April 16, 2010

Given the positive yield impact of planting soybeans as early as possible, producers must also recognize that there are three increased risks.

Risk 1. A late spring killing frost.

The probability of damage from a killing frost rises substantively with late April and early May planting dates. The key to assessing this risk is to think in terms of the expected date of seedling emergence rather than the planting date, since damage occurs when emerged soybean tissue is exposed to a freezing air temperature of 32°F.

To learn more about the timing and probability of late spring freezes in your local area, visit the High Plains Regional Climate Center Web Site, click on Historical Climate Data Summaries, select the red box representing the weather station closest your area, then on the left side select Spring Freeze Probabilities, and then Tabular Output. For example, at the Lincoln campus farm, the probability of a late spring 32°F frost is nearly zero on or after May 3, but increases to 50% on April 18, and becomes nearly 100% on March 30 (Table 1).

If you wanted your soybean seedlings to emerge no earlier than May 3, you could plant about seven days earlier (April 26), knowing that at least seven days would likely pass before the seedlings emerged. Given the cool soil temperatures that can prevail in late April, more than seven days will likely pass, but seven days is a conservative choice, given the variance over years in terms of warm or cold spells during this period.

Risk 2. Germination failure.

Today’s soybean varieties have much greater germination cold tolerance than older varieties. Indeed, this is why producers can now push the limits of early planting by sowing the new varieties into 40°F rather than 50°F seed beds. Germination failure in early planting is not so much due to cold temperature, but instead arises from soggy wet conditions coupled with cold temperatures immediately after planting. Soggy soil conditions favor fungal pathogens and, if accompanied by cooler temperatures that slow soybean seed germination, can give pathogens a favorable environment and more time to infect the seedlings before they emerge.

Seed fungicide treatments typically provide good protection against germination failure and seedling loss under such soggy, cold conditions, but if those conditions do not occur, producers typically grumble at the cost of applying an “unneeded” seed treatment. Still, given the potential yield reward that comes with a successful earlier soybean planting, most producers would consider a fungicide seed treatment to protect at least the earliest planted fields a worthwhile “insurance” cost.

Risk 3. The migration of overwintering bean leaf beetles (BLBs) to early planted soybean fields.

These beetles are attracted to the first emergent soybean seedlings of the growing season. They feed on the unifoliolate and trifoliolate leaves for a short time, mate, lay eggs in the soil around the base of soybean plants, and then die. The hatched larvae feed on soybean roots with a preference for root nodules, but little is known about the economic impact of underground feeding injury. In many instances, the seedling leaf feeding injury is not economically significant enough to warrant insecticide treatment; however, bean leaf beetles can transmit the bean pod mottle virus, and if enough plants are infected early, economic yield reductions can occur. If bean leaf beetles are common in your area every spring and incidence of the bean pod mottle virus is high, a systematic insecticide treatment will be necessary to “insure” that the yield potential of early planting is not diminished. For those of you growing soybeans just across the Missouri River from Iowa, see Iowa State University's 2010 prediction for bean leaf beetles in the nearly Iowa counties.

Recommendations

You may be tempted to plant soybeans even earlier than recommended (see Three Reasons Why Soybean Planting Date Matters), perhaps well into early April (and perhaps even March), but this is not recommended. The risk is that an abnormal warm spell after planting may lead to seedlings emerging in April instead of early May. Aside from the now greater risk of a crop-killing late spring freeze, the unifoliolate leaves of those seedlings will sense the much shorter photoperiods of late April instead of the longer photoperiods of mid-May. The resulting strong photoperiodic induction will convert all vegetative meristems in the developing plant stem nodes into reproductive apical meristems, causing the plant to flower at a much earlier date (perhaps in May). This could lead to a very early R5 stage, which in turn would lead to an earlier-than-normal cessation of node production and worse — a shorter reproductive period.

James Specht
Professor of Agronomy and Horticulture