Biotecnologías reproductivas en el ganado bovino: cinco décadas de investigación en México

The main bovine reproductive biotechnologies are recapitulated herein in five sections, and their historical development and current status are analyzed, including the results generated in Mexico. In the 1970s, estrus synchronization and ovulation induction began; thus, the reproductive cycle started to be controlled with the resources available at that time, based on the knowledge of bovine reproductive physiology. Over the years, hormone therapy evolved as new compounds were discovered, refining methods to standardize the effect and generating new methods for the release of hormones. The most widely used biotechnology in the world, artificial insemination, owes its expansion to advances in semen processing, among which the development of diluents, cryopreservation, semen sexing, and computer-assisted sperm analysis stand out. The embryonic era began with the development of multi-ovulation and methods for collecting, evaluating, transferring, and cryopreserving embryos. The second embryonic era came with the fully in vitro production of embryos from immature eggs and frozen sperm, known as in vitro embryo production. Great research and material resources have been invested in this procedure, rendering it a pillar of genetic improvement and productivity, in combination with two other tools: sexed semen and genomic evaluations. A golden age of in vitro embryo production is on the horizon, with the possibility to produce accurate modifications in the embryo genome, thanks to gene editing technology.A lo largo de cinco secciones, se recapitulan las principales biotecnologías reproductivas en el bovino, se analiza su desarrollo histórico, estado actual, y se incluyen resultados generados en México. En la década de 1970, se inició la sincronización estral e inducción de la ovulación donde, basados en el conocimiento de la fisiología reproductiva bovina, se empezó a controlar el ciclo reproductivo con recursos disponibles en aquel entonces. Con los años, la terapia hormonal evolucionó conforme se descubrieron nuevos compuestos, refinando métodos para estandarizar el efecto y generar nuevos métodos de liberación de las hormonas. La biotecnología más usada en el mundo, la inseminación artificial, debe su expansión a los avances en el procesamiento del semen, donde destaca el desarrollo de diluyentes, la criopreservación, el sexado del semen y el análisis espermático asistido por computadora. La era embrionaria inició con el desarrollo de la multiovulación y los métodos para colectar, evaluar, transferir y criopreservar los embriones. La segunda era embrionaria llegó con la producción de embriones completamente in vitro, partiendo de óvulos inmaduros y semen congelado, en lo que se denominó la producción in vitro de embriones. En ésta, se han invertido grandes recursos de investigación, y materiales, para hacerla un pilar del mejoramiento genético y la productividad, en combinación con otras dos herramientas, el semen sexado y las evaluaciones genómicas. Se vislumbra una época de oro de la producción in vitro de embriones con la posibilidad de modificar el genoma de embriones con precisión, gracias a la tecnología de edición de genes

Effects of age, weight and fatness slaughter end points on estimates of genetic parameters for carcass traits

The influence of slaughter end points of age, carcass weight and fat thickness (covariates) on estimates of phenotypic variances ([special characters omitted]), heritabilities (h2) for, and genetic correlations (rg) among, 14 carcass traits of purebred and composite steers from 12 breeds was studied. In addition, the effects of different levels of fat thickness and hot carcass weight as end points on estimates of breed effects and retained heterosis (individual and maternal combined) were also examined. Parameters were estimated by REML using animal models. In general, estimates of [special characters omitted] and h2 for carcass traits indicate that enough phenotypic and genetic variation exist to change measures of carcass merit by single-trait selection. For some carcass traits, however, the magnitude of change would depend on the effect of slaughter end point as estimates of [special characters omitted] and h2 were often different for different end points. The largest differences in estimates of h2 with different end points were for hot carcass weight and retail product weight. Among all parameters estimated, the greatest effects of end points were on estimates of r g for 54 of the 91 pairs of traits, which in some cases not only changed in magnitude (0.15 or more), but also in sign. Therefore, expected correlated responses to selection would also differ depending on end point. Ranking of breeds and expression of heterosis also varied for many carcass traits depending on slaughter end points. As an example, the estimate of retained heterosis for actual retail product as a percentage of hot carcass weight for the MARC III composite breed was significantly negative (-1.27%) at 0.7 cm, but was significantly positive (2.55%) at 1.5 cm of fat thickness. Furthermore, some estimates of heterosis also doubled in magnitude depending on levels of fat thickness end point. Age, weight and fatness end points influenced the sign and(or) the magnitude of estimates of genetic parameters for several carcass traits

Heritability estimates for carcass traits of cattle: a review

We present estimates of heritability for carcass traits of cattle published in the scientific literature. Seventy-two papers published form 1962 to 2004, which reported estimates of heritability for carcass traits, were reviewed. The unweighted means of estimates of heritability for 14 carcass traits by slaughter end point (age, weight, and fat depth) were calculated. Among the three end points, carcass weight, backfat thickness, longissimus muscle area, and marbling score were the carcass traits with the most estimates of heritability (56≤n≤66). The averages for these traits indicate that they are similarly and moderately heritable (0.40, 0.36, 0.40, and 0.37, respectively). However, heritability estimates for most traits varied greatly, which could be due to differences in breed groups, methods of estimation, and effects in the model, number of records, measurement errors, sex, and management. Few studies have compared heritability estimates for carcass traits adjusted to different end points. Results from such studies have been inconsistent, although some studies revealed that heritability estimates for several carcass traits are sensitive to the covariate included in the model for the end point, implying that direct response to selection would be different for some traits depending on slaughter end point. The effect of different end points on estimates of heritability for many carcass traits has not been studied

COMPARISON OF MODELS FOR THE ESTIMATION OF VARIANCE COMPONENTS FOR GROWTH TRAITS OF REGISTERED LIMOUSIN CATTLE

Six models to estimate genetic parameters for birth weight (BW), weaning weight adjusted to 205 days (W205), and yearling weight adjusted to 365 days (W365) were compared. Model A included direct genetic effects. Model AP allowed for direct genetic and permanent environmental effect of the dam. Model AM included direct genetic and maternal genetic effects. Models AMC and AMP were the same as Model AM but they also allowed for the covariance between direct and maternal genetic effects, and the common environmental effect due to the dam, respectively; and Model AMCP was fitted for all three random effects plus the covariance between direct and maternal effects. Models were compared using the likelihood ratio text. The AMC model was selected to be the most appropriate for BW and W205, whereas Model A was chosen for W365. When maternal effects were included, direct genetic variance and direct heritability estimates were reduced for BW and W205. Direct heritability estimates with appropriate models were: 0.13, 0.21 and 0.20 for BW, W205 and W365. Heritability of maternal effects with appropriate models was: 0.15 and 0.32 for BW and W205, and direct-maternal genetic correlations with appropriate models were: -0.67 and -0.69 for BW and W205, respectively.