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A divergent selection experiment on within-litter homogeneity of birth weight in rabbits was carried out at the INRA experimental farm at Auzeville. The two lines were created by selecting breeding does and bucks from the female strain AGP22 bred at the Grimaud Frères Sélection Company. This involved a new model incorporating a genotypic value for the mean of individual birth weight and a genotypic value for the environmental variance. This new "trait" was modelled in the usual infinitesimal framework, giving estimated breeding values for environmental variability. There was a favourable selection response with a significant difference in within-litter standard deviation of birth weight between the lines selected for increasing (HOM) or decreasing (HET) homogeneity. At the end of the third generation, 31 females from the HOM line and 33 from the HET line were sacrificed to collect the uterine horns and measure their initial length (L1) and their length after elongation with a weight of 50 g (L2) and then 70 g (L3). The length in the homogeneous line was significantly greater, whatever the weight (L1: + 1.3 cm, P = 0.02; L2: + 2.8 cm P b 0.001; L3: + 4.2 cm, P b 0.001). The absolute and the relative elongations were significantly higher in the HOM line. There was no significant effect of the line on the number of ova shed, the weight of the ovary, or the weight of the uterine horns. It is concluded that the divergence between lines for the within-litter homogeneity of birth weight is at least partly due to the characteristics of the genital tract, i.e. the length and capacity for elongation of the uterine horn.

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Résumé—Au total, 246 lapines de la souche INRA0029 ont été mises en place pour réaliser deux portées en insémination artificielle. La moitié avait préalablement subi une ovariectomie unilatérale pour doubler le nombre de foetus dans une corne utérine. Le nombre de foetus implantés était compté par endoscopie. Il existe une relation du nombre d'implants positive avec la taille de portée et négative avec le taux de survie foetale. Il y a une relation de la taille de portée positive avec le poids total des nés vivants et négative avec le poids moyen des lapereaux nés vivants. L'effet défavorable du doublement de la population dans une corne sur le taux de survie foetale (-5,2 points) et le poids moyen des lapereaux à la naissance (-2,6 g) est significatif, mais beaucoup plus faible. En deuxième parité, les portées étaient standardisées à six lapereaux à la naissance, qui étaient pesés chaque semaine jusqu'au sevrage, puis à 7 et 11 semaines pour les femelles conservées. La relation négative entre poids individuel et taille de portée à la naissance est significative quel que soit l'âge, malgré un effet favorable de la taille de portée à la naissance sur la production de lait pendant les deux premières semaines. L'effet défavorable du groupement de tous les fcetus dans une seule corne est beaucoup plus faible et de moins en moins marqué quand l'âge des lapereaux augmente ; l'effet défavorable sur la croissance est une conséquence de l'effet sur le poids à la naissance. Il n'y a aucun effet du surpeuplement utérin ou de la taille de portée sur la croissance après sevrage. La diminution du poids des lapereaux issus de grandes portées semble donc essentiellement due à une compétition nutritionnelle pendant la gestation entre tous les fcetus et non à un surpeuplement physique des cornes utérines. L'augmentation de la prolificité doit donc s'accompagner d'une meilleure couverture des besoins de la lapine gestante.lapin / reproduction / croissance / effets maternels Summary — Relations between the crowding of the uterine horns, litter size at birth and growth of young rabbits from birth to 11 weeks, after cross-fostering of litters. The effect of the number of foetuses during pregnancy and of litter size at birth on young rabbits' weight at birth and up to 11 weeks was investigated. Two hundred forty-six does from the INRA0029 strain were bred in artificial insemination. Half of them were unilaterally ovariectomized before puberty in order to achieve a double crowding of the uterine horn corresponding to the remaining ovary and so to challenge the uterine capacity. The number of implanted foetuses was counted by endoscopy. There was a positive relationship of the total number of implanted embryos with litter size at birth, in spite of a negative relationship with the foetal survival rate. There was a positive relationship of litter size at birth with the litter weight, in spite of a negative relationship with the average weight of young rabbits alive at birth. The unfavourable effect of double crowding of one h...

ABSTRACT:The aim of this experiment was to compare the ovary response, the fertilising aptitude and the embryo development of multiparous rabbit does having received during the whole career 0 (control: no injection, group 0), 8 or 25 IU of eCG (groups 8 and 25, respectively) 48 h before each 4 d post partum insemination. After the 11 th series of insemination, two groups of 60 does were sacrifi ced 30 h or 14 d after insemination. The percentage of ovulating females does not vary according to the eCG treatment, but the ovulation rate (number of corpora lutea per ovulating does) and the fertilising rate (number of segmented ova/number of corpora lutea×100) increases with eCG dose [10.2, 11.1, 12.3 corpora lutea (P=0.011); 61.6 vs 97.5, 81.2% (P<0.001), for groups 0, 8 and 25 IU respectively]. In a same way, at 14 d of pregnancy, the embryo survival (1-(number of corpora lutea -number of alive embryos/ number of corpora lutea)) increases with the injected amount of eCG (55.1 vs 69.9 and 83.1% for groups 0, 8 and 25, respectively, P=0.023). Moreover, eCG completely removes the fertilising failure independent of ovulation. Compared with the other does, lactating-non-receptive does have a lower fertilisation rate (70.0 vs 90.2%, P=0.008) and a weaker embryo survival (55.3 vs 83.5%, P<0.001). It is concluded that with intensive reproduction rhythm (4 d post partum insemination), an 8 IU of eCG injected 48 h before insemination is enough to enhance the ovulation rate, the embryo survival and the fertilisation rate of multiparous does.