Rural Energy for America Program (REAP) Improvement Act of 2021 This bill modifies and provides additional funding for the Rural Energy for America Program (REAP), with a particular focus on greenhouse gas reduction. Under the program, the Department of Agriculture (USDA) supports energy efficiency and renewable energy development for agricultural producers and rural small businesses. As part of the selection criteria for a grant or loan from the program, USDA must consider the potential of a project to reduce greenhouse gas emissions and provide other climate benefits. The bill also prohibits USDA from imposing restrictions on the quantity of energy that is generated under a grant or loan. Further, the bill expands the program by making agricultural producer cooperatives, electric cooperatives, and nongovernmental organizations eligible for financial assistance under the program. The bill also (1) increases from 25% to 50% the portion of a project's cost that may be covered by a grant, and (2) establishes a reserve fund to provide grants and other financial assistance related to underutilized renewable energy technologies. In addition, USDA must study dual-use energy systems (i.e., systems where both renewable energy and agricultural production occur on the same piece of land) and ensure that grant and loan recipients receive outreach, technical assistance, and education concerning such systems.