How will an early frost
  impact soybean yields?

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We've reached that time of year when an early frost can play a significant role in final yield and quality of a crop. Although usually not as big of a factor with soybeans versus corn, here's a summary of a University of Wisconsin study aimed at quantifying the effects of frost on soybeans.

No significant yield decrease should be expected from adapted soybean varieties (early Group II or I) if plants had reached the R7 growth stage (50% yellow leaves, pods yellowing, and one normal pod on the main stem having reached mature pod color). Beans on a frost damaged plant will mature even if they are green at least as early, if not earlier, than undamaged plants. Plants frosted between the R6 (pod containing a green seed the fills the pod cavity at one of the four uppermost nodes on the main stem with a fully developed leaf) and R7 growth stages may or may not suffer yield reductions depending upon variety maturity rating. Early maturing varieties suffered greater yield reductions at more advanced growth stages than later maturing varieties. In either case, the frosted green leaves will remain on the plant and make harvesting more difficult.

For producers that are planning to feed their soybeans, there may be some concern about the effects of frost on nutritional quality. The Wisconsin research indicates little impact on oil and protein concentrations when a killing frost occurs after the R6 growth stage. However, there may be serious reductions in quality for planting purposes because of low germination percent, seedling vigor, and a high susceptibility to disease.