Pre-harvest management options for wheat
There are two methods of pre-harvest management that can speed up the wheat harvest. Swathing or windrowing, at one time, was the default operation that signaled the beginning of harvest. The primary purpose of swathing is to speed up and even-out the dry down of the crop. However, swathing poses a risk, since grain in the swath is more prone to pre-harvest sprouting if adverse weather delays threshing.
A second pre-harvest option is an application of glyphosate at the hard dough stage. Research has shown that glyphosate applied with or without ammonium sulfate may hasten dry down of the wheat crop if conditions for dry down are adverse. With a pre-harvest interval of 7 days, a couple of days, at the most, may be gained. Since modern varieties allow for it, most wheat and barley is now straight cut instead of swathed. Table 1 summarizes advantages and disadvantages for each of these pre-harvest management options.
Table 1. Advantages and disadvantages of different methods of pre-harvest management.
Optimum time for pre-harvest management
The optimum time for pre-harvest management is right at or just after the crop has reached physiological maturity (PM). This applies regardless of whether the grain is swathed or glyphosate is applied as a pre-harvest treatment. At physiological maturity, the crop has the maximum kernel dry weight and no additional dry matter will be deposited into the grain. Similarly, grain protein cannot be increased after PM through glyphosate applications or other management practices.
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 does not harm grain yield or quality. However, this practice is not recommended when using glyphosate as a pre-harvest tool.
How to identify physiological maturity
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 along the crease of the wheat kernel (Figure 1). Kernels in the same spike will reach physiological maturity at different times, with the middle of the head maturing first.
Another visual indicator is the loss of green from the uppermost internode or peduncle. The uppermost portion of the peduncle, just below the spike, will have turned very light green or yellow at physiological maturity (Figure 2). At this time, transportation of water and nutrients to the head has been cut off and the crop has reached maximum grain fill.
Consequences of swathing or applying glyphosate too early
Swathing or applying glyphosate 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 the crop is cut or treated. Research at North Dakota State University (NDSU) in spring wheat and durum showed that swathing the grain at 45 percent moisture caused a 1 to 2 pound reduction in test weight and about a 10 percent 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.
Varietal differences in pre-harvest sprouting
Hard red spring wheat varieties differ in their resistance to pre-harvest sprouting, which can be concern for swathed grain during adverse weather conditions. This high-temperature dormancy peaks at physiological maturity. Repeated wetting and drying of the grain in a swath or even while standing will degrade this dormancy over time. In addition, the dormancy of some varieties break down sooner than others, potentially resulting in sprout damage. The pre-harvest sprouting ratings for current HRSW varieties can be found in the Minnesota Agricultural Experiment Station Variety Trials. The best remedy to avoid pre-harvest spring damage is to harvest in a timely manner, even if the grain moisture content is above 13.5 percent.