I have not had to deal with grass stains in a while. But we have red dirt that stains everything and cats that can make stuff smelly.
I pour equal parts of Tide w/ bleach and Cascade liquid detergent in the washer. Let it wash just for a second to get it mixed up good, put clothes in, and let it all soak for a couple hours. I have forgotten things and left them in overnight. Then run through regular wash cycle. My recipe calls for 1 cup of each, but I adjust according to number of items. You would not need to use that much for just 2 pairs of jeans obviously. I also put spray and wash or shout on every stain first, no matter how I wash it.