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Alfalfa (Medicago sativa L.) seed production differs from hay production and requires detailed research to determine appropriate water management strategies. Effects of surface-applied irrigation amount, frequency, and prereproductive soil water content on reproductive development, seed yield, and harvest index were studied for 3 yr in the field. In 1987, 1988, and 1989, six supplemental irrigation treatments representing combinations of two irrigation frequencies (daily and after 75 mm of accumulated evapotranspiration [[ET.sub.c]) and three rates of water replacement (40, 70, and 100% [ET.sub.c]) were used. These six treatments received 200 mm of water prior to foliage clipback in April. In 1988 and 1989, an additional 100 mm of water was applied prior to clipback to an additional three treatments that were irrigated after 75 mm of [ET.sub.c] and that had water replaced at either 40, 70, or 100% [ET.sub.c]. Seed yield response to amount of applied water differed between the establishment and two subsequent production years. For plants receiving 200 mm of water prior to clipback, increasing amounts of applied water decreased seed yield in the establishment year, but optimized yield at 70% [ET.sub.c] in the two subsequent years of production. Frequency of water replacement generally did not affect seed yield when 200 mm of water were applied to the soil prior to clipback. Replacement amount did not affect seed yield when 300 mm of water was applied prior to clipback. Increasing water replacement amounts increased number of floral buds plus racemes but decreased seed pod number through the growing season. Total season-end aboveground phytomass was a positive function of water application amount and was not affected by frequency of application or amount of water applied prior to clipback. Water management can affect alfalfa seed crop response and yield., The effects of different water management strategies on alfalfa seed yield was assessed over a three year period. Six irrigation treatments were used, based on combinations of different amounts of water supplied (40, 70 and 100% of accumulated evapotranspiration) and frequencies of application (daily and after 75 mm of accumulated evapotranspiration). During the first year, 40% accumulated evapotranspiration replacement rate treatments had the highest seed yields. During the second and third years, 70% treatments had the highest seed yields. The frequency of water replacement generally did not affect yield.