I am not sure that I understand your question correctly, but given more precise sunflower density estimation, the farmer has three options:
1. Plow the field and seed again (same or different variety or grain). This is a very crude measure, but it is sometimes the right thing to do, because as I said most of the expenses have not been realized yet (fertilizer, pesticide, fuel, payroll, paying rent for the land). It is also a time critical decision, because the window of opportunity for plowing and reseeding is not very wide.
2. Accept the lower yield if it is within a reasonable margin (e.g. comparable to the expenses to plow and reseed).
3. Do partial reseeding over the existing plants (without plowing). This is an emerging strategy with the proliferation of smart seeders, but it requires a precise seeding map to be created beforehand (i.e. based on the density estimate). As an advantage, you spare the expenses for seeds and plowing, however there is some disadvantage as well, due to the different rate of development of the newly seeded plants. Farmers usually need plants to be ready for harvest at the same time, otherwise the quality of the grains suffers and hence the selling price is lower.
In addition to these points, having precise density information after germination helps with the identification of problems, such as seeder malfunction (e.g. nozzles getting clogged), seed quality and meteo data (e.g. too much rain, low temperatures etc).
In the EU they use fertilizers and pesticides, but rarely use irrigation. However, pesticides are usually applied over the lifecycle of the plants. For fertilizer, there is some value to apply it on time, because it tends to migrate with water and applying now vs a month ago is not equivalent.
