Asian Soybean Rust - Impact of cultural practices.

Despite technology available to manage Asian soybean rust (SBR), the possibility of the disease will arrive in Iowa this year has some farmers thinking about reducing their soybean acreage. However, alternating corn and soybean typically results in a higher net return and lower variability than continuous corn or soybeans.

Crop rotation

The financial effects of crop rotations on the whole farm include the net return of the break crop itself, the effect on the variable costs of the continuously grown crop, and the beneficial effect on the yield of the crop following the break crop. These combined financial effects result in the difference in net return of different rotation schemes.

The results of a long-term corn and soybean rotation study I am conducting with Joseph Lauer, agronomist at the University of Wisconsin, show that corn rotated annually with soybean yielded 17 percent more than continuous corn when averaged across 15 years. Sure, there will be years where excellent environmental conditions help second-year corn perform as well as rotated corn. However, who can predict when that will be the case? Corn disease and insect pressure will rise if we increase our corn acreage. It is just a matter of time. Diversity is needed to minimize this risk and to increase our long-term economic sustainability.

What about row spacing, plant population, and planting dates?

It is still too early to say if an SBR epidemic will occur in Iowa in 2006. Therefore, I do not recommend dramatically changing your management practices until we get more experience with this fungus in the United States.

Planting soybeans earlier has also been a topic of discussion. Research from South America has shown the impact of SBR is greatest from flowering (R1) to the end of seed filling (R6). My recommendation remains that growers plant soybean at the optimum time in Iowa, which is the last week of April and the first week of May. Planting date has a bigger influence on the vegetative growth stages than on the reproductive stages. If you want your field to reach harvest maturity earlier, plant an earlier maturity group. However, this not always recommended since the yield of later-maturing varieties can be greater than very early varieties if an early frost is not a factor.

Overall, continue to do what you currently are doing to maximize yield. Plant in the optimum window (last week of April and first week of May), use row spacing narrower than 30-inch spacing, but don’t overseed to generate too dense of a canopy. Scout your fields frequently to monitor for yield-limiting factors such as SCN, Phytophthora, brown stem rot and other common soybean diseases, as well as SBR.

More rust topics

Asian soybean rust - overview
The effect of Asian soybean rust on soybean yield and grain quality
Spray equipment considerations for foliar fungicide application on soybean