Comparison of Tillage Implements to Incorporate Broadcast Cover Crop Seed for Small-Scale Vegetable Growers

Cover crops play a critical roles in soil and pest management in sustainable production systems for high-value vegetables.  A cover crop’s ability to scavenge nitrogen and suppress weeds early in the season requires an adequate population density of cover crop plants.  A grain drill is the ideal implement to plant cover crops because it plants the seed in evenly spaced rows at a uniform depth.  However, small-scale vegetable growers may not be able to justify the cost of a grain drill, and therefore may use a less-costly planting method such as broadcasting cover crop seed followed by shallow soil incorporation.  Growers need information on (1) the optimal method for shallow incorporation of broadcast cover crop seed, and (2) the seeding rate adjustments for broadcast versus drilled seed.  To address these needs, a study was conducted on a certified organic research farm in Salinas, California to evaluate cover crop population density of drilled seed (Drill), versus broadcast seed that was incorporated with either a tandem disc (Disc), a rototiller (Rototiller), or a spring/spike tooth harrow (Harrow).  The experimental design was a randomized complete block with the four planting methods and five blocks.  The cover crop was a mixture of 50% rye (Secale cereale) and 50% purple vetch (Vicia benghalensis), by seed weight.  The rainfed, winter cover crop was planted in early December at 140 kg ha^-1 on the bed top of 2 m wide beds (furrow to furrow).  Population densities of rye, vetch, and total cover crop were determined at 14 and 47 days after planting (DAP).  At 14 DAP, total cover crop densities were 301 plant m^-2 in the Drill treatment, and significantly less in the all broadcast treatments by 32% (Rototiller), 49% (Disc), and 50% (Harrow); the same patterns occurred for rye and vetch densities.   By 47 DAP, there were no difference in vetch density between treatments, however rye and total densities remained lower in Disc and Harrow treatments.  This study illustrates (1) that a Rototiller is the better implement than a Disc or Harrow to produce an adequate stand with broadcast seed, (2), up to 2 times higher seeding rates are needed for some broadcasting methods than drilling to achieve the same population density, (3) that cover crop densities were less variable in the Drill than broadcast treatments, and (4) that smaller seeds (rye) were more sensitive than vetch seed to soil incorporation method.