Current information is inadequate to define those environmental variables most important in alfalfa (Medicago sativa L.) summer dry matter production and those associated with successive yield declines in the Northern U.S.A. Such knowledge is useful in alfalfa modelling strategies requiring the use of production-environmental relationships and in management stategies aimed at optimizing production throughout the season. The purpose of this study was to define for a three-harvest management system those environmental variables most important to early and late summer production and those related to summer yield declines (i.e., summer slump). Multiple regression analysis was used on 21 years of climate-yield data from the University of Minnesota Agricultural Experiment Station at Rosemount to delineate those important variables in the first three production years. Early summer yields were positively and negatively affected by solar radiation and wind, respectively, accounting for 24% of the explained variability in the first production year. In the second production year, minimum air temperature had a negative effect on early summer yield and accounted for 37% of the variability in yields. Potential evapotranspiration (PET) had a positive effect on yield and accounted for 33% of the variability in the third production year. Late summer yields in the first production year were dependent on the early summer moisture ratio of precipitation to PET. The negative relationship indicated that a dry period preceding late summer growth was beneficial to yields. Minimum air temperatures had a positive effect on yields in the second production year and accounted for 48% of the yield variability. In the third production year, precipitation accounted for 61% of the yield variability and had a positive effect on yields. Reduction in yields from spring to early summer harvest was most closely associated with the occurrence of dry springs and high minimum air temperatures in the early summer period. The yield reduction between successive summer harvests was associated with low precipitation in the late summer period. Results indicate that the relative importance of environmental variables to yield varies as the stand ages and that their effect can change over the course of the season (i.e., the effect of minimum air temperature on yield was negative in early summer and positive in late summer). These considerations should be accounted for in alfalfa growth models.