By Sherri Roberts, PAg, Crops Extension Specialist, Weyburn
The majority of pulse crops in this province are desiccated. With the growing number of acres planted to soybeans, some may wonder if this is also a crop that needs to be desiccated. The answer is no. Pre-harvest herbicides do not speed up maturity, nor make soybean seed dry down faster; they only serve to drop remaining leaves and dry out green material. They will kill green tissue such as leaves, stems or pods on soybeans, but will not help speed up maturity or dry down green seed.
One reason to consider desiccating may be that crop quality can be increased because the harvest timing after the field has been sprayed is more predictable, and therefore harvest schedules can be better managed.
Should you decide to desiccate your soybeans, timing is the key. Studies done in the United States show a Gramoxone application that is applied too early will reduce seed weight and yield. Chemical residues can also be taken up into the seed and create marketing problems. If large patches of soybeans in the field remain green, it will be necessary to wait for those areas to mature before spraying. It is safe to spray when seed fill is complete. This can be determined by a colour change of the seed from green to yellow. The occurrence of yellow pods is an excellent indicator that the seed has also changed colour. Since the colour change of the pods is gradual, it may be difficult to determine when the pods have truly "changed" from green to yellow or brown. A change in colour is defined by the absence of green colour.
Another way to assess if the soybean field is ready is determining if the seed has separated from the pod. Once the seeds turn yellow, the next visual change is the seed shrinking inside the pod and separating from the white membrane inside the pod. This indicates that the seed is at full maturity. This test is most accurately done by collecting random pods from the top one-third of the plant throughout the field. If all the seed has separated from the pod, then the crop has reached physiological maturity and can be sprayed.
An additional consideration that needs to be addressed is the pre-harvest interval associated with the desiccation product you may decide to use. Also, if your soybean variety is such that you can legally save the seed, glyphosate should not be used, as it can negatively affect seed germination.