Plants grow in environments that favor their presence. A low area may be high in salts or have a high water table. If that is the case, planting a grass that cannot tolerate salinity or wet soils will not be successful. A soil test will tell you if salinity is a problem. I am guessing your undesirable grass might be tall wheatgrass.
Tall wheatgrass is more tolerant of salinity than many other species.
It also becomes coarse and unpalatable by late spring. Rather than trying to replace it, you may need to adjust your grazing management. Try placing a fence around the area to restrict your animals to the less palatable grass. If they have no choice, they are more likely to eat it. Watch the growth stage of the grass to determine when to start grazing.