Based on an extensive review of the literature, the most common selection criteria used in selection programs for maternal lines were related to litter size at birth or at weaning, while in other cases selection programs were practiced for litter size at birth and weight at nine weeks, number of teats, traits related to the ability of the doe to lactate and nourish the progeny (e.g., weight at weaning, litter weight at weaning or total milk production), and in few cases selection for hyper prolificacy and longevity have been introduced recently. Selection for ovulation rate and uterine capacity using new
reproductive techniques has been successfully performed, which can be used as an alternative to
improve litter size and prenatal survival. For paternal lines, post-weaning daily gain or marketing
weight are commonly selected on individual basis. New techniques, such as laparoscopy, ovariectomization, cryopreservation of embryos and semen, TOBEC (Total Body Electrical Conductivity) and X-ray scanning computerized tomography (CT), were used as tools to assist in selection programs. The application of molecular techniques in selection of rabbits so far has had a limited impact on farm animals. Major genes with large effects on litter size components have been identified. Family index or BLUP are the common procedures used to evaluate the animals genetically in selection experiments. Canalization selection model was recently used in evaluation of does and
bucks in selection experiments and this model incorporated the classical genetic effects acting on the mean production level, in addition to the other genetic effects acting on the residual variance. Several synthetic maternal, paternal and multi-purpose lines were developed using different criteria and methods of selection. Selection responses were estimated commonly by regressing the estimates of the
breeding values on the generation's number, or by using the control populations or the population selected divergently, or by comparing the contemporaries of two different generations using the frozen embryos of the same line. Selection responses obtained in crossbred rabbits could be periodically evaluated by estimating the crossbreeding parameters in the cross (e.g., direct and maternal additive,
direct and maternal heterosis, recombination effects, etc.), or by comparing heterosis values obtained from an experiment with those of contemporary commercial farms, or by evaluating the selection responses at different stages of the programme by carrying out contemporary comparisons among purebred and crossbreds.Studies that have compared selection responses in crossbreds with the responses in pure lines, have observed slightly higher responses in the crossbreds. Direct selection responses per generation estimated for litter size born or weaned were low or slightly moderate and ranged from 0.081 to 0.180
rabbits, while the correlated responses ranged from 0.03 to 0.18 ova for ovulation rate, and 2.0 to 3.7% for prenatal survival. Depending on modified components of litter size, selection for uterine capacity produced responses that were similar to that obtained in direct selection for litter size. Improvement in litter size caused by selection for uterine capacity was not greater than the improvement obtained from direct selection for litter size (approximately 0.1 rabbits per litter per generation). Does selected for litter size at weaning presented significant responses in feed intake (3%) and milk yield ( 6%). A response of 62 g per litter was recorded when selecting for litter weight at weaning, with a correlated response of 0.17 rabbits for litter size born and weaned. Estimates of direct selection responses per
generation were moderate and ranged from 8.7 to 12.6 g for weaning weight, 18 to 68 g for marketing weight, 0.45 to 1.73 g/d for weight gain from weaning to marketing, and 0.05 to 0.27 g feed per g gain for feed conversion from weaning to marketing, which was associated with an increase in correlated responses in adult weight and feed consumption, but with decreasing rate in feed conversion. Selection
for growth rate has little or somewhat moderate effects on carcass characteristics and meat quality when rabbits were selected at the same stage of maturity, which was associated with increases in intestinal content and decreases in dressing out percentage and fat deposits, and ultimately in pH in muscle and water holding capacity of the meat. Selection for litter weight at weaning achieved considerable responses in growth rate with maintaining high litter components and feed conversion. By selection, total fleece weight increased significantly associated with correlated improvements in live body weight and fleece qualities (bristle length and diameter, follicle ratios, compression, resilience, and fibre diameter). Selection responses estimated by different methods were in good agreement to most studies reviewed. |
