The Horse Drinking Trough in detail:
- Accelerates the production time of cavalry units by 1% per level
- Lowers the crop consumption of all Roman cavalry units in that village
- Equites Legati consume one crop less at level 10
- Equites Imperatoris consume one crop less at level 15
- Equites Caesaris consume one crop less at level 20
The level dependent attributes (such as construction costs and generated culture points) can be found here.
For the full table of construction times click this link for server with 1x speed and this link for server with 3x speed.