When commercial livestock are crossbreds, profit for the total operation including cost of production of the breeding stocks, is a function of the performance of both the crossbred offspring and their parents.
Since the performance level of the offspring is determined by its parents, it means that the parents contribute to profit determination in two ways. Reproduction cost is determined directly by the parents, that is through their own phenotypes, while productive efficiency (food conversion, growth rate, etc.) and quality are determined by the parents indirectly through the genetic determination of their offspring.
In most classes of livestock the number of males in the breeding herds, relative to that of the females, is very small. Consequently, variation in reproductive capacity of the male line contributes only little to the determination of reproductive costs, and this item can be ignored. As a result, the contributions of the parental lines to profit are unequal and of different kinds. Exploitation of this difference may produce ‘profit heterosis’ even when the component traits are genetically additive.
When profit is a non-linear function of one or more component traits another deviation of profit from the arithmetic mean of the parents is created. Unlike the previous one, this heterosis deviation is scale dependent.
Profit was presented graphically, as profit contours, on the two dimensional plane created by the variables productivity and reproductivity. This was found to be a helpful aid to an understanding of the underlying relationships.