Connexió genètica entre ramats bovins aparellats amb munta natural

L'encreuament entre ramats, per tal de millorar les característiques físiques del bestiar, ha de considerar els valors genètics que hi són implicats. L'efectivitat d'aquests encreuaments serà més gran quan més gran sigui la compatibilitat dels caràcters entre els ramats escollits. Per avaluar-ho, s'ha estudiat el grau de connexió entre ramats de bestiar boví de la "Bruna dels Pirineus", aparellats per munta natural.El cruce entre rebaños, con el fin de mejorar las características físicas del ganado, ha de considerar los valores genéticos implicados. La efectividad de estos cruces será mayor cuando más grande sea la compatibilidad de los caracteres entre los rebaños escogidos. Para evaluarlo, se ha estudiado el grado de conexión entre rebaños de ganado vacuno de la "Bruna dels Pirineus"

Estudi de la longevitat i la supervivència fins al deslletament en bovins de muntanya de la raça Bruna dels Pirineus

Descripció del recurs: 4 setembre 2006Consultable des del TDXEls caràcters amb major importància econòmica en el sistema de producció de la raça Bruna dels Pirineus són: interval entre parts, prolificitat, longevitat de les vaques, supervivència del vedell fins a l'escorxador, pes al deslletament, creixement durant l'engreix i conformació de la canal. Dins d'aquests, la longevitat de les vaques es una qualitat desitjable, ja que la seva millora permetria disminuir els costos de reposició i augmentar el percentatge de vaques en edats més productives. La supervivència dels vedells fins al deslletament incrementa els ingressos per la venta dels vedells i redueix significativament els costos. L'objectiu d'aquest treball es l'estudi dels factors que afecten la longevitat de les vaques de carn de la raça Bruna dels Pirineus i la supervivència dels seus vedells fins al deslletament mitjançant tècniques d'anàlisi de supervivència. El primer capítol és una revisió bibliogràfica que dóna una visió general de la situació i l'evolució de les tècniques d'anàlisi de supervivència en la seva aplicació a la producció animal. El segon capítol és l'anàlisi dels factors que influeixen la longitud de la vida productiva i les taxes de reposició de les vaques. La mitjana de la longitud de la vida productiva és aproximadament de 9 anys, que implicaria una taxa de reposició anual del 11%. Utilitzant un model de riscos proporcionals, hem vist que la taxa de reforma diaria augmenta amb edats al primer part tardanes, dificultats de part creixents, pesos al naixement dels vedells molt grans, guanys de pes fins al deslletament dels vedells molt petits i intervals entre parts molt llargs. A més, aquest increment també depèn de la seqüència de valors per a la mateixa variable en els parts previs. Una major taxa d'eliminació diària incrementa les taxes de reposició anuals, encara que la pèrdua de productivitat al reduir-se el percentatge de vaques en edats més productives no és molt important. El tercer capítol s'ha dedicat a l'anàlisi dels factors genètics i ambientals que afecten la supervivència dels vedells fins al deslletament, que és del 96.9%. Usant un model de riscos proporcionals, hem vist que la taxa de mortalitat diària no és constant durant tota la lactació sinó dins d'uns d'intervals: es major les primeres dos setmanes, disminueix les dos següents i a partir del mes de vida es gairebé negligible. Aquesta taxa s'incrementa per als vedells nascuts en la segona part del període de parts (de principis de març a agost), fills de mares joves, amb dificultat de part alta i amb pes al naixement petit. En el pla genètic, malgrat que el valor estimat de la variància genètica additiva sota un model pare és relativament alta (0.3), l'heretabilitat del caràcter al deslletament és només del 0.037. Finalment, el quart capítol és una validació per simulació d'un mètode aproximat amb dos passos per estimar correlacions genètiques entre caràcters lineals i de supervivència per a la seva aplicació a l'avaluació genètica. En el primer pas s'estimen les variàncies additives i es fa l'avaluació genètica de cada caràcter per separat amb el model univariant ideal per obtenir els pseudo-registres per cada caràcter i els seus pesos. En el segon pas, aquests registres s'utilitzen en l'avaluació genètica conjunta de los tres caràcters mitjançant la utilització de models lineals i mètodes de tipus BLUP y REML en la valoració genètica i l'estimació dels components de variància. El promig de les correlacions genètiques i les precisions obtingudes de les dades simulades confirma la validesa del procediment per a estimar correlacions genètiques en un ampli ventall de situacions (diferents correlacions genètiques, heretabilitats i nivells de censura).The most economically important traits in the production system of the Bruna dels Pirineus beef breed are: calving interval, prolificity, cow longevity, calf survival until sale, weaning weight, growth rate during fattening and carcass conformation. Within these, cow longevity is a desirable quality, because its improvement would allow to reduce the replacement costs and increase the percentage of cows in more productive ages. Calf survival up to weaning increases the incomes from calf sales and reduce significantly the costs. The aim of this work is to study the factors that afect longevity of Bruna dels Pirineus beef breed cows and survival up to weaning of its calves using survival analysis techniques. The first chapter is a bibliographic revision that gives a general view of the situation and evolution of survival analysis techniques in its application to animal production. The second chapter is the analysis of factors that influence cow's length of productive life and replacement rates. The average length of productive life is approximatelly of 9 years, which implies an annual replacement rate of 11%. Using a proportional hazard model, we have observed that the daily culling rate increases with late ages at first calving, increasing calving difficulties, very large calf birth weights, very small weight gains until weaning and very long calving intervals. Furthermore, this increase also depends on the sequence of values of the variable in previous calvings. A higher daily culling rate increases the annual replacement rates, in spite of the loss in productivity by reducing the percentage of cows in more productive ages is not very important. The third chapter analyses the genetic and environmental factors that afect calf survival up to weaning, which is 96.9%. Using a proportional hazard model, we have observed that the daily mortality rate is not constant during all the lactation but within intervals: is higher during the first to weeks, diminishes the next two weeks and after the first month of life is nearly depreciable. This rate increases for calf born in the second part of the calving period (from the beginning of march to august), born from young cows, with high calving difficulty and small birth weight. From a genetic point of view, despite the additive sire variance is relativelly high (0.3), the heritability of the trait at weaning is only 0.037. Finally, the fourth chapter is a validation via simulation of an approximate two-step approach to estimate genetic correlations between survival and linear traits for its aplication to genetic evaluation. In the first step, additive variances are estimated and a genetic evaluation is done separatelly for each trait with its ideal univariate model to obtain the pseudo-record for each animal and its weight. In the second step, these pseudo-records are used in the joint genetic evaluation of all traits using linear models and BLUP and REML methods in the genetic evaluation and the variance components estimation. The mean of the genetic correlations and accuracies obtained in simulated data confirmed the suitability of the approach to estimate genetic correlations in a wide range of situations (different genetic correlations, heritabilities and censoring levels)

