Summer Cover Crops and Lettuce Planting Time Influence Weed Population, Soil Nitrogen Concentration, and Lettuce Yields
Authors: Kruse Raymond , Nair Ajay 
Published: October 2019

Cover crops can be used as a sustainable weed management tool in crop production systems. Cover crops have the ability to suppress weeds, reduce soil erosion, increase soil organic matter, and improve soil physical, chemical, and biological properties. In the north-central region of the United States, including Iowa, much cover crop research has been conducted in row crop systems, mainly with corn (Zea mays) and soybean (Glycine max) where cover crops are planted at the end of the growing season in September or October. There is little information available on the use of cover crops in vegetable cropping systems, particularly on the use of summer cover crops for fall vegetable production. The choice of the cover crop will significantly impact the entire fall vegetable production enterprise. Vegetable growers need information to identify the right cover crop for a particular slot in the cropping system and to understand how cover crops would affect weed suppression, soil properties, and successive vegetable crop yield. The time interval between cover crop termination and vegetable planting critically affects the growth and successive yield of the vegetable crop. This study investigated how short-duration summer cover crops impact weed suppression, soil properties, and ‘Adriana’ lettuce (Lactuca sativa) yield. The study also examined appropriate planting times of lettuce transplants after soil incorporation of cover crops. The experimental design was a randomized complete block split-plot design with four replications. Whole plots consisted of cover crop treatments: ‘Mancan’ buckwheat (Fagopyrum esculentum), ‘Iron & Clay’ cowpea/southernpea (Vigna unguiculata), black oats (Avena strigosa), ‘Grazex II’ sorghum-sudangrass (Sorghum bicolor ssp. drummondii), and a control (no-cover crop) where weeds were left to grow unchecked. The subplot treatment consisted of two lettuce transplanting times: planted immediately or 8 days after cover crop soil incorporation. Fall-planted butterhead lettuce was used. Data were collected on cover crop biomass, weed biomass, soil nutrient concentration, lettuce growth, and yield. All cover crops significantly reduced weed biomass during the fallow period as compared with the control treatment. Highest degree of weed suppression (90% as compared with the no-cover crop control treatment) was provided by buckwheat. Southernpea, a legume, increased soil nitrogen (N) concentration and contributed to higher lettuce yield and improved quality. Southernpea also enhanced lettuce growth and led to an earlier harvest than other treatments. Sorghum-sudangrass showed evidence of detrimental effects to the marketable lettuce crop. This was not due to N immobilization but presumably due to alleopathic properties. There is no clear pattern within any cover crop treatment that lettuce planting time following cover crop termination affects plant growth; however, planting early or soon after cover crop incorporation ensures more growing degree days and daylight, thus leading to timely harvest of a higher quality product. This study demonstrates that cover crops can successfully be integrated into vegetable cropping systems; however, cover crop selection is critical.