Extension Educator, Local Food Systems and Small Farms
Is there a benefit to planting treated (insecticide and/or fungicide) soybean seed? I'm not sure there is a correct answer, but rather it depends upon several factors. If these protectants do their job, then you could conceivably reduce your planted population, which may or may not save you some money, depending upon the cost of the treatment compared to the cost savings of a reduced population.
There are really two types of fungicides you can select from. One protects against the "water molds", such as pythium and phytophthora. The other protects against rhizoctonia and fusarium seedling diseases. One thing to remember is that these seed treatments are not intended to provide season long protection. They will safeguard the seeds for 2 weeks or so. Metalaxyl will provide help against the water molds. Other products, such as fludioxonil, trifloxystrobin, pyraclostrobin can help against the others.
Which products should you use? Again, it depends upon the situation. The water molds are only found in moist soil conditions. Pythium is favored by cooler soil temperatures while phytophthora is found in warmer soils. Rhizoctonia and fusarium are opportunistic diseases. Which means they infect the plant when it's under stress. This could be from poor growing conditions or herbicide/insect injury or some other concern. Conditions such as early planting, planting into fields that have had incidence of disease in the past, or using poor quality seed may favor the use of a treatment.
Another point to consider is that you can't sell treated seed into the commodity market. Our advice for many years is that if you're going to use treated seed, use that seed first. After all, the first planted fields are more than likely the best candidates. Since it's more likely they will be cooler and wetter than those planted later.
Will insecticide treated seed be of benefit? When insecticide treated seed first became available, soybean aphid was beginning to make itself known. And for those growers who have early season soybean aphid pressure, then the answer is yes. But those areas are primarily in the northern soybean growing regions of the U.S. The soybean aphid does not overwinter here locally. So even though the insecticide treatment is systemic, it won't last season long. The other early season insect that could cause problems is the bean leaf beetle. However, I've only seen a handful of fields that have ever needed protection from early season bean leaf beetle populations.
The best advice on the use of treated seed would be to run some comparisons. Have some treated versus untreated and run them side by side. And better yet, do several side by sides in the same field. Don't compare field to field. The U of I has conducted fungicide treatment studies since 2001. Their results indicated a 0.8 bushel overall response. The response was greater the earlier the beans were planted.