In hot climate countries, little numbers of synthetic maternal, paternal and multi-purpose lines were synthesized using different criteria and methods of selection and crossbreeding. Synthetic lines of rabbits naming Saudi 2 and APRI (as maternal lines) and Alexandria and Saudi 3 (as paternal lines) and Moshtohor (as multipurpose line) were being formed to be convenient in hot climate in the Arabian areas. These synthetic lines have the adaptability to be reproducing efficiently in different systems of production of hot climates and they have also good capability to grow profitably in these hot areas. The most common selection criteria used in selection programs to develop new maternal lines were related with litter size at birth or at weaning and milk production, while in paternal lines, post-weaning daily gain or marketing weight are commonly selected individually. Spanish V-line rabbits genetically selected for more than 35 generations were introduced in various countries (as alive animals or as frozen embryos) and by using recent bio-techniques and applying selection and /or crossbreeding programs with local lines, this line was widely distributed in some hot countries of the world like Egypt, Saudi Arabia, Uruguay, etc. Involving V-line rabbits in crossbreeding plans was an encouraging factor since V line showed higher direct and maternal genetic effects for litter and lactational performances and most post-weaning growth and carcass traits than native rabbits. However, direct and maternal genetic effects for most carcass traits (e.g. weights of hot carcass, meat, offal, and bone) were in favour of V line, but these estimates were in favour of the local rabbits for meat quality traits. The estimates of direct and maternal heterosis reviewed for lactation, growth and carcass traits and heat-stress physiological parameters were favourable. Reviewed estimates of recombination effects for milk yields and components and growth traits were insignificant and negligible; indicating that crossing V-line with native breeds of rabbits in hot climate countries could be effective to develop synthetic maternal and paternal lines characterized by high milk production associated with rich milk components and high growth rate. Direct selection had little or moderate effects on litter size, prenatal litter components, meat quality traits, while it had considerable effects on post weaning growth and carcass traits, feed conversion. Direct selection for feed efficiency is less efficient than selection for growth rate for improving feed conversion ratio. Selection responses obtained in crossbred rabbits were slightly higher than responses obtained in the pure lines. In hot climate countries, new biological and molecular techniques commonly used as tools in selection programs are so far from what reached in developed countries.
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