The record rise in the New Zealand dollar means farmers are getting much less for commodities sold overseas.
The ANZ Commodity Price Index fell 0.1% in July, but is still 22% higher than a year ago.
However in New Zealand dollar terms, commodity prices fell 3.7%.
ANZ economist Steve Edwards says the dollar's rapid rise has stiffled returns at the farmgate.
The price of kiwifruit rose 7% in the month, followed by price rises in wool and beef. Prices for apples, dairy and aluminium fell.