Why can corn hybrids be planted only once and not saved?

All kinds of organisms produce hybrid advantages through crossbreeding, and the same is true for corn hybridization. Using two different genetic parents (autogamous lines), the progeny obtained through crossbreeding, often more than the original parent, showing a strong advantage. In the case of corn, the first generation of hybrids (F1) regardless of plant height, stems and leaves, root system, male ears, female ears and grains, as well as other drought and flood resistance, resistance to pests and diseases and the ability to resist the collapse of photosynthesis intensity, etc., than the parents are much superior, and thus the yield is also greatly increased, the superiority of these performances, that is, the generally speaking of the advantages of hybrids. This is also the theoretical basis for the use of hybrid advantage. But the hybrid advantage of corn to the first generation (F1) is the strongest. Production if the first generation of hybrids planted after the harvest to stay in the seed, the next build (year) to continue to plant, has been the second generation of hybrids (F2). The second generation of hybrid plants are not uniform in height, the size of the fruit spike is not consistent, the maturity of the early and late is not consistent, the hybrid advantage is significantly weakened, the yield is also greatly reduced. Because the second generation of hybrids (F2) is from the first generation of hybrids free pollination from the genetic basis of the population between the plants, is not the best with the best, but both good and bad, or even close to the genetic basis of the original parent, the parent of the self-intercrossing line, separated from a number of low-yield individuals, so that the yield declined significantly. The second generation of single crosses yield decline at most, other types of hybrids second generation are also showing different degrees of yield reduction. Therefore, corn hybrids can not stay seed, to year after year mating, year after year to utilize the first generation, in order to play a role in increasing yields