La vaca de l’Albera: necessitat d’un programa de conservació i expansió

La raça de l’Albera és una població bovina autòctona localitzada en el massís de l’Albera (Alt Empordà, Girona, Catalunya), ben adaptada i capaç d’aprofitar els recursos alimentaris de la zona. Són animals rústics, de petit format i que viuen en llibertat tot l’any. S’enquadra en el tronc cantàbric, encara que tradicionalment s’hagin descrit dues varietats: la Negra i la Fagina. Les femelles s’usen com a reposició del ramat i els vedells mascles són engreixats tot i el seu limitat potencial carnisser. L’edat al primer part de les vaques és entre els 3 i 4 anys. L’últim cens d’animals adults amb morfotip Albera ha classificant la població de raça en perill d’extinció d’acord amb la FAO. Per aquest motiu s’està realitzant un programa formal de conservació que inclou la conservació in situ, ex situ i in vitro.The Alberes cattle breed is an autochthonous bovine population located in the Alberes Massif (Alt Empordà,Girona, Catalunya), well adapted to the zone in which it mainly makes use of forest resources. Cows are small sized and rustic, living under a semi-feral management system with minimal human contact. This breed clusters within the Cantabrian trunk, although traditionally two coat colour varieties have been described, Black and Fawn varieties. All of the females are used for replacement and the males are destined for fattening despite their low meat potential. The age at first calving ranges between three and four years. The last census of adult animals considered as an Endangered Breed according to the FAO classification. This dramatic situation allows to start a formal conservation programme with the combination of in situ, ex situ and in vitro practices

Validation of an approximate approach to compute genetic correlations between longevity and linear traits

