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Genetic Parameters for Tunisian Holsteins Using a Test-Day Random Regression Model

H. Hammami^*,, B. Rekik, H. Soyeurt^*,, A. Ben Gara and N. Gengler^*,#,1

^* Animal Science Unit, Gembloux Agricultural University, B-5030 Gembloux, Belgium
Livestock and Pasture Office, 1002 Tunis Belvedere, Tunisia
Ecole Supérieure d’Agriculture de Mateur, 7030 Mateur, Tunisia
Fonds pour la Formation à la Recherche dans l’Industrie et l’Agriculture, B-1000 Brussels, Belgium
^# National Fund for Scientific Research, B-1000 Brussels, Belgium

¹ Corresponding author: gengler.n{at}fsagx.ac.be

Genetic parameters of milk, fat, and protein yields were estimated in the first 3 lactations for registered Tunisian Holsteins. Data included 140,187; 97,404; and 62,221 test-day production records collected on 22,538; 15,257; and 9,722 first-, second-, and third-parity cows, respectively. Records were of cows calving from 1992 to 2004 in 96 herds. (Co)variance components were estimated by Bayesian methods and a 3-trait-3-lactation random regression model. Gibbs sampling was used to obtain posterior distributions. The model included herd x test date, age x season of calving x stage of lactation [classes of 25 days in milk (DIM)], production sector x stage of lactation (classes of 5 DIM) as fixed effects, and random regression coefficients for additive genetic, permanent environmental, and herd-year of calving effects, which were defined as modified constant, linear, and quadratic Legendre coefficients. Heritability estimates for 305-d milk, fat and protein yields were moderate (0.12 to 0.18) and in the same range of parameters estimated in management systems with low to medium production levels. Heritabilities of test-day milk and protein yields for selected DIM were higher in the middle than at the beginning or the end of lactation. Inversely, heritabilities of fat yield were high at the peripheries of lactation. Genetic correlations among 305-d yield traits ranged from 0.50 to 0.86. The largest genetic correlation was observed between the first and second lactation, potentially due to the limited expression of genetic potential of superior cows in later lactations. Results suggested a lack of adaptation under the local management and climatic conditions. Results should be useful to implement a BLUP evaluation for the Tunisian cow population; however, results also indicated that further research focused on data quality might be needed.

Key Words: genetic parameter • random regression model • test-day yield • dairy cattle

This article has been cited by other articles:

				H. Hammami, B. Rekik, H. Soyeurt, C. Bastin, J. Stoll, and N. Gengler Genotype x Environment Interaction for Milk Yield in Holsteins Using Luxembourg and Tunisian Populations J Dairy Sci, September 1, 2008; 91(9): 3661 - 3671. [Abstract] [Full Text] [PDF]