The shear elastic constants cθ=(c11-c12)2 and c44 have been measured in DySb. c44 shows a small softening of about 0.6% at the phase transition, while cθ=(c11-c12)2 exhibits a 58% drop. The large change in cθ=(c11-c12)2 occurs over a temperature range of more than 200 °K. The dominant mechanism for the softening of the elastic constant cθ=(c11-c12)2 is a Jahn-Teller strain coupling. Strong-biquadratic pair interactions are present which compete with the strain coupling.