Summer Sorghum Cover Crop Reduces Fall Lettuce Growth and Yield

As part of the third year of a summer cover crop study for fall organic vegetable production in the Gulf States of the U.S., leaf lettuce (Lactuca sativa L.) was grown in Baton Rouge, LA and Crystal Springs, MS following summer cover crops. In this third year of a study, four cover crops were raised for during summer 2013: sorghum-sudan (Sorghum X drummondii var. Southland Honey Pasture hybrid), sesame ((Sesamum indicum L. var. sesame), sunn hemp (Crotalaria juncea L. var. sun hemp), and a sesame/sunn hemp blend.  After cover crop incorporation in September, lettuce was transplanted at 12 inch spacing onto raised beds 30 to 35 days after the cover crops were flail mowed and incorporated. Prior to transplanting, four rates of broadcast, incorporated composted chicken (Gallus gallus) broiler litter compost were superimposed on each cover crop plot as part of a split plot arrangement. The litter was incorporated and the soil cultivated to form narrow, raised rows 42 inches apart. Four week-old lettuce transplants were set into these rows 12 inches apart and watered in. At both locations, lettuce growth appeared to be impaired when grown after sorghum-sudan within the first two weeks after transplanting. This retarded growth continued through harvest. Increasing rates of CBL helped lettuce to partially overcome the negative effects of the sorghum-sudan but not completely. Almost no marketable lettuce was harvested from sorghum-sudan plots. Sorghum-sudan has well known allelopathic properties and this study provided very sound, regionally-specific evidence of this allelopathy. It also provided significant local educational opportunities for growers and others that might be considering using sorghum sudan as a summer cover crop for lettuce and perhaps other fall vegetable crops.