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Alfalfa (Medicago sativa L.) yield in the first and second years after establishment is typically much greater than yield in subsequent years under dryland production systems in semiarid regions. Alfalfa is a deep-rooted perennial that uses soil water stored at soil depths below the reach of shallow-rooted cereals and grasses. Since alfalfa yield is positively related to evapotranspiration, this study was conducted to determine the relationship between historical alfalfa yield data and weather variables as affected by sward age. Rambler alfalfa yields collated by sward age during cultivar yield trials from 1951 to 1994 at Swift Current, Saskatchewan, were statistically related to monthly precipitation (April to August) and monthly pan evaporation (May to September) during the growing season, and to the fall and winter total precipitation (September to March). One-year-old swards yielded more than 3-, 4- or 5-yr-old swards. For 1- and 2-yr-old alfalfa swards, weather accounted for 50% and 47% of the yield variability, respectively. However, weather accounted for 85, 87 and 96%, respectively, for 3-, 4- and 5-yr-old swards. We hypothesize that soil water stored deep in the profile accounted for much of the remaining yield variability in one and two year old swards. Researchers must measure soil water use from soil depths to at least 3 m when assessing dryland alfalfa yields.