The effect of stocking rate on forage growth has attracted much research attention in forage science. Findings show that forage growth may be affected by stocking rate, and there is a consensus that high stocking rates lead to soil compaction, which could also in turn affect forage growth because of the changing soil hydrology and increased soil impedance to forage root penetration. In this study we used a modeling approach to investigate the effect of stocking rates on the growth of sand-bluestem forage at Fort Supply, Oklahoma. The GPFARM-Range model, which was originally developed and validated for Cheyenne, Wyoming, was recalibrated and enhanced to simulate soil compaction effects on forage growth at Fort Supply. Simulations without the consideration of soil compaction effects overestimated the forage growth under high stocking rate conditions (mean bias [MBE]  =  −591 kg · ha⁻¹), and the agreement between the simulated and observed forage growth was poor (Willmott's d  =  0.47). The implementation in the model of soil compaction effects associated with high stocking rates reduced the bias (MBE  =  −222 kg · ha⁻¹) and improved the overall agreement between the observed and the simulated forage growth (d  =  0.68). It was concluded that forage growth under increasing soil compaction could be predicted provided such sensitivities are included in forage growth models.