The average kernel moisture content at black layer or physiological maturity is 30-35 percent. At this point, the ear has reached maximum dry matter accumulation or dry matter content.
On the average grain loses moisture at the rate of 0.6 to 0.7 point per day for the first half of September, and 0.4 to 0.5 point per day for the last half of September. Weather conditions and hybrid differences affect these “average” numbers.
Source: Illinois Agronomy Handbook