High levels of cereal rye biomass are necessary for adequate weed suppression when reducing or eliminating herbicide use in cover crop-based rotational tillage soybean production. Residual fall soil nitrogen levels influence cover crop biomass levels. If fall fertility levels are not adequate, growers can either supplement cereal rye with a spring nitrogen fertilizer application or use tillage to incorporate the cover crop prior to soybean establishment.  Early spring predictive cereal rye phenological traits that effectively estimate mature cereal rye biomass and responsiveness to nitrogen fertilizer applications can aid with farmer decision making. We tested the effects of fall and spring soil N levels on cereal rye biomass accumulation and the relationships between spring shoot density and mature cereal rye biomass. Rye biomass without fall or spring fertilizer was 4700 kg ha^-1 and increased by 25% with spring applied N fertilizer. Fall fertility can have a larger impact than spring on cereal rye biomass but this depends on fall planting date. At tillering, cereal rye shoot density corresponded with mature cover crop biomass. Future work should include evaluation of economic trade-offs between cost of cover crop fertilization and increase in crop yield from improved weed management.