"In the country, meanwhile, as the press forecast a host of benefits farmers would reap from motor vehicles, rural residents got over their earlier distaste for cars enough to consider putting horses out to pasture. [RLA: she means that metaphorically. I'm sure they were mostly slaughtered.] For one thing, switching to cars would free millions of acres -- 100 million just in the U.S. -- devoted to horse fodder: horses consumed 40 percent of the U.S. grain crop. The car would 'remove the last serious obstacle to the farmer's success,' said one writer in Harper's Weekly in 1907. 'It will market his surplus product, restore the value of his lands, and greatly extend the scope and pleasure of all phases of country life.' By eliminating isolation, said the press, cars would encourage succeeding generations to stay on the farm. The car's engine, with the strength of several horses in one small (albeit noisy) package, would ease farm labor. Motor vehicles would move food to market more cheaply, giving consumers lower prices as farmers gained more profits -- the best of all possible worlds."
In practice, of course, all that extra grain being available meant that food prices dropped and the farmer got screwed, simultaneously with having increased their capital costs. They now had to buy fertilizer (since they no longer had horses conveniently producing manure for them), as well. Also, they didn't need the kids to work on the farm, plus the car made it easier for the kids to leave.
In fact, while Alvord doesn't come right out and say it, in some ways, this was a big contributor to the over supply of food that caused catastrophically low prices for food that made the Depression so very, very painful.
Alvord's selective history of our relationship with cars (she's really serious about the titular metaphor) is remarkably even handed and accurate. She does a nice job covering the health problems associated with muscle powered transport (especially horses) in cities, and does not shy away from the issues the population at large had with rail. (Altho she does not adequately characterize the high death toll attributable to rail specifically.)