A data set with 5 tabs (TapRoots, FineRoots, WinRHIZO, CN, and TotalFiber) in a Microsoft Excel Workbook that contain data about coarse root biomass and fine root biomass; fine root length, surface area, volume, and root length density; fine root percent carbon (%C), fine root percent N (%N), and C:N ratio; as well as the structural carbohydrates and lignin in coarse roots. The data was collected from the clones of five modern alfalfa genotypes ranging from low to high stem fiber digestibility. It was hypothesized that the root traits of alfalfa genotypes with low stem fiber digestibility would differ from alfalfa genotypes with high stem fiber digestibility.

In May 2021, the clones of five alfalfa genotypes were transplanted from the USDA ARS greenhouse in St. Paul Minnesota at experimental plots located in St. Paul, Minnesota, USA (44°59′14′′ N, 93◦10′24′′ W, elevation 291 m) on June 26, 2024 and Rosemount, Minnesota, USA (44°42'37.3"N, 93°06'10.6"W, elevation 284 m). The soil at St. Paul was a Waukegan silt loam (fine-silty over sandy or sandy-skeletal, mixed, superactive, mesic Typic Hapludolls) and the soil at Rosemount was a Tallula silt loam (coarse-silty, mixed, superactive, mesic Typic Hapludolls). Total annual precipitation in Rosemount accumulated to 606, 648, 803, and 1,014 mm in 2021, 2022, 2023, and 2024, respectively. In St. Paul for the same period the total annual precipitation was 673, 604, 887, and 964 mm.

The experiment was established as a randomized complete block design with three replications at each of the two study locations. The five alfalfa lines were 4351 (low stem fiber digestibility), 55V12 (high standability), Megatron (reduced stem lignin), 54Q32 (high forage quality), and 4016 (high stem fiber digestibility). The 4351 and 4016 genotypes were experimental breeding lines developed by the USDA ARS in St. Paul, Minnesota while the other three genotypes were commercial varieties.

Root data was collected from St. Paul on June 26, 2024 and Rosemount on July 3, 2024. After the second cut at the early bud stage, two randomly selected alfalfa clones per experimental unit were excavated from a 15 cm x 15 cm area around the alfalfa crown using a shovel down to a 30 cm depth to evaluate the tap and coarse roots (diameter >2mm). The crown+tap roots were washed then the tap+coarse (TCo) roots were separated from the crown using a cutting shear. The fresh weight of the TCo roots was recorded, then TCo roots were dried in a forced-air oven at 60°C for 96 hours, and the dry weights were recorded to obtain the TCo root biomass (TapRoots tab).

The structural carbohydrates of the tap+coarse roots (TotalFiber tab) were obtained by passing the samples through a 1 mm screen in a cyclone-type mill (Cyclotec 1093, Tecator, AB, Högnäs, Sweden) for the Uppsala dietary fiber assay (Theander et al., 1995). This wet chemistry method quantified individual cell wall neutral sugars measured as alditol-acetate derivatives by gas chromatography and Klason lignin - as the unhydrolyzed, ash-free residue. The analysis was done in duplicate and chemical composition data were corrected to a 100°C dry matter (DM) basis.

The fine root traits were evaluated by collecting a subset of three alfalfa lines: 4016 (high stem fiber digestibility), 55V12 (high standability), and 4351 (low stem fiber digestibility) genotype. Fine roots were extracted from 4 cores using a 5 cm diameter probe to a depth of 20 cm. The soil cores were combined, the soil removed, and the fine roots washed, weighed, and dried in a forced-air oven at 60°C for 72 hours before re-weighing for dry matter content to obtain fine root biomass (FineRoots tab).

The WinRHIZO data (WinRHIZO tab) was obtained for the fine roots for St. Paul only. These roots were derived from the four cores mentioned in the previous paragraph. The fine roots were imaged on a flatbed scanner, and their length, surface area, and volume were quantified using WinRHIZO software (v2005, Regent Instruments, Montreal, QC, Canada).

The CN data (CN tab) was attained by grinding approximately 10 mg of the fine roots from both locations to 1 mm using a Spex SamplePrep 8000M Mixer/Mill (Cole-Parmer SamplePrep, Metuchen, NJ, USA) for subsequent analysis of total C and N in the fine root tissue via dry combustion at 900°C using a SoliTOC Cube (Elementar Analysensysteme, Hanau, Germany). The C:N ratio was calculated by dividing the C concentration by the N concentration.

Microsoft Excel Workbook Tabs:

TapRoots tab - (6 variables) Project = name of the study; Location = study location name; Variety = alfalfa genotypes evaluated; Rep = replications; PlotID = a unique identifier for each experimental unit; Tap_root_per_plant_dry_weight = the average biomass of tap+coarse roots

FineRoots tab - (5 variables) Project = name of the study; Location = study location name; Variety = alfalfa genotypes evaluated; Rep = replications; Fine roots dry weight = the average biomass of fine roots

WinRHIZO tab - (7 variables) Project = name of the study; Location = study location name; Variety = alfalfa genotypes evaluated; Rep = replications; length_LESSthan2mm = the length of fine roots with a diameter less than 2 mm; SA_LESSthan2mm = the surface area of fine roots with a diameter less than 2 mm; vol_LESSthan2mm = the volume of fine roots with a diameter less than 2 mm. The length_LESSthan2mm value is the sum of four values corresponding to the length of roots in the 0mm to <0.5mm, 0.5mm to <1mm, 1mm to <1.5mm, and 1.5mm to 2mm

CN tab - (6 variables) Project = name of the study; Location = study location name; Variety = alfalfa genotypes evaluated; Rep = replications; cent_N = the percent nitrogen concentration of fine roots; cent_C = the percent carbon concentration of fine roots; CN_ratio = the ratio of carbon to nitrogen in the fine roots

TotalFiber tab - (13 variables) Project = name of the study; Location = study location name; Variety = alfalfa genotypes evaluated; Rep = replications; Avg DM = average dry matter; Avg UA = average uronic acids (mg/g); Avg KL = average Klason lignin (mg/g); Avg RHA = average rhamnose (mg/g); Avg FUC = average fucose (mg/g); Avg ARA = average arabinose (mg/g); Avg XYL = average xylose (mg/g); Avg MAN = average mannose (mg/g); Avg GAL = average galactose (mg/g); Avg GLC = average glucose (mg/g)