (0.28) was estimated for NoHYC, compared to those for HYC (0.10) and LM (0.13). Pearson's correlations among EBV from different models for sires and cows ranged from 0.768 to 0.906 and from 0.668 to 0.847, respectively. When herd curves were ignored, the heritability was overestimated and bias was observed in EBV due to the confounding of herd effects with breeding values. The random regression model with herd-specific lactation curves (HYC) was found to be the most suitable for genetic evaluation of persistency because of less biased EBV, the precise adjustment for test-day effects, and the feasibility of simultaneous evaluation with 305-day milk yield.

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