This time of year there are many questions about harvest management and the use of pre-harvest chemicals in crops. Often the questions asked include “Should I desiccate my crop?” and “What should I spray and when is the best time to spray my crop?” In order to answer these questions, it is important to understand the difference between spraying a crop desiccant such as diquat and using a pre-harvest glyphosate for harvest management.
A crop desiccant is used when a crop or other green material needs to be dried down quickly for harvesting. Under good growing conditions, desiccants dry down the crop in four to seven days as well as any weeds that may be the field. It is important to note that a desiccant such as diquat is a contact herbicide. This means that the herbicide will not move through the plant and therefore it requires good coverage of the plant surface when sprayed to work effectively. However, there is no control of perennial weeds in fields when using a desiccant and crops can re-grow if there is any late season moisture after the plants have already dried down.
Pre-harvest glyphosate is often used as a good perennial weed control tool later in the season for some troublesome weeds such as Canada thistle, perennial sow thistle, quack grass and dandelions. Pre-harvest glyphosate can also help with uneven maturity crops and will assist with drying down weeds so they are easier to harvest. Dry down with pre-harvest glyphosate occurs at a much slower rate than desiccants, and generally occurs 10 to 14 days after spraying under good growing conditions. The longer time period is needed to allow the herbicide to move throughout the plant and into the roots.
Neither a crop desiccant nor the use of pre-harvest glyphosate will help the crop to finish maturing. Rather both diquat and glyphosate will dry plants down at whatever stage of maturity they are sprayed at. It is important that the majority of the seed in the stand has reached physiological maturity to reduce the risk of potential yield loss. The stage at which a seed has reached physiological maturity is different for each crop. For example, most cereal crops reach this stage and are ready for pre-harvest glyphosate application when the seeds are at the hard dough stage. This can be determined by pressing a kernel with your thumbnail. If the line from the impression remains on the seed then it is ready to be sprayed. In pulse crops such as peas, a crop desiccant is generally used when all of the bottom pods are ripe and the seeds are detached within those pods.
The decision whether or not to use a desiccant or preharvest glyphosate also depends on factors other than weed control and targeted dry down periods. It is important to know the pre-harvest intervals for applying glyphosate or a desiccant to a crop. This could affect the level of Maximum Residues Levels (MRL) in the crop and could impact where and how the crop can be marketed. If grain is going to be used for seed, then pre-harvest glyphosate should not be applied in order to minimize any negative impacts on seed germination and plant stand establishment the following year. Alternatively, swathing is another tool that can also be used to help dry crops for harvest but also comes with risks if conditions become wet. Each option comes with its own advantages and disadvantages and what works one year may not be the best solution the next year. Therefore it is important to evaluate each field individually to see which option will provide the best overall results and fit within your operation.