Environmental and Genetic Factors Affecting Litter Size Components in Rabbits

[EN] In rabbits, ovulation rate is, together with prenatal survival, one of the main limiting factors for litter size. Both components are affected by several factors related to females and their environment. Thus, understanding these components and their factors of variation is key in designing diets, optimisation of reproductive performance and genetic selection. In this review, authors summarise the main components of litter size and their environmental factors of variation. Genetic factors and the main results of genetic selection programmes on components of litter size are also summarised. In this regard, a negative effect of dietary restriction and reduced day light hours is found, as well as a positive effect of body condition, parity order and age of female on ovulation rate. However, an increase in deterioration of oocyte quality has been reported as ovulation rate increases, leading to decreased embryonic and foetal survival. Dietary restriction and heat stress also have a negative effect on embryonic and foetal survival, increasing the failures during gestation while good vascularisation and enough available space in uterine horn are keys to embryonic and foetal survival. Ovulation rate was proposed as indirect selection criterion to improve litter size due to higher heritability. However, this selection was relevant, but it did not modify litter size because of an increase in prenatal mortality. Uterine capacity has been directly related to prenatal survival, although its selection has also been unsuccessful in increasing litter size. Belabbas, R.; Ilès, I.; Argente, M.; Ezzeoug, R.; Ainbaziz, H.; García, M. (2023). Environmental and Genetic Factors Affecting Litter Size Components in Rabbits. World Rabbit Science. 31(2):117-131. https://doi.org/10.4995/wrs.2023.18680 Adams C.E. 1960. Studies on prenatal mortality in the rabbit, Oryctolagus cuniculus: the amount and distribution of loss before and after implantation. J. Endocrinol., 19: 325-344. https://doi.org/10.1677/joe.0.0190325 Adams C.E. 1962. Studies on prenatal moratality in rabbit Oryctolagus cuniculus: The effect of transferring varying numbers eggs. J. Endoc., 24: 471-490. https://doi.org/10.1530/jrf.0.0010036 Abecia J.A., Lozano J.M., Forcada F., Zarazaga L. 1997. Effect of level of dietary energy and protein on embryo survival and progesterone production on day eight of pregnancy in Rasa Aragonesa ewes. Anim. Reprod. Sci., 48: 209-218. https://doi.org/10.1016/s0378-4320(97)00021-3