References of "Hammami, Hedi"
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See detailMilk production of Holsteins under Mediterranean conditions: case of the Tunisian population
Ben Gara, Abderrahmen; Borni, Jemmali; Hammami, Hedi ULg et al

in Rekik, Boulbaba (Ed.) Milk Production (2012)

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See detailGenetic variation in heat stress tolerance of Holsteins producing under a continental temperate environment
Hammami, Hedi ULg; Arnould, Valérie ULg; Bormann, Jeanne et al

Poster (2012)

Under projected climate changes, considerations for heat stress tolerance but also for genotype by environment interactions should enter breeding decisions, both for importing, but also exporting ... [more ▼]

Under projected climate changes, considerations for heat stress tolerance but also for genotype by environment interactions should enter breeding decisions, both for importing, but also exporting countries. Most studies concerning the breeding strategies on heat stress tolerance used the temperature humidity index (THI) to assess the thermal stress in a given environment and assumed a specific threshold to each studied population with the same response to each individual above this threshold. Recently new comprehensive thermal indices (TI) integrating radiation solar, wind speed, in addition to temperature and relative humidity were developed. Therefore, the aim of this study is advanced modelling of milk yield trait to evaluate the genetic variation in heat stress tolerance of Holsteins in a temperate environment using random regressions models and six new comprehensive TI. A total of 107,350 test day milk records were available for 12,099 primiparous Holsteins calving from 2000 to 2010. Test-day milk records were merged with meteorological data from 14 public weather stations across Luxembourg. Daily values of 6 new comprehensive TI (3 THI and 3 apparent temperature indices) were calculated by averaging hourly TI over 24 hours. The average distance between herds and their meteorological reference station was of 16 km. Sensitivity of cows to the climate environmental conditions was modelled by applying a reaction norm for each animal, representing its EBV for milk yield on values of the TI on the day in milk (DIM). Six separate random regression models were applied using time-dependent (DIM) and specific TI-dependent covariates. Random effects were additive genetic, permanent environmental, and herd year modelled with Legendre polynomials of order 2 for both DIM and TI. Genetic variances of the different TI evaluated in this study represent 27 to 30% of their correspondent variances for DIM effect. Permanent environmental and genetic variances of TI effect were in the same range. Additive genetic variances and heritabilities for daily milk yield slightly decreased with increasing degrees of the different THI and apparent temperature indices. The average daily milk heritability overall the lactation at the mean of each of the six TI was of 0.28. Genetic correlations between adjacent points across the lactation were > 0.90. Correlations between extreme distant DIM (e.g. early and late lactation stage) were reduced at 0.38 to 0.52. The overall genetic correlations between the different values of each TI remain high (> 0.85). EBV’s summing regular additive effect (DIM effect) and tolerance to heat stress (TI effect) were computed for several combinations of DIM and TI values. Ranking for sires and cows did not change among the 6 heat stress indicators. The correlations between EBVs estimated with the six models were greater than 0.90. In conclusion, the different models depicted genetic variation of milk yield among combinations of DIM and TI levels without defining thresholds in advance. Genetic variation was slightly lower when TI values exceed the thermoneutral zone. Breeding for reduced thermal sensitivity using any of the six indicators of heat stress evaluated in this study is possible. The indicator TI2 representing the adjusted THI for radiation solar and wind speed is favoured because it was the indicator that did identify the highest yearly milk losses under this continental temperate environment. [less ▲]

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See detailAssessing heat stress effects on production traits of Holsteins in a temperate region
Hammami, Hedi ULg; Bormann, Jeanne; Gengler, Nicolas ULg

Conference (2012)

Heat stress impaired productive, reproductive and animal behaviour. Examination of milk production loss due to heat stress was mainly evaluated in USA and few tropical countries using the temperature ... [more ▼]

Heat stress impaired productive, reproductive and animal behaviour. Examination of milk production loss due to heat stress was mainly evaluated in USA and few tropical countries using the temperature humidity index (THI1) developed in the 1950’s. The main objective of this study was to evaluate the effects of six new environmental stress indices on production traits in a temperate region. These indices include solar radiation and wind speed in addition to the temperature and humidity only defining THI1. For that purpose, 530820 milk, fat, protein, and somatic cell count first-lactation test-day records (TD) collected between 2000 and 2011 in Luxembourg were used. TD records were merged with meteorological data from 14 public weather stations. Each TD was assigned to the average daily thermal index of the 3 days before. Firstly, broken-line regression models were applied to identify the threshold of heat stress (THR) for each of the thermal indices. The decline of daily production of the different traits above THR point was assessed by mixed linear models including regression on unit of each of the thermal index. Estimates of THR were specific to each thermal index and trait. Significant decrease of milk, fat and protein yields above THR was observed for all studied thermal predictors. Whereas, an increase of the somatic cell score was observed above the specific THR point. THR estimates for THI1 and THI6 were of 68 and 80 and 62 and 78 for milk and protein yields respectively. Respective rate of decline per unit were of 0.243 and 0.188 kg and 0.0045 and 0.0051 kg respectively. Results from this phenotypic analysis show that evaluating heat tolerance based on bio-meteorological indicators is promising and phenotypic and genetic implications should be investigated. [less ▲]

