A disadvantage of using GMO seeds is that GMO crops strips the soil of microbes. Since the crops are genetically modified, more of the microbes from the soil are used to create a harvest. The soil erosion can be so strong that farmers have to rotate plots of land from year to year to grow cover crops.
Once the cover crops are grown over that piece of farmland, the necessary microbes are replenished within the soil. Examples of cover crops are ryegrass, oats, beans, and peas. Basically, the profit margin for farmers actually goes down from year-to-year due to planting GMO crops manufactured by Monsanto.