Abstract Knowing a cover crop's carbon to nitrogen (C/N) ratio can help growers time termination to optimize N release. But growers lack simple methods to predict important C/N ratios like 20, an often‐cited “tipping point” between N mineralization and immobilization. We evaluated if Feekes growth stage, days after planting (DAP), and growing degree days (GDD) could predict C/N ratio changes of cereal cover crops. We tracked C/N changes monthly in rye ( Secale cereale L. ‘Merced’) and triticale (x Triticosecale Wittmack, ‘Pacheco’) in five biweekly trials sown from October 1 to November 30, 2021, and related these changes to farmer‐friendly metrics: DAP, GDD, and development. Growth stage and GDD were strongly correlated, as were DAP and C/N. The C/N ratios (1) increased with increasing GDD, DAP, and growth stage, (2) were similar for both cereals, and (3) became more variable over time. With exception of one trial, planted on November 15 where N was more limited, shoot biomass reached a C/N ratio of 20 when October‐planted cover crops were at Feekes 10.1 (heading, 97–103 DAP) and Feekes 8 (flag leaf just visible, 94–98 DAP) in November plantings. Excluding the 15 Nov. trial, fitted regressio curves up to ≈150 DAP showed that a C/N of 20 occurred by 94 to 102 DAP, for October‐ and November‐plantings, respectively. Accumulated GDD to reach a C/N of 20 varied with planting date ranging from approximately 650 to 900 GDD, and tended to decrease with later plantings. Our data illustrate the value and complexity of using growth stage, GDD, and DAP to predict the C/N ratio changes in cover crops. This will help farmers optimize cover crop management and comply with a new California regulation to reduce N leaching to groundwater.
Brennan et al. (Fri,) studied this question.