My humble 3 suggestions to reduce ammonia in urine:

- Drinking an ample amount of water everyday. If pets are the targets, their food ought to be wet  with water.

- Changing the eating habits. Too much nitrogen in food is bound to increase ammonia in urine of humans & animals; something detected by bad smell.

- Changing some medicines into others & this requires consulting a doctor or veteran.

Is there any herb to decreas the ammonia in urine ?

The answer in the attached link gives tips to decrease ammonia level in human blood & advises the use of herbal tea plus lemon or grapefruit seeds.

I think that drinking plenty of water is a better remedy for many diseases including this one.

http://www.home-remedies-for-you.com/askquestion/8632/remedies-and-cure-to-reduce-ammonia-level-in-blood.html