Monsanto Co. ’s top executive said the seed giant is struggling in its efforts to engage rival Syngenta AG in deal talks—despite statements last month by his counterpart that the Swiss company was open to discussing a sale.

Monsanto Chief Executive Hugh Grant said a deepening decline in the agricultural industry has strengthened the rationale for combining Monsanto, the world’s largest seed company by sales, and Syngenta, the top seller of pesticides, but “we haven’t seen much progress on that front.”

"I’d say that engagement has been difficult,” Mr. Grant told analysts on a conference call discussing Monsanto’s fiscal first-quarter results on Wednesday.

A spokesman for Syngenta declined to comment.

Seed and pesticide makers, buffeted by a three-year decline in major crop prices that has forced farmers to scale back spending on farm supplies, are pursuing deals as a way to continue growing and cut expenses. The group of so-called Big Six seed and chemical companies already is set to shrink after DuPont Co. and Dow Chemical Co. in December struck a deal to merge and then separate into three companies, one of which will become the world’s largest combined seed and pesticide company by revenue.