Optimal age at first calving for improved milk yield and lengh of productive life in Tunisian Holstein cows

in American-Eurasian Journal of Agronomy (2009), 2(3), 162-167

The effects of age at first calving on milk production and true herd life were studied in Tunisian Holstein cows. There were 33,407 first lactation records of cows born between 1987 and 2001 from 166 ... [more ▼]

The effects of age at first calving on milk production and true herd life were studied in Tunisian Holstein cows. There were 33,407 first lactation records of cows born between 1987 and 2001 from 166 herds. Firstly, age at first calving was analyzed using an animal model that included herd, calving year, herd-calving year interaction, calving month, and age of dam as fixed effects and the random additive genetic effect. Secondly, differences in first lactation and productive life milk yields and in true herd life were explained by age at first calving in addition to herd, year at first calving, herd-year at first calving interaction and month at first calving. A cow produced on the average 5669.8 kg milk (SD=1812 kg) during a 305-d first lactation period. The lifetime production of a cow was was 19,496.3 kg (SD=12,192 kg) during 3.3 lactations (SD=1.8 lactations). Coefficients of determination ranged from 14% for true herd life to 64% for first lactation milk yield. The mean of true herd life was 38.6 months (SD=24 months) and the mean age at first calving was 28.7 months (SD=3.4 months). Posterior mean of heritability of the age at first calving derived by a Markov Chain Monte Carlo Bayesien method via a Gibbs sampling algorithm was 0.08. The reduction of age at first calving to around 24 months may result in improved 305-d and lifetime yields and a longer herd life in Tunisian Holsteins. [less ▲]

Genetic parameters for Tunisian Holsteins using a test-day random regression model.

in Journal of Dairy Science (2008), 91(5), 2118-26

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 ... [more ▼]

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. [less ▲]