# Mixed-Species Cover Crop Biomass Estimation Using Planet Imagery
## 4 Discussion  
The relationship between seeding rate and biomass yield was explored to test the first hypothesis. A strong linear relationship exists between seeding rate and cover crop biomass regardless of whether single- or mixed-species cover crop planting is employed (Figure 3A). Biomass increase up to a certain seeding rate was expected, but we observed a linear increase in biomass up until the highest seeding rate, 280 kg ha\({}^{-1}\), in our study. A combination of seeding mixes was used at four study sites where multi-species mixtures were kept at low and high seeding rates (Table 1, Study sites 2 and 3). While seeding rates were not adjusted from single species to mixed species, and 100% rate of single species were added during the mix (study sites 1 and 4), we observed biomass increase with the mixed species. When reducing seeding rate with the mixed-species cover crop treatment [49], it was observed that the biomass produced with mixed species was similar to or on par with the highest biomass produced with single species. The rate of biomass returns per kg of seeds decreased with a higher seeding rate, and a stable return of around 20-30 kg biomass per kg of seeds was observed after a seeding rate of 135 kg ha\({}^{-1}\) (Figure 3B). Pure live seed counts per square meter of area differ with different cover crop species and could provide a more realistic evaluation compared to seed weight. Biomass was also positively correlated with PLS m\({}^{-2}\) (Figure 3C). The rate of return of biomass by PLS m\({}^{-2}\) decreased with a higher number of PLS and became stable around 1300 PLS m\({}^{-2}\). The PLS count of single cover crop vye was 670 m\({}^{-2}\) (163 kg ha\({}^{-1}\) seeds and 85% germination percentage), while  
Figure 4: Random forest model selection of the most important (**A**) ten out of 164 wavebands and (**B**) ten out of 115 vegetation indices for predicting CC biomass using hyperspectral data. (wv = wavebands, %incMSE = % increase in mean square error (MSE)). rye + clover + vetch was 1972 m\({}^{-2}\). This result supports mixed cover crop being a better option for higher biomass production and supports our first hypothesis that cover crop biomass increases with mixed species. It was reported in [50] that mixed species produced 1.7 times more biomass than single-species monoculture due to the complementarity effect. therefore, seeding rate recommendations on mixed species should consider not only the biomass produced by a higher seeding rate, but also an economic analysis associated with seed cost, as well as ecological service and functional performance as described by [49] for effective nutrient, moisture, weed suppression and carbon addition to the cropping system. Due to the smaller dataset (fewer study sites, similar planting date, and harvest date), the relationship between GDD and biomass reported in this study is not conclusive. Our second hypothesis was evaluated using the correlation strength of bands and VIs collected during different time periods with biomass yield. Across the study sites, cover crop biomass correlation with bands and VIs during the month of March was higher compared to that in November and April. During the month of November, ground cover by the cover crop was not sufficient due to the emergence and the small cover crop size, while during April, the reproductive stage (flowering, heading, and leaf yellowing due to translocation of photosynthates from leaves to seeds) with mixed-species cover crops might have contributed to the lower correlation. Vegetation index values increased with time from November to the first half of April, and decreased in the second half of April; however, the highest correlation was observed with VIs derived from the March imagery. Therefore, our second hypothesis--Planet imagery collected during the later growth stage is more strongly correlated with cover crop biomass--was rejected, although the trend showed a relationship until the month of March.