The top executive at Monsanto said Wednesday that the wet spring and tougher competition from China has hit sales of its best-selling herbicide and trimmed profit expectations.

CEO Hugh Grant said the volume and price of Chinese rivals to its Roundup range of products had been a "surprise" despite widespread expectations that earnings were set to decline.

Cooler, wetter weather in some parts of the Midwest also has delayed the application timing of Roundup and other glyphosate-based herbicides over the top of Roundup Ready crops this spring.

Grant said the Chinese generic rivals to RoundUp were being priced at around $20 a metric ton versus $30 for its own products.