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Fall-Grown Cover Crop Evaluations

By Richard Smith and Eric Brennanand

Fall-grown cover crops (planted August-September and incorporated October-November) provide a useful planting slot for a percent of vegetable crop acreage in the Salinas Valley. It is a time when some growers find an opportunity, after two crop rotations, to fit a cover crop in their operations. It has the particular advantage of allowing the grower to incorporate the cover crop and still have time to work the ground when it is still dry before the onset of winter rains.

In Ag Order 4.0 which was approved in April 2021, cover crops that meet the following criteria were granted a credit on the R side of the applied (A) minus removed (R) metric for nitrogen loading in vegetable production fields: 1) a non-legume cover crop grown for ³ 90 days during the winter fallow period (October to April); 2) accumulates more than 4,500 lbs/acre of oven-dry biomass; and 3) has a C:N ratio of ³  20:1 at incorporation. Unfortunately, fall-grown cover crops do not meet these criteria and therefore growers cannot claim a credit when reporting nitrogen loading in their fields.

In the fall of 2021, we conducted six on-farm evaluations of fall-grown cover crops to determine their productivity, nitrogen scavenging capability and C:N ratio at incorporation. Planting dates ranged from August 25 to October 3, and the average days to incorporation was 54 (ranged from 47 to 59). Two barley varieties UC 696 and UC 937, as well as Merced rye were planted in each evaluation. The barley varieties were included because we anticipated that they would reach the heading growth stage more quickly than rye when growers typically terminate cover crops. However, in these evaluations, barley did not reach this stage any faster than Merced rye as measured by the Feekes cereal growth and development scale (Table 1). In 54 days, all cover crops produced more than 4,500 lbs oven-dry biomass and took up from 150 to 161 lbs N/A. The C:N ratios of the cover crops ranged from 13.0 to 13.4.

Fall-grown cover crops grow and mature quickly due to the longer days and warmer weather that they experience in these early planting slots. An important question is, do fall-grown cover crops help to reduce nitrogen leaching during the winter?  To help answer this question, we intend to conduct mineralization studies of the cover crop residue to determine what amount of the residue remains unmineralized after twelve weeks. The nitrogen in the unmineralized portion of the cover crop residue is not immediately susceptible to nitrate leaching and could potentially be deserving of a credit. The Central Coast Regional Water Quality Control Board will update the criteria in the Ag Order each five years based on new scientific information. If there is evidence that fall-grown cover crops can help mitigate nitrate leaching, this may help to justify expanding cover cropping options for growers in the Ag Order. 


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