Risks and Rewards: Cover Crops Before Organic Dry Beans.

Organic systems aim to maximize potential benefits of cover crops, such as weed suppression and increased nutrient availability, to ultimately increase yields. Because Michigan is the number one organic dry bean producing state our goal is to determine the most beneficial cover crop for planting prior to dry beans.  To meet this objective, a field experiment was conducted at two locations in Michigan over two years. The cover crops studied included: medium red clover, oilseed radish, and cereal rye; a no cover treatment was also included. Within each cover crop treatment there were four bean varieties: ‘Zorro’ and ‘Black velvet’ black beans and ‘Vista’ and ‘R-99’ (non-nodulating mutant) navy beans. Cover crop biomass was recorded at peak production and included both above and belowground growth. Nitrogen availability was monitored using a chlorophyll meter (V2, R1, and R5), soil samples (fall, planting, V2, R1, R5, and harvest), ion exchange resin strips (changed every 2 weeks), and total N in the beans at harvest.  Dry bean populations (V2 and harvest) and yield were also recorded. Average peak cover crop biomass reached 7,900 kg/ha for clover, 3,900 kg/ha for oilseed radish, and 11,000 kg/ha for rye. The average biomass of weeds in the no cover treatment was 5,400 kg/ha. Interactions between bean variety and cover crop treatment were not observed, thus all varieties were grouped together for analysis. At both the V2, R1, and R5 stages, bean chlorophyll meter readings were highest following a clover cover crop. Later in the season, at R5, beans following a rye cover crop had lower readings than the other covers. Soils samples throughout the season (and ion exchange resin samples, at some timings and locations) also support that there was more available nitrogen following a clover cover crop. A rye cover crop reduced the amount of available soil nitrogen to beans, particularly in the form of nitrate. Bean populations at V2 and at harvest were higher following rye and oilseed radish cover crops than clover and no cover. However, yields were lowest following a rye cover crop, due to nutrient immobilization. Though beans following clover did not have improved yields compared with oilseed radish and no cover, analysis of the grain showed an increase of 6 μg N/mg of grain following clover. One final field season of this research is planned to clarify the impacts of cover crops on organic dry bean production systems.