Nutrient Management Research Database

Cover cropping is an age old agricultural practice that provides benefits to cropping systems such as increasing nitrogen supply to subsequent crops, conserving and cycling nutrients (Ingles et al. 1994), improving soil physical properties (Lal et al. 1991) and reducing erosion (Meisinger et al. 1991). However, cover crops have disadvantages that limit their use, such as direct cash costs and lost opportunity while the cover crop is growing. In the Salinas Valley, cover crops are utilized on about 5% of the acreage (Tom Hearne, personal communication). However, growers are interested in cultural practices that allow them to improve lettuce rotations and that may suppress Sclerotinia minor, Lettuce Drop, the key soilborne disease. Unfortunately, given economic pressures such as high land rents and lower returns for rotational crops, effective rotations are not always possible. Mustard cover crops (Brassica and Sinapis spp.) have been researched for a number of years in Europe, Australia and the Pacific Northwest as a means of suppressing nematodes and certain soilborne diseases, and they are now being examined in the Salinas Valley for benefits that they may provide lettuce rotations.

The objectives of this study were to:
    1) Evaluate the impact of mustard cover crops on lettuce drop prevalance and weed               growth.
    2) Evaluate how mustard cover crops influence the nitrogen cycle.

To achieve objective 1, both field and greenhouse experiments were conducted. The field experiments compared the use of mustard blend Caliete 119 to Merced Rye and leaving soil fallow in managing lettuce drop and weed growth. The greenhouse study investigated the effects of mustard and Merced rye residue incorporation on weed emergence.

Objective 2 was achieved by conducting three seperate field tirals. Trial 1 analyzed a variety of mustard mixes for total biomass and nitrogen content. Trial 2 compared a variety of cover crop mixes (mustard of several varieties, Merced rye, Cayuse oats, and a legume mix in terms of total biomass production, N content, and post-incorporation mineralization rates.

• Mustard cover crops showed a small decrease in the prevalence of lettuce drop. Weed emergence was also suppressed by using a mustard cover crop. However, the authors are careful to note that these results are preliminary.
• Mustard cover crops accumulated large amounts of N in their biomass, as high 250 lbs N/ac by maturity.
• The use of mustard cover crops resulted in faster N mineralization rates than cereals, but slower than legumes.
• The Indian mustard cover crop improved the yield and total N content of 40-day spinach crop following cover crop incorporation.