References of "Hammami, Hedi"
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See detailEstimation of Genetic Parameters for Test Day Milk Yields of Moroccan Holstein Cows Using Random Regression Test Day Model
Tijani, Aziz; Rgayai, Brahim; Hammami, Hedi ULg et al

in Proceedings of the 9th World Congress on Genetics Applied to Livestock Production (2010, August)

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See detailDevelopment of a new crossbred based evaluation for carcass quality of Piétrain boars in the Walloon Region of Belgium
Dufrasne, Marie ULg; Hammami, Hedi ULg; Jaspart, Véronique et al

in Proceedings of the British Society of Animal Science and the Agricultural Research Forum (2010, April)

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See detailEvaluation génétique des verrats Piétrain en croisement en Wallonie
Dufrasne, Marie ULg; Hammami, Hedi ULg; Gengler, Nicolas ULg et al

Diverse speeche and writing (2010)

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See detailDevelopment of a new crossbred based evaluation for carcass quality of Piétrain boars in the Walloon Region of Belgium
Dufrasne, Marie ULg; Hammami, Hedi ULg; Jaspart, Véronique et al

in Book of Abstract of the 61st Annual Meeting of the European Association for Animal Production (2010)

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See detailDevelopment of a new crossbred based evaluation for carcass quality of Piétrain boars in the Walloon Region of Belgium
Dufrasne, Marie ULg; Hammami, Hedi ULg; Jaspart, Véronique et al

Poster (2010)

The aim of this study was to develop a genetic evaluation model to estimate the genetic merit of Piétrain boars for some carcass quality traits. These boars are now evaluated on performances recorded on ... [more ▼]

The aim of this study was to develop a genetic evaluation model to estimate the genetic merit of Piétrain boars for some carcass quality traits. These boars are now evaluated on performances recorded on their crossbred progeny fattened in a central test station. Data provided by the on-farm performance recording system were utilized in this study. Traits analysed were backfat thickness (BF) and lean meat percentage (%meat). The data file contains 60 546 records measured on pigs between 150 and 300 days of age. Model developed was a multitrait animal model. Fixed effects were sex, contemporary groups and heterosis, modeled as regression on heterozygosity. Random effects were additive genetic and permanent environment, modeled by random regressions using linear splines, and residual. Variance components were estimated by restricted maximum likelihood (REML) method on random samples of the total dataset and then confirmed by a Gibbs sampling algorithm on the total dataset. Fit of the model was tested by computing residuals from a BLUP (Best Linear Unbiased Prediction) evaluation. BF and %meat have a high heritability that increase with age. These two traits are also highly genetically correlated. Mean residuals are not significantly different from zero for both traits. Given that BF and %meat had high heritability, genetic improvement of carcass quality is possible by selection on these two traits. Like residuals are close to zero for both traits, it seems that model developed explain a great proportion the variance in each traits. [less ▲]

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See detailDevelopment of a new genetic evaluation model for carcass quality based on crossbred performances of Piétrain boars in the Walloon Region of Belgium
Dufrasne, Marie ULg; Hammami, Hedi ULg; Jaspart, Véronique et al

Conference (2010)

In context of the development of the genetic evaluation system of Piétrain boars for crossbred performances, a genetic evaluation model was developed for the estimation of genetic merit of boars for ... [more ▼]

In context of the development of the genetic evaluation system of Piétrain boars for crossbred performances, a genetic evaluation model was developed for the estimation of genetic merit of boars for backfat thickness (BF) and meat percentage (%meat). The model developed was a random regression model using linear splines on age which knots at 175, 200 and 250 days. Variance components were estimated by restricted maximum likelihood and Gibbs sampling. Estimated heritability from 150 to 300 days increased from 0.56 to 0.75 for BF and from 0.55 to 0.69 for %meat. Genetic correlation between BF and %meat varied between -0.90 and -0.93 from 150 to 300 days. According to the study of residuals, the developed model was considered to fit well the data especially between 175 and 250 days of age. [less ▲]

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See detailDevelopment of a new genetic evaluation model for carcass quality based on crossbred performances of Piétrain boars in the Walloon Region of Belgium
Dufrasne, Marie ULg; Hammami, Hedi ULg; Jaspart, Véronique et al

in Proceedings of the 9th World Congress on Genetics Applied to Livestock Production (2010)

In context of the development of the genetic evaluation system of Piétrain boars for crossbred performances, a genetic evaluation model was developed for the estimation of genetic merit of boars for ... [more ▼]

