## 4 Discussion  
This study is important in sustaining the ongoing effort in TCC improvement, especially in low-resource breeding programs and off-site breeding stations. While the generation of the adequate calibration set from African germplasm that has trait values derived from a standard reference method such as HPLC might not be visible in the near future, the use of long-existing references from proxy alternatives for the calibration of the newly adopted near-infrared spectrometers will benefit the ongoing effort in TCC evaluation and genetic improvement. This study helps in developing strategies for the efficient phenotyping transition to NIRS technology, particularly the adoption of the portable NIRS. The early sampling approach needed for adequate sample homogeneity relies on the use of an electric-based blender [14; 15] or a special construction for field-based mashing of roots [16]. The calibration results, especially the correlation between the predicted and actual values using a validation set from both iCheck and Chroma Meter for TCC evaluation as well as DMC, with r values ranging from 0.76 in iCheck to 0.97 in Chroma Meter using PLS in both cases, are robust enough for the assessment of these traits based on the portable NIRS [12; 30; 31]. The results are comparable to earlier calibration efforts in cassava roots for these traits, although calibrations from fine-ground and homogenized samples provided higher calibration performance [15; 16; 24]. The use of random forest, a non-linear model for calibration, has been of benefit in some cases [20; 21], but the benefit over the conventional PLS model was not obvious in this study. The correlation between predicted values from the two models was highly correlated (r > 0.92), especially the correlation between the predicted DMC values in the validation set (r = 0.97). The range of values for both TCC and DMC was similar to those of previous studies and represents the range of values in most of the base germplasm collection of the African Consultative Group on International Agricultural Research (CGIAR) and National Agricultural Research Systems (NARS) centers [19; 23; 32]. For advanced populations with higher trait values, updating the models is always recommendable. There was an adequate mix of white- and yellow-fleshed roots in the calibration dataset, although iCheck provides more quantitative values. Quantitative evaluation techniques for both total and individual carotenoids are very important in the biofortification and genetic advancement efforts in cassava [17; 33; 34]. Figure 5: Correlation between actual and predicted DMC values using PLS and RF calibration models in the training (**a**) and validation sets (**b**). The negative correlation between TCC from the two evaluation methods and DMC has been reported several times in the African cassava germplasm compared to what has been reported in the South American germplasm [19; 34; 35]. There are ongoing efforts to understand and design an appropriate breeding approach in reversing this trend, as this has implications on cassava variety adoption [35; 36]. An ideal variety needs to meet both the target high carotenoids levels as well as the desired quality attributes sought by cassava consumers. Meeting this goal will require extensive and well-designed crosses, evaluation, and selection schemes which further highlight the importance of cheap but rapid and standardized evaluation protocols. The two proxy methods for TCC had high repeatability precision, with PRSD values higher than the corresponding RSD (Figure 2) and HorRat values falling within the fully acceptable \(0.3\leq\text{HorRat}\leq 1\) region [25; 27]. Therefore, these proxy methods, according to AOAC standards, are acceptable and repeatable since repeatability precision is inversely proportional to random errors [25]. Moreover, these methods have been shown to have a high correlation with the standard HPLC method [13; 18; 36].