Selection in Alfalfa for Forage Yield with Three Moisture Levels in Drought Boxes1

Although soil moisture availability is the most limiting factor in crop production in many areas of the United States, little progress has been made in developing cultivars tolerant to moisture stress. The objective of our study was to evaluate a drought box screening procedure for use in alfalfa (Medicago sativa L.) breeding to improve performance under less than optimum moisture conditions. Seven diverse alfalfa cultivars and three germplasm sources were evaluated for forage yield and root characteristics under three moisture treatments in drought boxes in a shadehouse. Divergent selection for forage yield was practiced in the low and intermediate moisture treatments, and for high forage yield in the high moisture treatment. Selected plants were intercrossed within groups and progenies were evaluated under similar conditions. Average forage yield was reduced 30 and 55% in the intermediate and low moisture treatments, respectively, when compared to the high moisture treatment. Entries did not differ for yield in the low moisture treatment, but they varied in their yield response to increasing rates of irrigation. Variability among plants for forage yield was not reduced by moisture treatments. Root weight decreased and root fibrousness increased as moisture stress increased. Entries differed for all root characteristics measured. Forage yield was correlated with all root characteristics, but we concluded that the added information did not justify the time, labor, and expense of root excavation. Selection and evaluation in the intermediate moisture treatment resulted in the greatest progress and differentiation among populations.