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See detailAssessing heat stress effects on production traits of Holsteins in a temperate region
Hammami, Hedi ULg; Bormann, Jeanne; Gengler, Nicolas ULg

in Book of abstracts of the 63rd Annual Meeting of the European Association for Animal Production (2012)

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See detailPrediction of cow pregnancy status using conventional and novel mid-infrared predicted milk traits
Hammami, Hedi ULg; Bastin, Catherine ULg; Gillon, Alain et al

in Book of Abstracts of the 62nd Annual Meeting of the European Association for Animal Production (2011, August)

The objective of this study was to determine the ability of conventional milk cow characteristics and novel traits predicted by mid infrared (MIR) obtained from milk recording to predict the pregnancy ... [more ▼]

The objective of this study was to determine the ability of conventional milk cow characteristics and novel traits predicted by mid infrared (MIR) obtained from milk recording to predict the pregnancy status once the cow was inseminated. Conventional milk recording, spectral, and reproductive data collected in Luxembourg Hoslteins between 2008 and 2010 were used. Cows were defined as pregnant if they were positively checked and calved between 267 and 295 d later after the last AI or if they had calved between the later intervals when no checks were recorded. Pregnant or not within 3 intervals after last AI (<=35 d, 45-60 d, and 60-90 d) was modeled using logistic regression models firstly as a function of conventional cow milk characteristics and extended to fatty acids as novel traits predicted by MIR in a second step. The lactation curve characteristics for milk, fat, protein, and lactose yields were estimated using modified best prediction method. Test-day fatty acid contents were estimated from collected MIR spectra using an appropriate calibration equation. Two third proportion and one third of the whole data set were randomly selected for calibration and validation models respectively. The relation between the predicted and observed probabilities of cow pregnancy was approximately linear for calibration and validation models. The sensitivity-specificity combination for cow pregnancy increased when fatty acids were added to conventional milk characteristics as inputs to the different models (from 78 to 85% for sensitivity and from 40 to 52% for specificity). Results based on those models showed that it would be possible to help breeders to manage cow fertility using such tool implemented in the milk recording organizations. [less ▲]

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See detailOptiMIR- Développement et mise en place d'outils innovants de gestion des troupeaux et de conseil personnalisé pour une meilleure durabilité du secteur laitier
Hammami, Hedi ULg; Massart, Xavier; Bertozzi, Carlo et al

in 16ème Carrefour des Productions animales: La filière laitière bovine européenne est-elle durable? (2011, March)

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See detailShort communication: Genetic variation of saturated fatty acids in Holsteins in the Walloon region of Belgium
Arnould, Valérie ULg; Hammami, Hedi ULg; Soyeurt, Hélène ULg et al

in Journal of Dairy Science (2010), 93

Random regression test-day models using Legendre polynomials are commonly used for the estimation of genetic parameters and genetic evaluation for test-day milk production traits. However, some ... [more ▼]

Random regression test-day models using Legendre polynomials are commonly used for the estimation of genetic parameters and genetic evaluation for test-day milk production traits. However, some researchers have reported that these models present some undesirable properties such as the overestimation of variances at the edges of lactation. Describing genetic variation of saturated fatty acids expressed in milk fat might require the testing of different models. Therefore, 3 different functions were used and compared to take into account the lactation curve: (1) Legendre polynomials with the same order as currently applied for genetic model for production traits; 2) linear splines with 10 knots; and 3) linear splines with the same 10 knots reduced to 3 parameters. The criteria used were Akaike’s information and Bayesian information criteria, percentage square biases, and log-likelihood function. These criteria indentified Legendre polynomials and linear splines with 10 knots reduced to 3 parameters models as the most useful. Reducing more complex models using eigenvalues seemed appealing because the resulting models are less time demanding and can reduce convergence difficulties, because convergence properties also seemed to be improved. Finally, the results showed that the reduced spline model was very similar to the Legendre polynomials model. [less ▲]

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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

Poster (2010, August 02)

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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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