Sudangrass is more similar to US commercial sorghums than to cultivated sorghums from Africa sequence-wise and contain significantly lower dhurrin than sorghums. CYP79A1 is linked to dhurrin content in sorghum. Sudangrass [Sorghum sudanense (Piper) Stapf] is a hybrid between grain sorghum and its wild relative S. bicolor ssp. verticilliflorum and is grown as a forage crop due to its high biomass production and low dhurrin content compared to sorghum. In this study, we sequenced the sudangrass genome and showed that the assembled genome was 715.95 Mb with 35,243 protein-coding genes. Phylogenetic analysis with whole genome proteomes demonstrated that the sudangrass genome was more similar to US commercial sorghums than to its wild relatives and cultivated sorghums from Africa. We confirmed that at seedling stage, sudangrass accessions contained significantly lower dhurrin as measured by hydrocyanic acid potential (HCN-p) than cultivated sorghum accessions. Genome-wide association study identified a QTL most tightly associated with HCN-p and the linked SNPs were located in the 3' UTR of Sobic.001G012300 which encodes CYP79A1, the enzyme that catalyzes the first step of dhurrin biosynthesis. As in other grasses such as maize and rice, we also found that copia/gypsy long terminal repeat (LTR) retrotransposons were more abundant in cultivated than in wild sorghums, implying that crop domestication in the grasses was accompanied by increased copia/gypsy LTR retrotransposon insertions in the genomes.