There are two methods of pre-harvest management for wheat that can speed up harvest. Swathing or windrowing is one method. An application of glyphosate is a second option. Several brands of glyphosate are labeled for preharvest weed control. Research has shown that glyphosate can also quicken the dry down of the wheat crop if conditions for dry down are adverse. The preharvest interval for preharvest glyphosate is seven days and expects only to gain a couple of days at the most. More time can be gained with swathing.
The optimum time for either pre-harvest management tool is right at or just after physiological maturity of the crop. At physiological maturity, the crop has the maximum kernel dry weight and no additional dry matter will be deposited in the grain. The kernel moisture percentage at physiological maturity is relatively high and can vary from 20 to 40%. Research has shown that swathing just before physiological maturity does not harm the grain yield or quality. This practice, however, is not recommended when using glyphosate as a pre-harvest tool.
There are two visual indicators that can be used to determine whether the crop has reached physiological maturity. The first indicator is the loss of green in the kernel and the appearance of a dark layer of cells or pigment strand along the crease of the wheat kernel (Photos 1). Kernels in the same spike will reach physiological maturity at different times with the middle of the head maturing first.Photo 1 - Wheat kernels before (above) and at (below) physiological maturity.
Photo 2 - Wheat spikes before (left) and at (right) physiological maturity
Another visual indicator is the loss of green from the peduncle and glumes. If the peduncle just below the head becomes straw-colored, transportation of water and nutrients to the head has been cut off and the crop has reached physiological maturity (Photo 2). The advantages and disadvantages of pre-harvest glyphosate and swathing are listed in Table 1.
Table 1 - Advantages and disadvantages of different methods of pre-harvest management.