For direct-seeded crops, seed treatments are the least expensive and highest return application a grower can apply.
Seed treatments can contain bacterial inoculants, fungal inoculants, microbial biostimulants, plant biostimulants, and trace minerals. The most effective treatments usually are a synergistic stack that contains ingredients from several or all of these categories.
Trace mineral seed treatments are most important with poor quality seed that is small in size and light in weight (most commercial corn seed). High-quality seed often contains enough of the more common trace minerals.
Many seeds, at least those grown on healthy parent plants, vector their own symbiotic endophytic microbes, both bacteria and fungi. In addition to those microbes vectored on the seed, seedlings also recruit symbiotic fungi and bacteria from the soil, particularly mycorrhizal fungi, as well as others, even some of the same species they carry along on the outside of the seed.
Applying mycorrhizal fungi and other bacterial inoculants as a seed treatment is generally an effective delivery method to achieve early root colonization, particularly for monocots, where the seed remains in the ground.
For dicotyledon plants, I have always wondered how effective it is to treat the seed when the seed is soon pushed up and out of the ground. Many growers have used seed treatments on these crops effectively, but I suspect we may get better responses from applying them in-furrow right with the seed when the option exists.
One other thought, applying a fungal inoculant on fungicide treated seed doesn’t seem like the brightest idea under the sun. We know the crop does benefit because this is a (surprisingly) common combination. How much bigger might the benefits be if the fungicide was removed and the beneficial fungi permitted to flourish.