References of "Hammami, Hedi"
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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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See detailGenetic parameters for Tunisian Holsteins using a test-day random regression model.
Hammami, Hedi ULg; Rekik, Boulbaba; Soyeurt, Hélène ULg et al

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 ▲]

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See detailGenotype x environment interaction for milk yield in Holsteins using Luxembourg and Tunisian populations.
Hammami, Hedi ULg; Rekik, Boulbaba; Soyeurt, Hélène ULg et al

in Journal of Dairy Science (2008), 91(9), 3661-71

Test-day (TD) milk yield records of first-lactation Holstein cows in Luxembourg and Tunisia were analyzed using within-and between-country random regression TD models. Edited data used for within-country ... [more ▼]

Test-day (TD) milk yield records of first-lactation Holstein cows in Luxembourg and Tunisia were analyzed using within-and between-country random regression TD models. Edited data used for within-country analysis included 661,453 and 281,913 TD records in Luxembourg and Tunisia, respectively. The joint data included 730,810 TD records of 87,734 cows and 231 common sires. Both data sets covered calving years 1995 to 2006. Fourth-order Legendre polynomials for random effects and a Gibbs sampling method were used to estimate variance components of lactation curve parameters in separate and joint analyses. Genetic variances of the first 3 coefficients from Luxembourg data were 46 to 69% larger than corresponding estimates from the Tunisian data. Inversely, the Tunisian permanent environment variances for the same coefficients were 52 to 65% larger than the Luxembourg ones. Posterior mean heritabilities of 305-d milk yield and persistency, defined as estimated breeding values (EBV) at 280 days in milk-EBV at 80 days in milk, from between-country analysis were 0.42 and 0.12 and 0.19 and 0.08 in Luxembourg and Tunisia, respectively. Heritability estimates for the same traits from within-country analyses, mainly from the Tunisian data, were lower than those from the joint analysis. Genetic correlations for 305-d milk yield and persistency between countries were 0.60 and 0.36. Product moment and rank correlations between EBV of common sires for 305-d milk yield and persistency from within-country analyses were 0.38 and 0.41 and 0.27 and 0.26, respectively. Differences between genetic variances found in both countries reflect different milk production levels. Moreover, low genetic and rank correlations suggest different ranking of sires in the 2 environments, which implies the existence of a genotype x environment interaction for milk yield in Holsteins. [less ▲]

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See detailGenetic diversity and joint-pedigree analysis of two importing Holstein populations.
Hammami, Hedi ULg; Croquet, Coraline; Stoll, Jean et al

in Journal of Dairy Science (2007), 90(7), 3530-41

Genetic diversity and relatedness between 2 geographically distant Holstein populations (in Luxembourg and Tunisia) were studied by pedigree analysis. These 2 populations have similar sizes and structures ... [more ▼]

Genetic diversity and relatedness between 2 geographically distant Holstein populations (in Luxembourg and Tunisia) were studied by pedigree analysis. These 2 populations have similar sizes and structures and are essentially importing populations. Edited pedigrees included 140,392 and 151,381 animals for Tunisia and Luxembourg, respectively. To partially account for pedigree completeness levels, a modified algorithm was used to compute inbreeding. The effective numbers of ancestors were derived from probabilities of gene origin for the 2 populations of cows born between 1990 and 2000. The 10 ancestors with the highest contributions to genetic diversity in the cow populations accounted for more than 32% of the genes. Eight of these 10 ancestors were the same in both populations. The rates of inbreeding were different in the 2 populations but were generally comparable to those found in the literature for the Holstein breed. Average inbreeding coefficients per year, estimated from the data, ranged from 0.91 and 0.50 in 1990 to 3.10 and 2.12 in 2000 for the Tunisian and Luxembourg populations, respectively. Genetic links have also strengthened with time. Average additive relationships between the 2 populations were as high as 2.2% in 2000. Results suggest that it would be possible to investigate genotype by environment interactions for milk traits using the Tunisian and Luxembourg dairy populations. [less ▲]

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See detailLinear and curvilinear effects of inbreeding on production traits for walloon Holstein cows
Croquet, Coraline; Mayeres, Patrick; Gillon, Alain ULg et al

in Journal of Dairy Science (2007), 90(1), 465-471

The nonlinear effects of inbreeding were studied by comparing linear and curvilinear regression models of phenotypic performances on inbreeding coefficients for production traits (milk, fat, and protein ... [more ▼]

