Summer annual grass such as forage sorghum, sorghum-sudangrass hybrid, and millet can grow in a broad range of environmental conditions and produce forage in a short growing period, which donates more flexibility in cropping systems over a large part of the US. Summer annual forage crops are beneficial to dairy producers in areas where high quality forage is scarce during summer months. Currently, bmr gene mutant varieties are being marketed targeting the demand of highly digestible summer annual forage crops. Improved feeding values of BMR forage crops due to lower lignin concentrations has been documented (Gerhardt et al., 1994; Grant et al, 1995). Recently, there has been an increase in availability of summer annual BMR varieties.  Some summer annual forage crops were developed using different bmr genes. Evaluation of the different bmr genes and their lines demonstrated significant variations in fiber concentration, lignin, and digestibility (Oliver et al., 2005). Information is limited comparing actual rumen fermentation rates of different bmr gene type forages (sorghum, sudangrass, or sorghum × sudangrass hybrid ). An experiment was conducted to study the in vitro rumen fermentation behaviors of bmr 6 or 12 type summer annuals harvested at vegetative, full bloom, and regrowth using a fermentation gas measuring technique.