Cash rents for some of the most fertile farmland in the United States fell in the second quarter and are likely to decline again between July and September, according to a survey published on Thursday by the Federal Reserve Bank of St. Louis.The quarterly report, which covers the northern delta and southern Midwest region, added that incomes for farmers continued to fall in the April to June period and further decreases are likely in the third quarter.The rural economy has been hit by recent bumper harvests that have pushed grain prices to the lowest level in five years, and by a strong dollar that has hurt exports.