For what it's worth, the decline in use of horses was much slower than you might expect. The model T Ford motor car reached peak production in 1925 [0], and for an inexact comparison (I couldn't find numbers for the US) the horse population of France started to decline in 1935, but didn't drop below 80% of its historical peak until the late 1940's down to 10% of its peak by the 1970's [1].

[0] https://en.wikipedia.org/wiki/Ford_Model_T#Mass_production

[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC7023172/