The estimation of genetic correlations between a nonlinear trait such as longevity and linear traits is computationally difficult on large datasets. A two-step approach was proposed and was checked via simulation. First, univariate analyses were performed to get genetic variance estimates and to compute pseudo-records and their associated weights. These pseudo-records were virtual performances free of all environmental effects that can be used in a BLUP animal model, leading to the same breeding values as in the (possibly nonlinear) initial analyses. By combining these pseudo-records in a multiple trait model and fixing the genetic and residual variances to their values computed during the first step, we obtained correlation estimates by AI-REML and approximate MT-BLUP predicted breeding values that blend direct and indirect information on longevity. Mean genetic correlations and reliabilities obtained on simulated data confirmed the suitability of this approach in a wide range of situations. When nonzero residual correlations exist between traits, a sire model gave nearly unbiased estimates of genetic correlations, while the animal model estimates were biased upwards. Finally, when an incorrect genetic trend was simulated to lead to biased pseudo-records, a joint analysis including a time effect could adequately correct for this bias

Connectedness among herds of beef cattle bred under natural service

Background: A procedure to measure connectedness among herds was applied to a beef cattle population bred by natural service. It consists of two steps: (a) computing coefficients of determination (CDs) of comparisons among herds; and (b) building sets of connected herds. Methods: The CDs of comparisons among herds were calculated using a sampling-based method that estimates empirical variances of true and predicted breeding values from a simulated n-sample. Once the CD matrix was estimated, a clustering method that can handle a large number of comparisons was applied to build compact clusters of connected herds of the Bruna dels Pirineus beef cattle. Since in this breed, natural service is predominant and there are almost no links with reference sires, to estimate CDs, an animal model was used taking into consideration all pedigree information and, especially, the connections with dams. A sensitivity analysis was performed to contrast single-trait sire and animal model evaluations with different heritabilities, multiple-trait animal model evaluations with different degrees of genetic correlations and models with maternal effects. Results: Using a sire model, the percentage of connected herds was very low even for highly heritable traits whereas with an animal model, most of the herds of the breed were well connected and high CD values were obtained among them, especially for highly heritable traits (the mean of average CD per herd was 0.535 for a simulated heritability of 0.40). For the lowly heritable traits, the average CD increased from 0.310 in the single-trait evaluation to 0.319 and 0.354 in the multi-trait evaluation with moderate and high genetic correlations, respectively. In models with maternal effects, the average CD per herd for the direct effects was similar to that from single-trait evaluations. For the maternal effects, the average CD per herd increased if the maternal effects had a high genetic correlation with the direct effects, but the percentage of connected herds for maternal effects was very low, less than 12%. Conclusions: The degree of connectedness in a bovine population bred by natural service mating, such as Bruna del Pirineus beef cattle, measured as the CD of comparisons among herds, is high. It is possible to define a pool of animals for which estimated breeding values can be compared after an across-herds genetic evaluation, especially for highly heritable traits

Carcass conformation and fat cover scores in beef cattle : a comparison of threshold linear models vs grouped data models

Background: Beef carcass conformation and fat cover scores are measured by subjective grading performed by trained technicians. The discrete nature of these scores is taken into account in genetic evaluations using a threshold model, which assumes an underlying continuous distribution called liability that can be modelled by different methods. Methods: Five threshold models were compared in this study: three threshold linear models, one including slaughterhouse and sex effects, along with other systematic effects, with homogeneous thresholds and two extensions with heterogeneous thresholds that vary across slaughterhouses and across slaughterhouse and sex and a generalised linear model with reverse extreme value errors. For this last model, the underlying variable followed a Weibull distribution and was both a log-linear model and a grouped data model. The fifth model was an extension of grouped data models with score-dependent effects in order to allow for heterogeneous thresholds that vary across slaughterhouse and sex. Goodness-of-fit of these models was tested using the bootstrap methodology. Field data included 2,539 carcasses of the Bruna dels Pirineus beef cattle breed. Results: Differences in carcass conformation and fat cover scores among slaughterhouses could not be totally captured by a systematic slaughterhouse effect, as fitted in the threshold linear model with homogeneous thresholds, and different thresholds per slaughterhouse were estimated using a slaughterhouse-specific threshold model. This model fixed most of the deficiencies when stratification by slaughterhouse was done, but it still failed to correctly fit frequencies stratified by sex, especially for fat cover, as 5 of the 8 current percentages were not included within the bootstrap interval. This indicates that scoring varied with sex and a specific sex per slaughterhouse threshold linear model should be used in order to guarantee the goodness-of-fit of the genetic evaluation model. This was also observed in grouped data models that avoided fitting deficiencies when slaughterhouse and sex effects were score-dependent. Conclusions: Both threshold linear models and grouped data models can guarantee the goodness-of-fit of the genetic evaluation for carcass conformation and fat cover, but our results highlight the need for specific thresholds by sex and slaughterhouse in order to avoid fitting deficiencies