The nonlinear effects of inbreeding were studied by comparing linear and curvilinear regression models of phenotypic performances on inbreeding coefficients for production traits (milk, fat, and protein yields) of Holstein cows in their first lactation. Three different regression models (linear, quadratic, and cubic) were introduced separately into a single-trait, single-lactation, random regression test-day model. The significance of the different regression coefficients was studied based on a t-test after estimation of error variances and covariances associated with the different regression coefficients. All of the tested regression coefficients were significantly different from 0. The traditional regression coefficients of milk, fat, and protein yields on inbreeding were, respectively, -22.10, -1.10, and -0.72 kg for Holstein cows in their first lactation. However, the estimates of 305-d production losses for various classes of animals based on inbreeding coefficients showed that the effect of inbreeding was not a linear function of the percentage of inbreeding. The 305-d milk yield loss profiles attributable to inbreeding, obtained by the various regression models, were different. However, for inbreeding coefficients between 0 and 10%, these differences were small. [less ▲]

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See detailIntegration of longevity into the Walloon genetic evaluation system
Vanderick, Sylvie ULg; Croquet, Coraline; Soyeurt, Hélène ULg et al

Conference (2006, June)

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See detailEtude de la variabilité et des liens génétique entre les populations Holsteins Luxembourgeoise et Tunisienne
Hammami, Hedi ULg

Master of advanced studies dissertation (2006)

L’analyse d’un pedigree combiné a permis d’étudier la variabilité et les liens génétiques entre les 2 populations luxembourgeoise et tunisienne. L’extraction et l’ajout de plusieurs ancêtres a permis une ... [more ▼]

L’analyse d’un pedigree combiné a permis d’étudier la variabilité et les liens génétiques entre les 2 populations luxembourgeoise et tunisienne. L’extraction et l’ajout de plusieurs ancêtres a permis une amélioration des profondeurs des 2 pedigrees. Cependant, la connaissance des pedigrees reste partielle pour les 2 populations malgré cette consolidation. L’intervalle de génération de la voie mère-fille reste similaire pour les 2 populations. Par contre, l’intervalle de génération père-fille était différent entre les 2 populations. En effet, il était plus élevé et de l’ordre de 9 ans pour le cas de la population Holstein tunisienne, mais il n’a pas dépassé les 7 ans chez la population luxembourgeoise. Les coefficients de consanguinité ont été estimés selon une approche modifiée de la méthode de Meuwissen et Luo (1992). Les niveaux de consanguinité enregistrés sont similaires à la majorité des valeurs et trends obtenus dans la plupart des études menées sur la Holstein dans le monde. Ces niveaux et pour le cas des 2 populations restent faibles jusqu’à la fin des années 1980, mais augmentent considérablement après les années 1990 d’une manière plus accentuée chez la population femelle tunisienne. Les 2 populations sont caractérisées par des nombres réduits de fondateurs efficaces et d’ancêtres efficaces. 8 parmis les 10 ancêtres ayant la contribution marginale la plus élevée à chacune des 2 populations femelles de référence restent identiques mais avec des contributions appropriées à chacune d’elles. Ces ancêtres ont contribué à plus de 32% au patrimoine génétique des 2 populations femelles de référence. La parenté moyenne entre les 2 populations femelles ainsi que celle estimée entre les taureaux utilisés par ces 2 populations entre 1990 et 2000 montre que ces dernières sont bien connectées. Ceci permettra une investigation des interactions génotype x environnement basée sur la similarité et la parenté génétique entre ces 2 populations. [less ▲]

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See detailInvestigation on genotype by environment interaction for milk yield of Holstein cows in Luxembourg and Tunisia
Hammami, Hedi ULg; Croquet, Coraline; Soyeurt, Hélène ULg et al

in INTERBULL Bulletin (2006), 35

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See detailGenetic evaluation of female fertility for Walloon dairy and dual purpose cows using a parity random regression model: first results
Mayeres, Patrick; Vanderick, Sylvie ULg; Croquet, Coraline et al

in INTERBULL Bulletin (2006), 34

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See detailIntegration of longevity into the Walloon genetic evaluation system
Vanderick, Sylvie ULg; Croquet, Coraline; Soyeurt, Hélène ULg et al

in Interbull Bulletin (2006), 35

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See detailSimplification of milk recording in dairy cattle for low input systems.
Hammami, Hedi ULg; Romdhani, Sonia Bedhiaf; Djemali, M'naouer et al

in Guellouz, Mustapha; Dimitriadou, Andie; Mosconi, Cesare (Eds.) Performance recording of animals. State of the art, 2004 (2004)

The simplification of recording procedures constitutes an economic alternative to reduce operational costs when national recording programs are concerned in low input production systems. A total of 54 940 ... [more ▼]

