D.C. Code § 9-1209.03
All horses, harness, horse-drawn vehicles necessary for use in connection with construction and supervision of sewer, street, street lighting, road work, and street-cleaning work, including maintenance of said horses and harness, and maintenance and repair of said vehicles, and purchase of all necessary articles and supplies in connection therewith, or on construction and repair of buildings and bridges, or any general or special engineering or construction work authorized by District of Columbia appropriations, may be purchased, hired, and maintained, and motortrucks may be hired exclusively to carry into effect said appropriations, when ordered by the Mayor of the District of Columbia in writing; and all such expenditures necessary for the proper execution of said work, exclusive of personal services, shall be paid from and equitably charged against the sums appropriated for said work; and the Mayor in the budget estimates shall report the number of horses, vehicles, and harness purchased, and horses and vehicles hired, and the sums paid for same, and out of what appropriation; and all horses owned or maintained by the District shall, so far as may be practicable, be provided for in stables owned or operated by said District; provided, that such horses, horse-drawn vehicles, and carts as may be temporarily needed for hauling and excavating material in connection with works authorized by appropriations may be temporarily employed for such purposes under the conditions named in § 9-1209.02 in relation to the employment of laborers, skilled laborers, and mechanics.
D.C. Code § 9-1209.03