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Response of Birdsfoot Trefoil and Alfalfa to Various Levels of Shade¹

In 1963 and 1964, birdsfoot trefiol (Lotus corniculatus L.) and alfalfa (Medicago sativa L.) were grown in the field under full sunlight, and under 51, 76, and 92% shade. Growth of the two species was evaluated in terms of relative growth rate and other attibutes of classical growth analyses.

Relative growth rate (RGR, g/g/week) of birdsfoot trefoil was greater than that of alfalfa under all shading intensities due to a greater leaf area ratio (LAR, dm²/g plant weight). Net assimilation rate (NAR, g/dm²/week) of alfalfa was greater than that of birdsfoot trefoil at high light intensities, but similar at low light intensities. The greater NAR of alfalfa at high light intensities was more than offset by the greater LAR of trefoil. The greater RGR of birdsfoot trefoil did not overcome the initial advantage of alfalfa for greater seed size and initial advantage of alfalfa for greater seed size and initial amount of phgotosynthetic tissue. This adavantage was directly responsible for the final greater yields of alfalfa.

Formulas were derived expressing LAR as the product of leafiness by weight of a plant (leaf weight/plant weight, LWR) and the leaf area per unit of leaf weight (leaf area/ leaf weight, LALW)., Dirrerences iin LAR between species were due to differences in LWR, and those between shading intensities were due to differences in LALW.

Shading reduced yield and plant height proportionately more for alfalfa than for birdsfoot trefoil. It reduced mean area of individual leaves of both species in 1 year, but only slightly affected it in the other. Shading did not affect stands of legumes.

² Agronomist, Crops Research Division, ARS, USDA.

Received for publication August 23, 1965.