Strategies of introgressing a transgene into a pig nucleus undergoing mass selection for net merit were deterministically evaluated. They consisted of systematically backcrossing hemizygous transgenic sires to females from the selection nucleus, or vice versa, followed by intercrossing of hemizygous individuals, assuming different levels of heritability (h2), polygenic breeding value of the founder animal, inbreeding depression and constraints in availability of resources. The polygenic breeding value of the founder transgenic animal and inbreeding depression were of negligible importance if backcrossing lasted for at least three generations, and there was little advantage in extending backcrossing much further. The best introgression strategy examined was to backcross selection nucleus sires to hemizygous females, but this was a less efficient strategy in terms of testing transgene effects. Testing the survival of homozygous carriers requires approximately five and 100 matings among hemizygous individuals to detect a reduction in viability of 0.5 and 0.1, respectively. Comparing several candidate transgenes in the first generations of backcrossing is feasible, and does not result in substantial delays in improvement of polygenic breeding value in the selected transgene. If resources are limited, the magnitude of the transgene effect (as a proportion of the mean) that compensates for the genetic lag incurred by its introgression is about 0.1 for most economic traits in pigs. To compensate for less selection while backcrossing and for risk in use, transgenes must have an appreciable effect on economic merit to make their introgression worthwhile, even when the additional costs of transgene production are ignored.
