When your rice land becomes inundated with salt water and how long it stays waterlogged you have not mentioned. On the other hand the soil salinity level of your crop land is unknown. That is why appropriable technical suggestion is very difficult. If you want to grow fodder crop you have to conduct with Bangladesh Livestock Research Institute who deals with fodder crops. So far we understand you need salt tolerant fodder variety. We did not study the performance of Nepiar grass against salimity. So, we are not able to advise you to cultivate Nepiar grass instead of rice.