Improving low‐pH and high‐aluminum field tolerance in alfalfa ( <i>Medicago sativa</i> L.)

Abstract Alfalfa ( Medicago sativa L.) is the fourth most valuable field crop in the United States. Alfalfa production is limited in regions predominated by acidic soils, including the southeastern United States. The purpose of this study was to characterize the effect of low‐pH soils on yield and fall dormancy ratings and select top performers for the next breeding cycle. A population of 133 genotypes was selected from 966 diverse accessions in the National Plant Germplasm System collection evaluated for 3 years in a low‐pH field environment. Half‐sib families resulting from intermating these selections were evaluated in row plots in a natural low‐pH soil (4.9–5.2, extractable aluminum = 10.41–11.38 mg kg −1 ) and an adjusted‐pH soil (pH = 6.37–7.07, extractable aluminum = 0.01–2.29 mg kg −1 ) across six environments (year–location combinations). The results revealed that acid stress significantly reduced yield by 61 g dry matter per cut per plot ( p &lt; 0.001) and fall dormancy ratings by 1.47 classes ( p &lt; 0.01). The adjusted‐pH field condition did not show significantly higher survivorship compared to the low‐pH field during the relatively short study period. An Acid Soil Adaptation Index based on yield data across low and adjusted‐pH conditions identified 56 acid‐tolerant half‐sib families in at least 4 of the 6 tested environments, indicating high stability. Despite significant variability across environments and genotype × environment interactions, high heritability estimates were observed for yield (0.80–0.90) and acid soil tolerance (0.854), suggesting that selections from this germplasm panel can be leveraged to develop regionally adapted, acid and aluminum‐tolerant cultivars.