I think it is extremely important that when we are making comparisons of the advantages and disadvantages of artificial rearing, as compared with other systems of rearing, that the comparisons should be made on a valid basis.

There are three ways in which the systems can be compared: firstly, they can be compared on a basis of the biological targets of the system. These represent a limit to production in the appropriate systems. Secondly, they can be compared on a basis of maximum performance under laboratory conditions, or in those situations in which scientific personnel have clear control over the system being undertaken. Thirdly, systems can be compared on the basis of the best performance achieved under purely practical conditions. This point can be illustrated by three examples:

1. Number of pigs born per sow per year

Since gestation length cannot be affected, the important factors involved in the number of pigs born per sow per year are: number of pigs born, length of lactation period and the interval between weaning and service.