Fill your thermos with coffee. Slip on your insulated coveralls. Don your long johns.

That's because planting soybeans earlier than you used to can bag more beans come fall.

"We've planted beans as early as April 20," says Rick Rosenboom, who farms with his family near Herscher, Illinois. "It's worked out pretty well."

Farmers used to ease into soybean planting following corn planting. In Iowa, mid-May with a 55- to 60-degree soil temperature was the recommended planting time. Other states mimicked this guideline.

Good reasons existed. Cold and wet springtime soils spawn seedling diseases. Late-spring frosts can pummel young plants. Bean leaf beetles target early-planted soybeans.

However, odds are that the higher yields you'll garner by planting soybeans early dwarf these drawbacks.

"If you want to aim for 75-bushel yields, plant early," says Jim Specht, a University of Nebraska soybean geneticist and member of the High Yield Team (HYT) expert panel.

If soils are fit to plant, the optimum planting time in central and southern Iowa is the last week of April, says Palle Pedersen, Iowa State University (ISU) Extension agronomist and HYT expert panel member. Northern Iowa farmers may plant soybeans the first week of May.

"There is an 84% probability that early-planted soybeans will yield more than soybeans planted under the old recommendations," says Pedersen. That's based on an average of 32 trials across Iowa since 2003. In ISU tests, yields dipped .40 to .90 bushels per acre per day for each day following the new planting recommendations.

Specht agrees. Nebraska planting date tests he's conducted reveal that yields decline from .25 to .62 bushels for each day following May 1.

Early planting gives plants more time to garner solar radiation that fuels photosynthesis and spurs yield. The resulting early canopy conserves water by nixing evaporation of water close to the soil surface.

Early planting also quickens the time plants take to reach the V1 (first node) stage. Plants that flower 28 to 30 days later key the critical seed-filling stage. The longer the flowering and seed-fill stage, the higher the yield potential, says Specht.

You can't plant early the same way you could with later dates. Cold and wet soils are a disease haven.

"When I was in graduate school, we talked about emergence being a race between the seed and disease and insects," says Jack Walker, an NK Seeds agronomist. That's why he recommends planting good-quality seed into a seedbed that is not excessively wet.

Pests include the overwintering generation of bean leaf beetles that target early-planted soybeans. Specht says that a fungicide and insecticide seed treatment is the best option for systemic protection against fungal seedling disease and bean leaf beetles.

Variety selection is key, as early-planted soybeans are vulnerable to soilborne pathogens like sudden death syndrome (SDS) and stressors like soybean cyst nematode (SCN).

"Planting a variety early with no resistance to SDS and SCN is like playing roulette," says Pedersen.

Late frost is a concern, although not a big one. "I've been an agronomist for 24 years, and very few times has there been a risk from late April planting," says Walker.