Technical Abstract: The E-Screen assay was used to evaluate the estrogenicity of sugar beet by-products obtained from a dairy farm experiencing low success rates of embryo transfer. The beet tailings had ~ 3 fold the estradiol equivalents of the pelleted beet pulp (3.9 and 1.2 µg estradiol equivalents or E2Eq/kg dry matter, respectively). Whole sugar beets, sugar beet pellets and shreds from several Midwest US locations were also evaluated by E-Screen. All pellets examined were found to have some estrogenic activity (range ~ 0.1 – 2.0 µg E2Eq/kg DM) with a mean of 0.46 µg/kg dry matter and median of 0.28 µg/kg dry matter. Relative E2Eq ranked as follows: pellets > shreds > most unprocessed roots. Using recommended feeding levels and conservative absorption estimates (10%), the estrogenic activity in the original samples could result in blood estradiol equivalents = those found at estrus (10 pg/mL, cows). Chemical analyses revealed no known phytoestrogens, but the estrogenic mycotoxin, zearalenone, was found in 15 of 21 samples. Of significance to those using the E-Screen, are our findings that contradict previous reports: ß-sitosterol has no proliferative effect and genistein’s glucuronidated form – genistin – is equal to genistein in proliferative effect. The latter is the result of deconjugation of genistin to genistein in the presence of fetal bovine serum (determined by LC MSMS). These data show the usefulness and caveats of the E-Screen in evaluation of feedstuffs, and indicate a potential for sugar beet by-products to contain zearalenone at concentrations that may impact reproduction.