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Photosynthetic Efficiency, Yields, and Leaf Loss in Alfalfa¹

Field and controlled-environment studies were conducted to determine the causes of differential production of alfalfa (Medicago sativa L.) cultivar ‘Vernal’ under four cutting schedules to determine specifically why alfalfa cut three times prior to early September yields more hay than when cut twice per season in the Great Lakes area. Net production, leaf area index, leaf distribution and light interception were measured weekly. Net photosynthetic rates of excised alfalfa leaflets from a controlled-environment chamber and the field were measured in a Warburg respirometer.

Alfalfa cut three times per season yielded 17% more hay than alfalfa cut twice, primarily due to a net leaf loss of 1.19 mt/ha more in the 2- than in the 3-cut schedule. This accounted for two-thirds of the difference in total yield. The remainder, accounting for 4.4% of the total yield, was due in part to higher rates of net photosynthesis in the younger leaves of the 3-cut as compared with the 2:cut schedule.

Net photosynthetic activity of leaves decreased with age. Leaves more than three weeks old were less than one-seventh as active photosynthetically as 5-day old leaves.

LAI reached a maximum of 5.44 during the late bud stage in the first cutting. LAI did not serve as an adequate predictor of seasonal production, but showed a positive relationship to yield for individual cuttings in the frequently-cut alfalfa. Increasing LAI and LI in the second or third cuttings would be an approach to increased yields of alfalfa.

Key Words: photosynthesis • leaf area index

² Formerly graduate assistant in Crop Science (now Associate Professor of Agriculture, Illinois State University, Normal, Illinois) and Professor of Crop Science.

Received for publication January 25, 1967.

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