The simplification of recording procedures constitutes an economic alternative to reduce operational costs when national recording programs are concerned in low input production systems. A total of 54 940 daily milk yields from a herd of 63 Holstein cows recorded in 3 years were used in this study. The effect of reducing tests to measure the accuracy of 305 milk yields was investigated. Two methods of total milk yield computation (the Test Interval Method (TIM) and the Interpolation using Standard Lactation Curves (ISLC)) were compared based on 495 combinations of milk recording alternatives. These alternatives were based on 2 milkings/day, one alternated milking (AM/PM) and one non-alternated milking (AM or PM). All of them were used with 9 intervals between consecutive tests (3 to 11 weeks) and 11 intervals between calving date and the first test day. Results showed that the differences between the actual 305 d milk yield and milk estimated by TIM were the smallest on consecutive test days of maximum 6 weeks of intervals. Differences increased in both sides positively back to A3 and negatively toward A11. Standard estimation errors increased from 4 to 9 from A3 to A11. Phenotypic correlations decreased from .99 to .94 when intervals between the calving date and the first test day increased and this for all test methods (A3-A 11). The ISLC led to a better accuracy of milk yield estimation when intervals between consecutive tests got longer. [less ▲]

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See detailFitting lactation curves of dairy cattle in different types of herds in Tunisia
Rekik, Boulbaba; Ben Gara, Abderrahmen; Ben Hamouda, Mohamed et al

in Livestock Production Science (2003), 83(2-3), 309-315

The incomplete gamma function was used to fit lactation curves of Holstein-Friesian cows in four types of herds in Tunisia. A total of 8640 records were used in the analysis. These included 1269, 637, 239 ... [more ▼]

The incomplete gamma function was used to fit lactation curves of Holstein-Friesian cows in four types of herds in Tunisia. A total of 8640 records were used in the analysis. These included 1269, 637, 239, and 498 first lactation and 2986, 1441, 650, and 920 second and later lactation records in four herd groups namely investors, state, cooperative, and farmers' herds, respectively. The effects of environmental variables, production sector, herd, parity, first test-day date, calving year, and calving season on the main lactation curve traits were analysed. The factors associated with milk yield at the beginning of lactation and the decreasing phase of the curve, persistency, and peak yield varied significantly (P<0.01) with all variables. The ascending phase of the lactation curve was not affected by parity and calving year, while days in milk until peak depended only on the rank of lactation. The state herds had the lowest peak and total yields. The summer season was unfavourable for milk production. In contrast to first lactation cows, third lactation cows had the highest peak and total yields. Milk yield was highly correlated with peak yield (r = 0.79) and was not related to persistency measure. (C) 2003 Elsevier B.V. All rights reserved. [less ▲]

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See detailAssessment of milk yield losses and subclinical mastitis risk factors using bulk milk somatic cell counts in dairy herds
Mtaallah, Brahim; Oubey, Zied; Hammami, Hedi ULg

in Revue de Médecine Vétérinaire (2002), 153(4), 251-260

A prospective study, involving 21 selected dairy farms in north of Tunisia, was conducted to asses the loss of milk production due to high levels bulk tank somatic cell count and to find some relationship ... [more ▼]

A prospective study, involving 21 selected dairy farms in north of Tunisia, was conducted to asses the loss of milk production due to high levels bulk tank somatic cell count and to find some relationship between risk factors and subclinical mastitis. We found that the mean of bulk milk somatic cell count was 626 103 cell/ml. Using a simple statistical model, the average loss of milk due to levels bulk tank somatic cell count was 524 kg per cow per year. Using method of mean comparisons, risk factors associated to high levels bulk tank somatic cell count was: Livestock farming risk factors : inadequacy bedding area; inadequacy cleaning bedding and scrapping area. Milking risk factors : washing teat with a shower no adjustable flow and without wipe with individuel towel milk shift work above five per cow-herd; no stripping squirts of milk before milking; milking healthy and mastitis cows at the same time; no teat dipping. [less ▲]

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See detailAcquis de la recherche en matière d'évaluation génétique des ovins et des caprins en Tunisie
D'Jemali, M'naouer; Jamal, Sofiene; Ben-Dhiaf, Sonia et al

in Cahiers Options Méditerranéennes (1995), 11

Work carried out on performance recording for growth and milk yield in sheep and goats for use in genetic evaluation is briefly summarized, and preliminary data on the growth and reproductive traits of 9 ... [more ▼]

Work carried out on performance recording for growth and milk yield in sheep and goats for use in genetic evaluation is briefly summarized, and preliminary data on the growth and reproductive traits of 9 flocks of Barbary sheep in 1987-92 are presented in 14 tables and discussed. [less ▲]

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