## 2 Methods  
### Study Area and Sampling Plan  
We sampled 27 cover crop fields (~5 sampling locations per field) over three winter-to-spring seasons (2018-2019, 2019-2020, and 2020-2021) at the USDA-BARC research facility (Figure 1). These fields were planted with one of the three cereal grass cover crop species: wheat (\(n=431\) across 10 fields), 'Aroostok' cereal dye (_Secale cereale_; \(n=153\) across 14 fields), and triticale (_Triticale hexapolide_ Lart.; \(n=185\) across six fields). Cover crops were planted between 26 September and 18 October of a given year, after harvest of the previous summer crop (usually corn, soybeans, or alfalfa (_Medicago sativa_)). Some of the fields--those in triticale and some wheat--were fertilized in the fall, as these fields were harvested later in the spring of the following year (typically in April) for dairy silage production.  
During the 2019-2020 and 2020-2021 seasons, we sampled approximately five locations per field at each sampling time. The sampling dates were distributed throughout the cover crop growth period: once per month from October-February, twice per month from March-April, and once in May. We performed more frequent sampling in the springtime due to the faster cover crop growth and development. Sampling locations were kept consistent within seasons. During the spring of 2019, we collected cover crop biomass twice (late April-early May). We flagged five sampling locations per field with the goal of capturing spatial variability in cover crop performance (i.e., capturing areas of low and high biomass). Across all seasons, we selected sampling locations with locally uniform growth towards the center of each field to avoid edge effects and mixed pixels. Samples were spaced at least 30 m apart to avoid collections within the same pixel.  
Destructive biomass samples were collected near flagged locations within each field. At each flag, we sampled aboveground cover crop biomass from \(\sim\)0.57 m\({}^{-2}\) areas in paper bags (i.e., harvested 1-m length cover crop from three adjacent drilled rows that were 19 cm apart). Paper bags containing fresh biomass samples were then dried in an oven for approximately one week at 60 \({}^{\circ}\)C and the dry weights were recorded. We then scaled these values from g m\({}^{-2}\) to kg ha\({}^{-1}\).  
During the 2019-2020 and 2020-2021 growing seasons, we also collected a series of nadir photographs from a red, green, blue (RGB) Canon PowerShot G16 camera (12.1 megapixels). In previous work, nadir photographs were used to estimate fractional vegetative cover [14]. However, as we did not have complete time series photographs associated with every  
Figure 1: Overview of the study area located at the Beltsville Agricultural Research Center (BARC) in Maryland (**A**,**B**). Study fields (2019–2021) are colored by species. Some fields (purple) were used by wheat and triticale in different years of the study. Approximately five flagged locations were sampled within each field at a given sampling time or event (**C**). (**A**) contains a Sentinel-2 satellite image from 1 May 2020; (**B**) uses a map of the United States, courtesy of the Federal Government.  
sampling event, we could not include fractional vegetative cover as a covariate in our analysis. Instead, we used photographs and estimates of fractional vegetative cover where possible to interpret the relationship between the remotely sensed data and the destructively sampled biomass. We calculated fractional vegetative cover for each photograph by applying threshold values to the digital numbers for red/green ratio [50] and excess green [51] VIs.