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Bivariate Analysis of Liability to Clinical Mastitis and to Culling in First-Lactation Cows

B. Heringstad^*, R. Rekaya, D. Gianola^*,, G. Klemetsdal^* and K. A. Weigel

^* Department of Animal Science, Agricultural University of Norway, P.O. Box 5025, N-1432 Aring;s, Norway
Department of Animal and Dairy Science, University of Georgia, Athens, GA 30602, USA
Department of Dairy Science, University of Wisconsin, Madison 53706, USA

Corresponding author:
B. Heringstad; e-mail:
bjorg.heringstad{at}ihf.nlh.no.

Records taken on 13,070 first-lactation daughters of 250 Norwegian Cattle sires were used to examine associations between susceptibility to clinical mastitis and to culling. Clinical mastitis was defined as a binary trait, whereas culling was treated as either binary (culled or not culled) or continuous (length of opportunity period) for two sampling periods (120 or 300 d of lactation). Two Bayesian models were employed; 1) a bivariate threshold model with both mastitis and culling as binary traits, and 2) a bivariate model with mastitis as a threshold binary variable and time to culling as Gaussian. The heritability of liability to clinical mastitis was not affected by either the length of sampling period (120 vs. 300 d) or by whether culling (binary) or length of opportunity period was the second trait in the bivariate analysis. The posterior mean (standard deviation) of heritability of liability to clinical mastitis was 0.06 to 0.07 (0.02) in all analyses. The heritability estimate of length of opportunity period was less than 0.001. Culling (threshold trait) in first lactation had a low heritability, but a high genetic correlation with clinical mastitis. The posterior means (standard deviation) for heritability of liability to culling were 0.01 (0.006) for 120 d and 0.02 (0.009) for 300 d, and the posterior means (standard deviation) of the genetic correlation between liability to clinical mastitis and to culling were 0.48 (0.24) and 0.53 (0.21) for 120 and 300 d, respectively.

Key Words: Bayesian methods • clinical mastitis • (co)-variance components • threshold model

Abbreviation key: CM = clinical mastitis, CU = culling defined as a binary trait, LOP = length of opportunity period, NRF = Norwegian cattle

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