There is research that suggests that rice bran has toxic levels of inorganic arsenic (from pesticides) arsenic also occurs naturally (organic arsenic) and rice is more susceptible to organic arsenic from the earth due to the way it's cultivated in paddies -so there has been concern about rice bran and arsenic due to the fact that it has higher levels of naturally occurring arsenic than other types of grains.
That said regardless of the arsenic it is the RICE BRAN that is good for putting on weight not the arsenic. The arsenic is not a 'secret ingredient' that the government adds to the feeds to put weight on! LOL!
Rice bran is widely used in both human and animal feeds whether we like it or not - it's actually amazing how often it's used and what it's in.
All though arsenic isn't good organic or (especially) inorganic millions of us consume it every day in water,bran /rice products etc. It's just there it's hard to get away from it organic or otherwise. Unless you /we are buying organic foods most of the foods we eat have pesticides which has arsenic in it. Don't really like to think about but it's out there!
If it were me and my horse I would put her back on the Senior feed. There is a good chance that there is a rice bran derivative in the feed but it is in most feeds. I agree with what Reining_Lover said as well.
I wouldn't worry about it - she'll be fine =;->
Edited to add that - most Senior feeds are made with a lower 'sugar' content as part of the nutritional make up for a Senior horse. If you are concerned about molasses most companies make lower sugar feeds that are appropriate for horses in all stages of life.