Short communication Study on functional longevity of Tunisian Holstein dairy cattle using a Weibull proportional hazard model Naceur M'hamdi a, ⁎, Rafik Aloulou a , Mahdi Bouallegue b , Satinder K. Brar c , Mohamed Ben Hamouda d a Département des Sciences Animales, Institut Supérieur Agronomique de Chott-Mariem, BP47, 4042 Sousse, Tunisie b Faculté des Sciences Mathématiques, Physiques et Naturelle de Tunis, Campus Universitaire, 2092 Tunis, Tunisie c Institut National de la Recherche Scientifique, Québec, Canada d Institution de La Recherche et de l'Enseignement Supérieur Agricole (IRESA), Ministère de l'Agriculture et des Ressources Hydrauliques, Tunisie article info abstract Article history: Received 11 October 2009 Received in revised form 6 May 2010 Accepted 19 May 2010 Available online xxxx Records of functional longevity (FL), from first calving to last recording (or culling), of 36,888 Tunisian Holstein cows in 169 herds were studied using survival analysis. Using a Weibull proportional hazard model, the influences of age at first calving, herd–year–season, lactation number–stage of lactation, milk production within the herd and year, annual change in herd size and random genetic effect of sire on FL were studied. Besides a random time-dependent effect of herd–year–season, the model included fixed effects of parity and stage of lactation, within herd and parity production level (all time-dependent), and age at first calving (time-independent). All factors had a significant effect on FL at a level of P b 0.001. Milk yield was by far the most important factor influencing FL and the hazard increased as the milk production decreased. Within the first three lactations, the relative culling risk increased stepwise with lactation stage. The highest risk of culling was found for cows at the beginning and at the end of the first lactation and at the end of any other lactation. The risk of culling decreased with parity. © 2010 Elsevier B.V. All rights reserved. Keywords: Heritability Holstein cow Length of productive life Tunisia Survival analysis Weibull model 1. Introduction Functional traits have received increasing attention in breeding programs for dairy cattle in many countries. Because of well adaptation characteristic of Holstein population to temperate climatic conditions and its high production level, this breed is the mainly demanded one among culture cows in Tunisia (Ben Salem et al., 2006). However, in Tunisia, as in most other countries, dairy cattle breeding programs were mainly focused towards yield traits. In the last decade interest has increased in broadening breeding objective to include func- tional traits such as fertility, mastitis resistance and longevity (Sewalem et al., 2008). Functional longevity is an important component of profits from milk production because high longevity reduces rearing costs and purchasing replacement females (Forabosco et al., 2004), enables the animals to reach their age dependent maximum of milk production (Strapák et al., 2005), and increases the proportion of higher yielding cows in the herd (Vukasinovic et al., 2001). It is assessed as days in milk or days between first calving and culling (Tsuruta et al., 2005; VanRaden et al., 2006), and can be measured in different ways such as Weibull proportional hazard model (Sewalem et al., 2006), Cox regression (Prentice and Kalbfleisch, 1979) and Survival Kit (Ducrocq and Soelkner, 1994, 1998b) and its genetic evaluations are not standardized across countries (Du Toit et al., 2009). In Tunisia, for instance, FL is recorded as a binary trait and evaluated with a multiple-trait linear animal model (Ajili et al., 2007). The interest in implementing longevity in the breeding goal is mainly due to the need of farmers to have cows with higher economic efficiency. Selection for FL is attractive for reducing involuntary culling rate which is becoming a high cost in Tunisian dairy herds. The Livestock Science xxx (2010) xxx–xxx ⁎ Corresponding author. Tel.: +216 73 348 544; fax: +216 73 327 591. E-mail address: naceur_mhamdi@yahoo.fr (N. M'hamdi). LIVSCI-01176; No of Pages 4 1871-1413/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.livsci.2010.05.011 Contents lists available at ScienceDirect Livestock Science journal homepage: www.elsevier.com/locate/livsci Please cite this article as: M'hamdi, N., et al., Study on functional longevity of Tunisian Holstein dairy cattle using a Weibull proportional hazard model, Livestock Science (2010), doi:10.1016/j.livsci.2010.05.011