In context of the development of the genetic evaluation system of Piétrain boars for crossbred performances, a genetic evaluation model was developed for the estimation of genetic merit of boars for backfat thickness (BF) and meat percentage (%meat). The model developed was a random regression model using linear splines on age which knots at 175, 200 and 250 days. Variance components were estimated by restricted maximum likelihood and Gibbs sampling. Estimated heritability from 150 to 300 days increased from 0.56 to 0.75 for BF and from 0.55 to 0.69 for %meat. Genetic correlation between BF and %meat varied between -0.90 and -0.93 from 150 to 300 days. According to the study of residuals, the developed model was considered to fit well the data especially between 175 and 250 days of age. [less ▲]

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See detailSustainable dairy farming - A case study of Holsteins in a developed and an emerging country
Hammami, Hedi ULg; Rekik, Boulbaba; Bormann, Jeanne et al

Conference (2009, September 23)

Sustainability of breeding programs under the flow of exchange among various environments are conditioned by the ability of genotypes to adjust sufficiently their phenotypes in response to changes in ... [more ▼]

Sustainability of breeding programs under the flow of exchange among various environments are conditioned by the ability of genotypes to adjust sufficiently their phenotypes in response to changes in their new bio-physical conditions, and also by the capabilities of farmers to balance between environmental, ethical, social. and economic aspects. The objective of this paper was to quantify the effectiveness of genetic responses from indirect selection in high- and low- to medium- input systems using Luxembourg and Tunisian Hoisteins. The magnitude of genotype-by-environment interaction (GxE) for production traits was firstly investigated based on the country border delimitation as environmental character state. Secondly, three herd management levels reflecting feeding and management intensity were identified in each country and GxE was investigated within and across country environments. Significant GxE was detected for milk yield and persistency with large differences in genetic and permanent environmental variances between the 2 countries. Genetic correlations for 305-d milk yield and persistency between Luxembourg and Tunisia were 0.60 and 0.36. Low rank correlations obtained between estimated breeding values of common sires translated a significant re-ranking between the 2 countries. Within-country environments analysis show that the magnitude of GxE varied from only scaling effects resulting from heterogeneous variances in high-input systems to considerable re-ranking of common sires under limited feeding resources, low management care, and stressful conditions in low-input systems. Overall, this study shows that substantial differences exist between Hoisteins in terms of their sensitivity to production environ ment suspecting the sustainability of the dairying system. In high-input systems, GxE effects are expected to be easily managed but harmful effects of intensive dairy systems should be considered. Selection of breeds under less intensive production systems respecting the ruminant's specificities should be encouraged. In law-input systems, selection for adaptive traits under their specifie conditions and improvement of management conditions and husbandry practices are needed. [less ▲]

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See detailGenotype by Environment Interaction for Production Traits of Holsteins Using Two Countries as Model: Luxembourg and Tunisia
Hammami, Hedi ULg

Doctoral thesis (2009)

Under globalization, breeding organizations are selecting animals and exchanging germplasm across various environments. Ignoring genotype by environment interaction (G x E) may affect the efficiency of ... [more ▼]

Under globalization, breeding organizations are selecting animals and exchanging germplasm across various environments. Ignoring genotype by environment interaction (G x E) may affect the efficiency of breeding strategies and limit outcomes from cooperation between breeding programs. Quantifying the effectiveness of indirect selection and effects of G x E for different breeds is therefore necessary. The objective of this thesis was to evaluate the magnitude of G x E for milk yield using Luxembourg and Tunisian Holstein populations. In fact, these two countries rely considerably on importation of superior genes from diverse origins for their breeding programs. This study needed records on both the genotype and the environment. In the first part of this thesis, genetic ties between the two populations were studied. Additive relationships and genetic similarity were important and genetic links have been strengthened with time which allowed the analysis of the phenotypic expression of daughters of common sires under each of these tow production environments. In the second part, genetic parameters for production traits of Tunisian Holsteins were estimated by a test-day random regression model (RRTD). Heritability estimates for 305-d milk, fat and protein yields were low to moderate (0.12 to 0.18) suspecting difficulties of high-producing cows to express their potential under limiting production conditions. In the third part, G x E for milk yield and persistency were investigated using character state models, where milk yield in each country was considered as a separate trait, and where the country border delimitation was designed as an environmental character state. A RRTD sire model was applied and was extended to a RRTD animal model. Significant G x E was detected for milk yield and persistency by both models. Large differences in genetic and permanent environmental variances between the two countries were observed. Genetic correlations for 305-d milk yield and persistency between Luxembourg and Tunisian Holsteins were 0.50 and 0.43 (sire model) and 0.60 and 0.36 (animal model). Moreover, low rank correlations obtained between estimated breeding values of common sires translate a significant re-ranking between the two environments. At the end of this thesis, a herd management (HM) parameter reflecting feeding and management intensity was defined. Three HM levels were identified in each country and G x E was investigated within- and across-environments. Significant G x E was detected between the Tunisian HM levels, whereas, only heterogeneous genetic variance for milk yield with limited re-ranking of sires across the three Luxembourg environments was observed. Overall, this thesis shows that under constraining environmental effects, selection for adaptive traits among economically valuable traits under their specific conditions is needed for low-input systems. When satisfactory feeding resources, management and husbandry practices are available, high degree environmental sensitivity is desired and the use of a high yielding breed may be encouraged. [less ▲]

