Adapting Field Crop Seeding Rates for Organic Rotational Tillage Systems in the Mid-South.

A limited body of research in organic field crop systems generally supports the premise that higher crop seeding rates relative to seeding rates recommended for conventional production systems are beneficial to crop yields and weed control.  Higher crop seeding rates and stand density may result in quicker canopy closure and greater efficiency of resource use by the crop species early in crop growth stages, leaving fewer available niches for weed colonization and reproduction. Increased seeding rates may also compensate for greater crop seed and seedling mortality due to lack of synthetic seed protection inputs. However, environmental conditions typical to organic systems are different and more heterogeneous compared to conventional systems, suggesting that a number of interacting cropping system factors (e.g., tillage system, existing weed pressure, cultivar choice, soil fertility, crop rotation, etc.) may influence the impact of seeding rate/stand density on crop performance to a greater extent.  Field studies have been implemented at the Organic Crops Unit at the University of Tennessee to evaluate response of organic field crop systems to increased seeding rates and stand densities. In late maturity group soybean, data over two field seasons suggests that for high residue, reduced tillage organic systems, narrow row spacings and seeding rates 1.5 and 2 times the typical recommendation of 370,500 seeds ha^-1 can increase yields by 29 and 45%, respectively (from 2,744 kg ha^-1 to 3,533 and 3,971 kg ha^-1). Early season crop populations were typically near 50% of seeding rates, suggesting that biological (e.g. pathogens and other pests) and environmental conditions in this system can limit stand establishment to a greater extent than is typical in conventional systems. Impacts of increased seeding rates on weed competition were less clear due to the relatively high level of weed suppression observed from the cover crop residue in this production system. Research in winter wheat, maize, grain sorghum, forage cowpea, and perennial forage mixture seeding rates and stand densities is also in progress at the University of Tennessee’s Organic Crops Unit.