The optimum time for pre-harvest management in small grains is right at or just after the crop reaches physiological maturity (PM). This applies regardless of whether you swath the grain or apply glyphosate as a pre-harvest treatment. At PM, the crop has the maximum kernel dry weight. No additional dry matter will be deposited into the grain. The kernel moisture percentage at physiological maturity is relatively high, and can vary from 20 to 40 percent. Research has shown that swathing just before PM doesn’t harm grain yield or quality. However, this practice isn’t recommended when using glyphosate as a pre-harvest tool.
There are two visual indicators you can use to determine whether the crop has reached PM:
1) Loss of green from the kernel - The first indicator is the loss of green in the kernel, and the appearance of a dark layer of cells or pigment along the crease (Figure 1). Kernels in the same spike will reach physiological maturity at different times, with the middle of the head maturing first.
Source : umn.edu