How did government policy change American agriculture in the period 1865-1900?
Government policy changed American agriculture during the time that you mention by helping it to expand dramatically.
Part of this expansion was geographical. There were two main government policies that contributed to this expansion. These policies were the Homestead Act and the building of the transcontinental railroad system. Before the Civil War, the American Great Plains were largely empty of farms. Then, the government started to subsidize railroad construction and to give land to settlers. This encouraged many Americans to move west and start to farm in areas that are now very productive.
Part of the expansion came about because farmers became more efficient at producing crops and animals. Some of this was due to technological changes that the government had little to do with. But some of the improvements in farming were due to research into better farming practices. Much of this research was conducted at land grant colleges that had been set up by government action.
In these ways, government policies helped American farmers be more productive and it helped them have access to more land on which to farm.