RESUMEN
Genetic parameters were estimated for first lactation survival defined as a binary trait (alive or dead to second calving) and the curve shape traits of milk yield, fat and protein percentages using information from 25â¯981 primiparous Tunisian Holsteins. For each trait, shape curves (i.e. peak lactation, persistency), level of production adjusted to 305 days in milk (DIMs) for total milk yield (TMY), and average fat (TFâ¯%) and protein (TPâ¯%) percentages were defined. Variance components were estimated with a linear random regression model under three bivariate animal models. Production traits were modelled by fixed herdâ¯ × â¯test-day (TD) interaction effects, fixed classes of 25 DIMsâ¯ × â¯age of calvingâ¯ × â¯season of calving interaction effects, fixed classes of pregnancy, random environment effects and random additive genetic effects. Survival was modelled by fixed herdâ¯ × â¯year of calving interaction effects and age of calvingâ¯ × â¯season of calving interaction effects, random permanent environment effects, and random additive genetic effects. Heritability ( h 2 ) estimates were 0.03 ( ± 0.01 ) for survival and 0.23 ( ± 0.01 ), 0.31 ( ± 0.01 ) and 0.31 ( ± 0.01 ) for TMY, TFâ¯% and TPâ¯%, respectively. Genetic correlations between survival and TMY, TFâ¯% and TPâ¯% were 0.26 ( ± 0.08 ), - 0.24 ( ± 0.06 ) and - 0.13 ( ± 0.06 ), respectively. Genetic correlations between survival and persistency for fat and protein percentages were - 0.35 ( ± 0.09 ) and - 0.19 ( ± 0.09 ), respectively. Cows that had higher persistencies for fat and protein percentages were more likely not to survive.
RESUMEN
OBJECTIVE: The objective of this study was to estimate genetic parameters of milk, fat, and protein yields within and across lactations in Tunisian Holsteins using a random regression test-day (TD) model. METHODS: A random regression multiple trait multiple lactation TD model was used to estimate genetic parameters in the Tunisian dairy cattle population. Data were TD yields of milk, fat, and protein from the first three lactations. Random regressions were modeled with third-order Legendre polynomials for the additive genetic, and permanent environment effects. Heritabilities, and genetic correlations were estimated by Bayesian techniques using the Gibbs sampler. RESULTS: All variance components tended to be high in the beginning and the end of lactations. Additive genetic variances for milk, fat, and protein yields were the lowest and were the least variable compared to permanent variances. Heritability values tended to increase with parity. Estimates of heritabilities for 305-d yield-traits were low to moderate, 0.14 to 0.2, 0.12 to 0.17, and 0.13 to 0.18 for milk, fat, and protein yields, respectively. Within-parity, genetic correlations among traits were up to 0.74. Genetic correlations among lactations for the yield traits were relatively high and ranged from 0.78±0.01 to 0.82±0.03, between the first and second parities, from 0.73±0.03 to 0.8±0.04 between the first and third parities, and from 0.82±0.02 to 0.84±0.04 between the second and third parities. CONCLUSION: These results are comparable to previously reported estimates on the same population, indicating that the adoption of a random regression TD model as the official genetic evaluation for production traits in Tunisia, as developed by most Interbull countries, is possible in the Tunisian Holsteins.