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See detailEvaluation génétique du porcs en Wallonie
Dufrasne, Marie ULg; Hammami, Hedi ULg; Gengler, Nicolas ULg

Diverse speeche and writing (2009)

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See detailEvaluation génétique du porc en Wallonie
Dufrasne, Marie ULg; Hammami, Hedi ULg; Gengler, Nicolas ULg

Diverse speeche and writing (2009)

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See detailMilk production of imported heifers and Tunisian-born Holstein cows
Rekik, Boulbaba; Bouraoui, Rachid; Ben gara, Abderrahmen et al

in American-Eurasian Journal of Agronomy (2009), 1(3), 36-42

Test day (TD) records of milk, fat and protein yields and somatic cell scores (SCS) were studied in Holstein cows in Tunisia. There were 43114, 32923 and 24633 lactation records collected on first, second ... [more ▼]

Test day (TD) records of milk, fat and protein yields and somatic cell scores (SCS) were studied in Holstein cows in Tunisia. There were 43114, 32923 and 24633 lactation records collected on first, second and third parity cows between 1992 and 2004 in 182 herds. Records were of cows born in Tunisia (22000 cows) and those imported from Europe (10830 cows) and North America (850 cows). Variation of total days in milk (DIM) per lactation was studied in function of the herd, calving year x calving season interaction and the origin of the cow. Test-day records were analyzed using a linear model that included calving year x calving season and herd x test-day date interactions, calving season, calving year and origin of the cow. The effective length of lactation was affected by all factors included in the model (p< 0.0001) in all lactations. Test- day milk, protein and fat yields and TD SCS varied (p< 0.01) with management and climatic factors (calving year x calving season and herd x test-day date interactions and year and season of calving). A cow produced 18.8 kg, 0.61 kg and 0.58 kg of milk, fat and protein yields on a daily basis in all lactations, respectively. Average SCS was 2.8 in the three lactations. The origin of the cow was an important (p < 0.05) source of variation for DIM, yields and SCS in all lactations except for first lactation cell scores (p>0.05). Cows born in Tunisia seemed to perform better than imported cows in the first lactation while imported cows showed clearly better performances in later lactations. North American cows produced the highest yields and had the lowest SCS among all cows in the second and third lactations. Imported high producing cows seemed able to adjust to Tunisian management conditions following their first lactation. [less ▲]

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See detailModeling milk urea of Walloon dairy cows in management perspectives.
Bastin, Catherine ULg; Laloux, Laurent; Gillon, Alain ULg et al

in Journal of Dairy Science (2009), 92(7), 3529-40

The aim of this study was to develop an adapted random regression test-day model for milk urea (MU) and to study the possibility of using predictions and solutions given by the model for management ... [more ▼]

The aim of this study was to develop an adapted random regression test-day model for milk urea (MU) and to study the possibility of using predictions and solutions given by the model for management purposes. Data included 607,416 MU test-day records of first-lactation cows from 632 dairy herds in the Walloon Region of Belgium. Several advanced features were used. First, to detect the herd influence, the classical herd x test-day effect was split into 3 new effects: a fixed herd x year effect, a fixed herd x month-period effect, and a random herd test-day effect. A fixed time period regression was added in the model to take into account the yearly oscillations of MU on a population scale. Moreover, first autoregressive processes were introduced and allowed us to consider the link between successive test-day records. The variance component estimation indicated that large variance was associated with the random herd x test-day effect (48% of the total variance), suggesting the strong influence of herd management on the MU level. The heritability estimate was 0.13. By comparing observed and predicted MU levels at both the individual and herd levels, target ranges for MU concentrations were defined to take into account features of each cow and each herd. At the cow level, an MU record was considered as deviant if it was <200 or >400 mg/L (target range used in the field) and if the prediction error was >50 mg/L (indicating a significant deviation from the expected level). Approximately 7.5% of the MU records collected between June 2007 and May 2008 were beyond these thresholds. This combination allowed for the detection of potentially suspicious cows. At the herd level, the expected MU level was considered as the sum of the solutions for specific herd effects. A herd was considered as deviant from its target range when the prediction error was greater than the standard deviation of MU averaged by herd test day. Results showed that 6.7% of the herd test-day MU levels between June 2007 and May 2008 were considered deviant. These deviations seemed to occur more often during the grazing period. Although theoretical considerations developed in this study should be validated in the field, this research showed the potential use of a test-day model for analyzing functional traits to advise dairy farmers. [less ▲]

