When you spray herbicide in water or liquid fertilizer, good coverage of the weed is important. There is a recommended range of gallons per acre of water and recommended pressure range on the nozzle for effective results. When herbicide is applied in granular fertilizer, the herbicide is mostly going on the soil. Moist soil or rain will move the herbicide into the ground where the roots can take up the herbicide. Seems to me that most of the results has to come from the residual chemical instead of the 2, 4-D. Unless the granular fertilizer is spread with a very heavy dew or in the rain. Think about spraying 2,4-D on the ground around a weed, will the 2,4-d kill the weed from the soil?
The herbicide manufacturer says that weed control is less compared to conventional application.