What can i give my mare to help her gain weight quickly?

I have a 24 year old appaloosa mare that has been a particularly hard keeper for the past few years. Normally she'll loose weight during the summer but gain it back during the winter, however, this summer that hasn't happened and with only a few more months until winter comes I'm desperate to get as much weight on her as I can. She is currently getting about 13 pounds of pounds plus (which is ground up hay, corn and a little bit of sweet feed mixed together) and two pounds of Purina Ampliy (a rice bran pellet) and I also add about 3/4 cup of vegetable oil to her feed because she gets really bad diareaha if she gets too much. I also started adding a bit of salt to both of my horse's feed to stimulate drinking so they'll stay hydrated since its been so humid lately. She's normally had no problem finishing all the food but today she only ate about half of it and wouldn't eat the rest of it. Is there something else I can give her to help increase her appetite so she'll eat all her food? Or any other suggestions on what to do to help her gain more weight quickly? She probably needs to gain about 200-300 pounds in order to survive the winter, so any suggestions will be greatly appreciated!