By Jim Herbek and Chad Lee, Kentucky Pest News

The potential threat of Asian soybean rust this year is on the minds of soybean producers. In discussions with producers, some are considering a change in their production practices because of the potential threat of this disease.

There are still many uncertainties about soybean rust and its impact including: our lack of experience in dealing with this disease; or, when it may arrive during the growing season (early or late); or, if it will even be present in Kentucky; or, how widespread and severe its distribution will be; or, if the weather will be favorable for the disease. If rust becomes an issue the only certain management strategy is proper fungicide applications (products, timing, coverage, etc.). Some other management strategies may be effective, but without further evidence or experience, a change in production methods is not a recommended strategy.

Most production factors will not affect whether a field does or does not become infected with soybean rust; however, a change in certain production practices could reduce yield potential, even if soybean rust is not present. The best management approach for this disease is to use production practices that maximize yield potential. If rust occurs, the additional costs for fungicides will be a worthwhile investment to protect that high yield potential.

Production practice changes being considered by some producers are: soybean varieties/maturity groups, planting date, row spacing, and plant populations.

Soybean Variety/Maturity Group: Currently, there are no varieties available with resistance to soybean rust and it will be at least 5 to 10 years before resistant varieties are developed. Therefore, select varieties with maximum yield potential, based on performance tests, from maturity groups that are adapted to your area or region. In Kentucky, adapted varieties include those from late MG III, MG IV, and early MG V for various regions of the state.

While there may be some logic for selecting varieties from ultra-early maturity groups to possibly reduce the impact of soybean rust (or to reduce the number of needed spray applications), the yield potential of these unadapted maturity groups is reduced. Even without soybean rust, their yield typically would be less than that of varieties from adapted maturity groups.

Planting Date: Plant during the optimum planting period. Begin planting when soil temperature is at least 60-65° F to promote rapid emergence and uniform stands. This usually occurs from late April to early May in Kentucky. Complete planting by early June to avoid a yield decline.

While there may be some logic for extremely early plantings (before soil temperature reaches 60° F) to reduce the impact of soybean rust, planting date studies show no yield advantage (and often a yield loss) for extremely early plantings over traditional planting dates. With very early plantings, stand uniformity and plant vigor is often reduced.

Spreading out planting dates within the optimum planting period (along with some variation in soybean variety maturity) is a good practice that would result in differential stages of soybean development among your fields. If soybean rust occurred, this would provide a better opportunity (particularly with limited sprayer capacity) to spray soybean acres in the time necessary to protect yield. Based on experiences in Brazil, and unless our experience in the U.S. proves otherwise, significant rust infection usually does not occur until R1 (beginning bloom) or later and fungicide applications prior to R1 are usually not beneficial.

A University of Kentucky publication (AGR-184) predicts when soybeans will first flower (growth stage R1) based on different planting dates and maturity groups. It can also be found at the Grain Crops Extension website: www.uky.edu/Ag/Agronomy/GrainCrops/.

Row Spacing: There was a 12-15% average yield advantage for narrow rows (15 inches or less) in previous row spacing research at the University of Kentucky. This yield advantage was greater in high-yield environments and also greater and more consistent in late (double-crop) plantings. The great majority of soybeans are planted in narrow rows in Kentucky.

Some producers are considering a switch back to 30-inch rows to better facilitate equipment for late-season spray applications. The yield loss of run-over rows from sprayer traffic (which will vary according to sprayer boom width) in narrow row soybeans will be much less than the yield gained for using narrow rows. A good alternative, by coordinating planter and sprayer size, is to establish skip-rows (unplanted rows) in your narrow row system to facilitate wheel tracks for late-season spray applications. The yield loss for unplanted rows would be very minimal (or non-existent) with a slight savings in seed costs.

Even without soybean rust, wide rows would have a 5 to 6 bu/acre yield disadvantage. If soybean rust occurs, the yield advantage for narrow rows should more than compensate the cost of a fungicide application to protect that yield gain.

Plant Populations: It is not certain what effect plant density may have on soybean rust; most likely it will not be significant. However, many soybean fields are planted using excessive seeding rates. Now may be a good time to reduce seeding costs, without sacrificing yield.

Studies have shown that soybean plant populations as low as 100,000 plants per acre, in many cases, yield as well as 200,000 plants per acre. Reducing seeding rates to achieve final plant densities of 110,000 to 130,000 plants per acre would result in a seed cost savings of over $10 per acre, particularly with higher priced seed. If soybean rust occurs, the seed cost savings could be better invested in fungicides.

In summary, without further evidence, or until our experience in the United States proves otherwise, the best management approach for soybean rust is to use production practices that maximize yield potential. To change certain production practices because of the potential threat of soybean rust, would likely result in reduced yield potential. This inherent loss in yield potential would occur, irregardless of whether soybean rust became an issue this year. It would be more justifiable to maintain those production practices that maximize yield potential. If rust occurs, then additional costs for fungicides would be a good investment to protect that maximum yield potential.