Discussion on the importance, definition and genetic components of the number of animals born in the litter with particular emphasis on small ruminants in tropical conditions

According to the recent literature, litter size (LS) produces a large number of negative direct and collateral effects on individual female lifetime productivity or flock economy. If all these influences were considered together, the improvement of LS would be far from the expected economical advantage. However, the LS can be used as an indicator of fitness or adaptability of the animals to their environment, which is very important under tropical conditions. Using a simple example, the expected genetic components of LS estimated by two procedures was compared. In the first and classical approach (CP), the LS is considered as a single trait belonging to the female who gave birth to the litter. The results of CP were compared to the new alternative procedure (AP), which considered the number of young animals born in the litter as equivalent to LS, their individual birth weight and their status (dead or alive) as a function of the synergistic action of the animals born in the litter and their mother. This exercise showed that the genetic parameters for LS estimated by the CP were biased, since a large number of genetic (co)variances were hidden. For the same type of related animals, the AP can estimate eight times more genetic components than CP. The use of AP is highly recommended, allowing the best and most efficient way of using the collected data and also estimating the breeding values of all animals born in the litter at a very young age, irrespective of their sex. Shorter generation interval and higher selection intensity can be obtained by this new procedure. All these possibilities are very important to breeding programs under tropical conditions. © 2003 Elsevier B.V. All rights reserved