Dairy Farm Sustainability Act of 2018 This bill amends the Agricultural Act of 2014 to establish a Dairy Farm Sustainability Price Loss Coverage Program. Dairy operations that enter into contracts under the program are eligible to receive payments from the Department of Agriculture (USDA) when milk prices fall below the minimum milk price. The bill specifies that the minimum milk price is $23.34 per hundredweight of milk and requires USDA to adjust the limit for inflation for each half of each calendar year. The bill also reauthorizes the existing Dairy Margin Protection Program through 2023. (The program is a voluntary program that makes payments to participating dairy operations when a formula-based national margin [the difference between the national all milk price and the national average feed cost] falls below a producer-selected insured margin.)