Horses remained a common mode of transport and agricultural labor at the dawn of the 20th century before the widespread adoption of automobiles and tractors, making this a matter of common historical observation requiring no specific citation.
The claim is a well-established historical fact regarding transportation and labor modes at the turn of the 20th century. While historical literature discusses industrialization and the transition of power sources, the fact itself is everyday historical knowledge and the retrieved papers do not specifically address this historical transport transition.