Winter wheat is approaching maturity and early planted barley is not far behind and therefore, harvest will soon be upon us. Glyphosate can be used as a pre-harvest aid in small grains to control green weeds and to speed up uniform ripening of the crop. Control of perennial and other green weeds with glyphosate just prior to harvest is usually disappointing as weeds at this time are tall, nearing maturity and slow-growing. Glyphosate has been shown to reduce the amount of time that it takes for a crop to reach harvest moisture if conditions are not favorable for drying. The reduction in the time to harvest is usually on the order of a couple of days, however. Since glyphosate is a systemic herbicide it can take from seven to ten days to effectively kill the growing parts of the crop, and therefore the dry down process is not immediately visible. Applying glyphosate too early can reduce yield, test weight and seed germination. Because germination of the developing kernel can be affected when glyphosate is applied too early, it should not be applied to fields that will be used for seed or on barley intended for malt. The optimum time to apply glyphosate pre-harvest is when the crop has reached physiological maturity. For most varieties of wheat this occurs at a grain moisture content of about 30% (20% moisture is the recommended timing for barley). At this moisture content the grain will be in the hard dough stage and if you run your thumb nail across the kernel, the indentation will remain. Since not all kernels arrive at physiological maturity at the same time, be sure to sample multiple kernels to be sure that you are not spraying too early. A visual indicator that can also be used to determine physiological maturity is when the peduncle, the portion of the stem just below the spike, turns from green to yellow. Jochum Wiersma (University of Minnesota Extension Service) has published photos that nicely show these differences. Pre-harvest applications of glyphosate must be made at least seven days before harvest. Always read and follow label directions.
(Source – http://www.farms.com/news/glyphosate-as-a-pre-harvest-aid-in-small-grains-79208.aspx)