For a horse that has a "kidney sensitivity", you can supplement it with plant extracts with a diuretic effect. Indeed, stimulating the kidneys allows them to support them.
There are many food supplements in this section.
Personally, I am particularly fond of Equistro Elimination which contains orthosiphon. With this product you can carry out regular cures at the rate of 50 to 100 g per day in 7-10 day cures.
Here is the product link : https://www.cheval-energy.com/fr/complement-alimentaire-chevaux-foie-rein/195-equistro-elimination.html
Do not hesitate to come back to me if you need more information.
Have a good day.
Marc, the veterinarian of the site cheval-energy.com