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Alfalfa Yield, Specific Leaf Weight, C0₂ Exchange Rate, and Morphology¹

Alfalfa (Medicago sativa L.) clones differ widely in CO₂ exchange rates (CER) and in specific leaf weights (SLW); CER is correlated with SLW. If high forage yield could be shown to be associated with high CER, selection for high CER or SLW should result in improved forage yield.

We selected plants with extremely high or extremely low SLW from several alfalfa populations and measured forage yields of clones propagated from these plants in the greenhouse and the field. Stems/plant, length and weight of the longest stem, the percentage of leaves on the longest stem, and SLW were determined at some harvests.

The 116 clones studied in four experiments exhibited a wide range of CER, SLW, morphological characteristics, and yield. Correlations between yield and CER or SLW were small and in some cases negative. Yield per dm² of leaf area was more closely correlated with CER (r = 0.26 to 0.28) than was total yield. SLW and CER were positively correlated in all experiments (r = 0.50 to 0.92). Yield was much more closely correlated with stem number, stem weight, and estimated leaf area (r = 0.60 to 0.91) than with CER or SLW. Neither CER nor SLW is an effective selection index for forage yield in alfalfa.

Key Words: Tiller number • Tiller weight • Leaf area • Photosynthesis • Forage production • Medicago sativa L.

² Research agronomist, High Plains Grasslands Res. Stn. USDASEA, Cheyenne, WY 82001; associate professor, Agronomy Dep., Iowa State Univ., Ames, IA 50011; research plant physiologist, USDA-SEA, Beltsville, MD 20705; research agronomist, USDASEA, Beltsville, MD 20705; research geneticist, USDA-SEA, St. Paul, MN 55101; and former research agronomist (retired), USDA-ARS, Beltsville, MD 20705, respectively.

Received for publication September 10, 1977.