My guess is that she is not getting enough fats/oils in her diet, but unfortunately you say she is overweight, so adding those would probably not be such a good idea. Hopefully our resident nutrition experts could help you. The only thing I can think of is to use heavy-duty conditioners.
They've been on M30 for about 4 months now. My black horse looks great on it, but she just doesn't shine anywhere, and has had this straw like mane/tail condition for a couple or 3 years now.
I was just avoiding the topical suggestions in case there was something else like copper deficiency or such that someone would come up with first. I find topicals tend to just collect dirt, too, but I may go that way if it continues to bug me.
"As a rule we disbelieve all the facts and theories for which we have no use."- William James
~~~~~~~~~~~~~~~~ Proud member of the Wheat Loss Clique.
What I do with mine is wash and condition weekly. If the tail isn't terribly dirty, I just condition, and then rinse. It collects much less dirt than the leave-in topicals. Plus it helps protect the hair from breakage, so you lose less hair when they are swishing at flies.
I've also noticed a huge improvement in my mare's tail since I put her on SmartHoof. Her diet was excellent before, but the SmartHoof has helped a lot. She also no longer gets thrush. So you might try something like that for a couple months or so. SmartHoof is pretty inexpensive and is flax based, so you're getting some fat and omega fatty acids in addition to the copper, zinc, and biotin.