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See detailFacteurs de variation non génétique de la productivité des brebis de race Noire de Thibar
Ben Salem, Imène; Rekik, Mourad; Hammami, Hedi ULg et al

in Revue d'Elévage et de Médecine Vétérinaire des Pays Tropicaux (2009), 62(1), 59-66

An initial database containing growth records of 101,000 lambs obtained within the context of the national meat sheep improvement program was used to quantify reproductive and productivity (total litter ... [more ▼]

An initial database containing growth records of 101,000 lambs obtained within the context of the national meat sheep improvement program was used to quantify reproductive and productivity (total litter weight) traits of ewes of the Noire de Thibar breed, and to determine the influence of non-genetic factors on these traits. A database was then generated: it comprised 64,137 lambings over a 12-year period (1991-2002), and 25 farms and 48 flocks. The mean fertility rate in the flocks was 80.1±12.9% with highly significant effects (p<0.001) of the farm and lambing year. At birth, the average litter size per lambing ewe was 1.32±0.47. It dropped to 1.23±0.55 at 70 days after lambing. All main sources of variation (farm, lambing year, lambing month, and dam age) retained in the model as well as their interactions significantly affected the litter size at different stages after lambing. Survival of all the lambs in the litters between birth and 70 days post-lambing was observed in 89.9% of the ewes. Entire loss of the litter was observed in only 5.8% of the ewes. An analysis of variance revealed that factors such as the lambing year, lambing month, dam age, sex of the litter, and litter size at birth had a highly significant effect on the proportions of ewes with different levels of lambs' survival within litters. On average, the productivity of the Noire de Thibar was 6.6±2.2, 10.3±3.5 and 18.6±6.6 kg at 10, 30 and 70 days after lambing, respectively. The main factors of variation (farm, lambing year, lambing month, dam age, litter size), and some second-level interactions had highly significant effects (p<0.001) on the studied productivity traits. Ewes aged four years and which had male triplets yielded the highest productivities at 70 days after lambing. This preliminary work on the reproductive and productivity traits of Noire de Thibar ewes is essential to develop a ewe selection index as part of a genetic improvement scheme for this breed. [less ▲]

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See detailSystéme d'évaluation génétique pour les verrats Piétrain en croisement en Wallonie
Dufrasne, Marie ULg; Rustin, Maïté; Hammami, Hedi ULg et al

Computer development (2009)

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See detailGenotype by environment interaction in dairy cattle
Hammami, Hedi ULg; Rekik, Boulbaba; Gengler, Nicolas ULg

in Biotechnologie, Agronomie, Société et Environnement = Biotechnology, Agronomy, Society and Environment [=BASE] (2009), 13(1), 155-164

The aim of this literature review was to identify the existence and scope of genotype by environment interaction (G x E) from reports on dairy cattle populations in different management systems, Methods ... [more ▼]

The aim of this literature review was to identify the existence and scope of genotype by environment interaction (G x E) from reports on dairy cattle populations in different management systems, Methods applied to deal with G x E (controlled experiments and large data modeling) were discussed. A G x E was confirmed essentially when high differences between production environments and/or genotypes (genetically distant genotypes) were observed. Environmental effects were aggregated in most studies and identification of the components of the environment was largely unresolved, with only a few studies based on more definite-descriptors of environment. The implications of G x E on breeding decisions are discussed. Breeders should select genotypes on production traits within environmental conditions comparable to where candidate animals are intended to perform. [less ▲]

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See detailEnvironmental sensitivity for milk yield in Luxembourg and Tunisian Holsteins by herd management level.
Hammami, Hedi ULg; Rekik, Boulbaba; Bastin, Catherine ULg et al

in Journal of Dairy Science (2009), 92(9), 4604-12

Milk production data of Luxembourg and Tunisian Holstein cows were analyzed using herd management (HM) level. Herds in each country were clustered into high, medium, and low HM levels based on solutions ... [more ▼]

