The Need to Integrate Plant Populations Into Cover Crop Seeding Recommendations: Case Study with Two Oilseed Radish Cultivars with Contrasting Seed Masses

Seed size, seed mass, and seeding rate have been shown to have profound impact on the performance of plants, yet those characteristics have not been integrated into most cover crop recommendations. Most recommendations continue to call for lb/A or kg/ha as the seeding rate despite the large variations in seed size among cultivars and sometimes among seed lots of the same cultivar.  A study using two oilseed radish cultivars with contrasting seed masses was conducted to demonstrate the need for using plant populations as the most appropriate seeding recommendation.  The cultivars were ‘Defender’ with small seeds (13.9 g/1,000 seeds) and ‘Daikon’ with large seeds (20.4 g/1,000 seeds).  Planting densities were 444,416; 333,312; and 222,208 seeds/A (14, 10, 7 lb/A, respectively for Defender and 20, 15, 10 lb/A, respectively for Daikon).  Both cultivars had a germination rate greater than 95%.  The experiment was conducted in Hamilton, Michigan on a high organic matter (muck) soil in 2008 used previously for celery production.  A randomized complete block design with 8 replications was used. The cover crops were planted on 30 April using a no-till drill with row spacing of 9 inch.  The cover crops were mowed and incorporated at flowering stage on 13 June. Celery ‘Dutchess’ was planted on 1 July. Cover crop biomass was estimated on 13 June by destructively harvesting plants in a 50 by 50 cm quadrat placed randomly in each plot.  Biomass production of both cultivars showed significant response to planting density.  Increasing planting density caused high intraspecific completion which resulted in reduced biomass.  The extent of the response varied between cultivars with Defender exhibiting the steepest curve.  Total biomass decreased from 4.2 to 2.5 T/ha for Defender and from 4.1 to 3.3 T/A for Daikon when the seeding rated increased from 222,208 to 444,416 seeds/A.  For cultivar Daikon the decline in biomass with increasing plant population was less dramatic suggesting a greater resilience to intraspecific competition, likely due to larger seed mass.  This study suggests that in situations where the primary objective is to maximize biomass production by oilseed radish, seeding rate should take into account seed mass to mitigate effects of cultivar and seed lots.  This study also raises the broader need to integrate plant population/density into cover crop seeding recommendations.