Swathing or windrowing of wheat, barley and oats were, at one time, the default operations that signaled the beginning of harvest. The primary purpose of swathing is to speed up and even out the dry down of the crop. Swathing always posed a risk as grain in the swath is more prone to preharvest sprouting if threshing is delayed due to adverse weather
Therefore, most wheat and barley is now straight cut in large part because modern varieties allow for it. Preharvest applications of glyphosate have further reduced need to swath wheat. In oats swathing remains more common place.
Swathing is becoming, however, something of a lost art. First, you have to decide when the crop is ready to be swath. The optimum time to swath is when the crop has reached physiological maturity. This is the same time to consider the application glyphosate. This is the point in the development when the crop has reached its maximum dry weight and the grainfill period has come to an end. Moisture content of the grain will vary but the ranges from 30 to 40 percent. In the absence of a moisture meter, there are other cues that signal the crop has reached physiological maturity. One of the easiest is to look at the color of the uppermost internode, or peduncle. The upper most portion of the peduncle, just below the spike or panicle, will have turned very light green to yellow when the crop reaches physiological maturity. There still may be some green in the canopy below or in the glumes but the least mature kernels will no green left in them, when threshed out by hand, .
Swathing before the crop reaches physiological maturity will result in yield and test weight losses and green kernels in the harvested grains. The losses get progressively worse the earlier you cut the crop. Research at NDSU in spring wheat and durum showed that swathing the grain at 45% moisture caused a 1 to 2 lb reduction in test weight and about a 10% reduction in grain yield. Swathing after physiological maturity increases the risk of shattering and will equally cause yield losses but no losses in grain quality. Shattering losses can be reduced by swathing in the early morning or late evening when some dew is present in the crop.