Relations entre le nombre de fœtus par corne utérine, la taille de portée à la naissance et la croissance pondérale des lapereaux

Abstract

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 horn on foetal survival rate (-5.2 points) and average young rabbits weight at birth (-2.6 g) was significant, but weaker. In the second parity, litters were standardized at six young rabbits after birth by cross-fostering. They were weighed every week up to weaning, and at 7 and 11 weeks. The negative effect of litter size at birth on weight and average daily gain was significant whatever the age of the young rabbits, in spite of a favourable effect of litter size at birth on milk production. The unfavourable effect of overcrowding was weaker and less and less significant when the age of young rabbits increased. It is concluded that: i) Competition for uterine space, due to overcrowding in large litters, has an effect on foetal survival rate; however, it is the limiting factor of litter size only in extreme conditions (more than 11 foetus in one uterine horn). It has a very limited effect on weight and growth of young rabbits from birth to weaning. ii) The decrease of weight of young rabbits from large litters, which remains after birth and even after weaning, was mainly due to a nutritional competititon, according to the total number of foetus; therefore, improvement of prolificacy must be accompanied by a better response to the nutritional needs of pregnant does.