Low Residue Cover Crops Evaluated for Use for Intensive Vegetable Production.

The Salinas Valley is an intensive vegetable production area which supplies over 90% of US lettuce from April to October. On average >2.0 crops/ha/year are produced.  High land rents and the need for keeping ground open during the winter to accommodate spring planting schedules have limited the use of winter cover crops. In addition, high residual soil nitrate levels in the soil at the end of the production season and the lack of deep-rooted winter rotation crops have resulted in substantial nitrate losses from leaching and runoff during the winter storm events. This project evaluated the use of low-residue winter cover crops which have potential to allow the use of a cover crop while not disrupting spring planting operations. Trials were conducted during the winters of 2008-09 and 2009-10.  Cover crops were either broadcast on fallow beds or planted in the furrow, grown for 55-70 days and then killed with an herbicide before they produced too much residue that would disrupt subsequent tillage and lettuce planting operations.  Two cover crop species were evaluated: rye ‘Merced’ which was broadcast on winter beds and winter dormant varieties of triticale ‘Trios 102’ in 2008-09 and ‘888’ in 2009-10 which were seeded in the furrow bottoms. The cover crop treatments were compared with bare fallow and evaluated for the following: biomass production, nitrogen uptake, nitrate leaching, quantity of runoff and the content of nutrients and sediment in the runoff. The trials were conducted with cooperating growers in winter-fallow commercial vegetable production fields. Rye ‘Merced’ grew more rapidly than triticale; it was killed at 55 days after germination (DAG) with glyphosate and reached 1.08 and 1.94 metric tons/ha in 2008-09 and 2009-10, respectively. Triticale was killed at 70 DAG in 2008-09 and 55 DAG in 2009-10 and reached 0.76 and 0.63 metric tons/ha, respectively.  Nitrogen uptake by the cover crops was modest and was highest in rye in 2009-10 which reached 81 kg N/ha.  Levels of nitrate-N in the soil in the fall at the beginning of the 2009-10 trial were high (36 ppm), but the grower applied 181 kg of 15-15-15 which further elevated residual soil nitrate-N levels (72 ppm). A series of winter storms in mid-January 2010 leached a large proportion of nitrate-N from the top foot of soil from all treatments. Cover crop biomass accumulated for 13 days following glyphosate treatment then began to decline steadily due to decomposition until the cover crop was incorporated on March 22, 2010 (66 days after glyphosate treatment). As cover crop biomass decomposed, the quantity of biomass nitrogen also declined.  Nitrate leaching data indicated that the low residue cover crops initially reduced nitrate leaching, but nitrate released by decomposition was lost in subsequent rain events. Surface runoff from rye and triticale plots was 94% and 80% less than the bare fallow, respectively. Prior to planting, the furrows were chiseled and the beds lillistoned, and cover crop residue did not impede either operation. These results indicate that low residue cover crops can be used successfully in intensive vegetable production systems without disrupting planting schedules. They are capable of initially sequestering modest amounts of residual soil nitrate, but are most effective in reducing runoff and sediment loss during winter storm events.