Milk production data of Luxembourg and Tunisian Holstein cows were analyzed using herd management (HM) level. Herds in each country were clustered into high, medium, and low HM levels based on solutions of herd-test-date and herd-year of calving effects from national evaluations. Data from both populations included 730,810 test-day (TD) milk yield records from 87,734 first-lactation cows. A multi-trait, random regression TD model was used to estimate (co)variance components for milk yield within and across country HM levels. Additive genetic and permanent environmental variances of TD milk yields varied with management level in Tunisia and Luxembourg. Additive variances were smaller across HM levels in Tunisia than in Luxembourg, whereas permanent environmental variances were larger in Tunisian HM levels. Highest heritability estimates of 305-d milk yield (0.41 and 0.21) were found in high HM levels, whereas lowest estimates (0.31 and 0.12, respectively) were associated with low HM levels in both countries. Genetic correlations among Luxembourg HM levels were >0.96, whereas those among Tunisian HM levels were below 0.80. Respective rank orders of sires ranged from 0.73 to 0.83 across Luxembourg environments and from 0.33 to 0.42 across Tunisian HM levels indicating high re-ranking of sires in Tunisia and only a scaling effect in Luxembourg. Across-country environment analysis showed that estimates of genetic variance in the high, medium, and low classes of Tunisian environments were 45, 69, and 81% lower, respectively, than the estimate found in the high Luxembourg HM level. Genetic correlations among 305-d milk yields in Tunisian and Luxembourg HM environments ranged from 0.39 to 0.79. The largest estimated genetic correlation was found between the medium Luxembourg and high Tunisian HM levels. Rank correlations for common sires' estimated breeding values among HM environments were low and ranged from 0.19 to 0.39, implying the existence of genotype by environment interaction. These results indicate that daughters of superior sires in Luxembourg have their genetic expression for milk production limited under Tunisian environments. Milk production of cows in the medium and low Luxembourg environments were good predictors of that of their paternal half-sisters in the high Tunisian HM level. Breeding decisions in low-input Tunisian environment should utilize semen from sires with daughters in similar production environments rather than semen of bulls proven in higher management levels. [less ▲]

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See detailAccessing genotype by environment interaction using within- and across-country test-day random regression sire models
Hammami, Hedi ULg; Rekik, Boulbaba; Soyeurt, Hélène ULg et al

in Journal of Animal Breeding & Genetics (2009), 126(5), 366-377

First-lactation test-day (TD) milk records of Luxembourg and Tunisian Holsteins were analysed for evidence of genotype by environment interaction (G · E). The joint data included 730 810 TD records of 87 ... [more ▼]

First-lactation test-day (TD) milk records of Luxembourg and Tunisian Holsteins were analysed for evidence of genotype by environment interaction (G · E). The joint data included 730 810 TD records of 87 734 cows and 231 common sires. Random regression TD sire models with fourth-order Legendre polynomials were used to estimate genetic parameters via within- and across-country analyses. Daily heritability estimates of milk yield from within-country analysis were between 0.11 and 0.32, and 0.03 and 0.13 in Luxembourg and Tunisia, respectively. Heritability estimates for 305-day milk yield and persistency (defined as the breeding value for milk yield on DIM 280 minus the breeding value on DIM 80) were lower for Tunisian Holsteins compared with the Luxembourg population. Specifically, heritability for 305-day milk yield was 0.16 for within- and 0.11 for across-country analyses for Tunisian Holsteins and 0.38 for within- and 0.40 for across-country analyses for Luxembourg Holsteins. Heritability for apparent persistency was 0.02 for both within and across-country analyses for Tunisian Holsteins and 0.08 for within and 0.09 for across-country analyses for Luxembourg Holsteins. Genetic correlations between the two countries were 0.50 for 305-day milk yield and 0.43 for apparent persistency. Moreover, rank correlations between the estimated breeding values of common sires for 305-day milk yield and persistency, estimated separately in each country, were low. Low genetic correlations are evidence for G · E for milk yield production while low rank correlations suggest different rankings of sires in both environments. Results from this study indicate that milk production of daughters of the same sires depends greatly on the production environment and that importing high merit semen for limited input systems might not be an effective strategy to improve milk production. [less ▲]

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See detailOptimal age at first calving for improved milk yield and lengh of productive life in Tunisian Holstein cows
Ben Gara, Abderrahmen; Bouraoui, Rachid; Rekik, Boulbaba et al

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 ▲]

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