Hello JoJo :)
Sugar Beet tends to
fill horses up which may add to the reason why your horse dosent want to eat much! Instead, why not try
fibrebeet if your wanting something to add to chop/fibre cubes. Also, what type of feed are you using? is it boring and bland or is it really tasty and sweet smelling? Once you have feed sorted, try adding a small amount of Molasses onto the top which is basically just like golden syrup to us! If this dosent work, maybe Soya Oil could do the trick. The reason i mention Soya oil is the fact that it is
very fatty. Soya oil is basically an oil for horses which does a number of things.. it helps the horses joints which would be perfect for your veteran, it gives the horse a glossy coat and if fed in large amounts gives the horse abit more energy so be careful on how much you give!
I tend to feed my horses haylage, the difference between hay and haylage is that hay is cut off the fields and then left to dry out where as haylage is cut from the field and then wrapped up to keep moist and tasty! Haylage contains alot more sugar as opposed to hay and is therefore alot tastier to most horses! I would certainly give haylage a try if your horse is a fussy feeder!
Good Luck!!!

Let me know how you get on.
Sarah xx