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Characterization of the variable cow's age at last calving as a measurement of longevity by using the Kaplan–Meier estimator and the Cox model

Published online by Cambridge University Press:  04 October 2012

S. L. Caetano ,
G. J. M. Rosa ,
R. P. Savegnago ,
S. B. Ramos ,
L. A. F. Bezerra ,
R. B. Lôbo ,
C. C. P. de Paz  and
D. P. Munari
S. L. Caetano
Affiliation:
Departamento de Ciências Exatas, Faculdade de Ciências Agrárias e Veterinárias (FCAV), Universidade Estadual Paulista (UNESP), CEP 14884-900, Jaboticabal, SP, Brazil
G. J. M. Rosa
Affiliation:
Department of Animal Sciences, University of Wisconsin, Madison, WI 53706, USA
R. P. Savegnago
Affiliation:
Departamento de Ciências Exatas, Faculdade de Ciências Agrárias e Veterinárias (FCAV), Universidade Estadual Paulista (UNESP), CEP 14884-900, Jaboticabal, SP, Brazil
S. B. Ramos
Affiliation:
Departamento de Ciências Exatas, Faculdade de Ciências Agrárias e Veterinárias (FCAV), Universidade Estadual Paulista (UNESP), CEP 14884-900, Jaboticabal, SP, Brazil
L. A. F. Bezerra
Affiliation:
Departamento de Genética, Faculdade de Medicina de Ribeirão Preto, Universidade de São Paulo, CEP 14049-900, Ribeirão Preto, SP, Brazil
R. B. Lôbo
Affiliation:
ANCP – Associação Nacional de Criadores e Pesquisadores, CEP 14020-230, Ribeirão Preto, SP, Brazil
C. C. P. de Paz
Affiliation:
Departamento de Genética, Faculdade de Medicina de Ribeirão Preto, Universidade de São Paulo, CEP 14049-900, Ribeirão Preto, SP, Brazil APTA – Agência Paulista de Tecnologia dos Agronegócios, CEP 14001-970, Ribeirão Preto, SP, Brazil
D. P. Munari*
Affiliation:
Departamento de Ciências Exatas, Faculdade de Ciências Agrárias e Veterinárias (FCAV), Universidade Estadual Paulista (UNESP), CEP 14884-900, Jaboticabal, SP, Brazil
*
† E-mail: danisio@unesp.fcav.br
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Abstract

In most studies on beef cattle longevity, only the cows reaching a given number of calvings by a specific age are considered in the analyses. With the aim of evaluating all cows with productive life in herds, taking into consideration the different forms of management on each farm, it was proposed to measure cow longevity from age at last calving (ALC), that is, the most recent calving registered in the files. The objective was to characterize this trait in order to study the longevity of Nellore cattle, using the Kaplan–Meier estimators and the Cox model. The covariables and class effects considered in the models were age at first calving (AFC), year and season of birth of the cow and farm. The variable studied (ALC) was classified as presenting complete information (uncensored = 1) or incomplete information (censored = 0), using the criterion of the difference between the date of each cow's last calving and the date of the latest calving at each farm. If this difference was >36 months, the cow was considered to have failed. If not, this cow was censored, thus indicating that future calving remained possible for this cow. The records of 11 791 animals from 22 farms within the Nellore Breed Genetic Improvement Program (‘Nellore Brazil’) were used. In the estimation process using the Kaplan–Meier model, the variable of AFC was classified into three age groups. In individual analyses, the log-rank test and the Wilcoxon test in the Kaplan–Meier model showed that all covariables and class effects had significant effects (P < 0.05) on ALC. In the analysis considering all covariables and class effects, using the Wald test in the Cox model, only the season of birth of the cow was not significant for ALC (P > 0.05). This analysis indicated that each month added to AFC diminished the risk of the cow's failure in the herd by 2%. Nonetheless, this does not imply that animals with younger AFC had less profitability. Cows with greater numbers of calvings were more precocious than those with fewer calvings.

Keywords

beef cattlecow's age at calvingCox modelKaplan–Meier estimator
Type
Breeding and genetics
Information
animal , Volume 7 , Issue 4 , April 2013 , pp. 540 - 546
Copyright
Copyright © The Animal Consortium 2012

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