Monsanto Co, the world's largest seed company, said on Wednesday it plans to divest Syngenta Ag's seeds and genetic traits businesses as well as some overlapping chemistry assets to win regulatory approval for a takeover of its Swiss rival.

"We intend to make this a really clean deal ... really easy to get done," Monsanto President Brett Begemann said in a presentation to investors. He said U.S.-based Monsanto is confident it can address all regulatory concerns about a combination of the agrichemical and seed giants.

Syngenta already has rejected a $45 billion offer, but Monsanto continues to pursue a deal.

"We made an offer," said Begemann. "They rejected that offer, we continue to talk and we'll continue to talk with them and see where we move this along to." He said if Syngenta continues to reject an offer, Monsanto is "not precluded from developing relationships with other chemical suppliers."

Syngenta officials said they did not agree that a sell-off of its seeds business would be sufficient to appease regulators.

"The regulatory hurdles are more challenging than implied by the announcement," a Syngenta spokesman said in a statement.

Syngenta shares rose 4.4 percent to $91.09, while Monsanto shares were off 0.65 percent at $119.13 in midday trade on Wednesday.

Acquiring Syngenta is a compelling need for Monsanto, according to those who follow the industry, as the company known best for its Roundup herbicide and biotech seeds faces mounting threats from both regulatory scrutiny and consumer opposition.

Though Monsanto has sought to diversify its business, the bulk of its profits are tied to its long-held glyphosate-based Roundup herbicide and genetic alterations to seeds that make crops impervious to glyphosate herbicides.

But widespread weed resistance to glyphosate has created problems for farmers, and health concerns about glyphosate and glyphosate-tolerant crops are growing.

"I view the deal as being partly a defensive maneuver around managing the chemical glyphosate exposure," Jefferies analyst Laurence Alexander said.

Alexander said glyphosate chemistry and the related crop traits contribute roughly 70 percent of Monsanto earnings before interest and tax (EBIT).

If Monsanto takes over Syngenta, it would gain a broad portfolio of fungicides, insecticides and other herbicides.

Syngenta's new agrichemical products include one that combats rust disease on soybeans in Brazil, another that uses natural soil bacteria in a seed treatment to fight soybean and sugar beet pests, and a seed treatment that combats pest damage to soybeans, corn and sunflower.

Syngenta's agrichemical portfolio brought in more than $11.3 billion in revenue last year, compared to Monsanto's $5.1 billion from its herbicides. Each company saw total revenue exceeding $15 billion in 2014.

An industry banker familiar with the situation said developing just one new agrichemical can take a decade or more and cost $200 million to $300 million, and Monsanto needs an entire portfolio of new offerings.

"A deal with Syngenta is a necessity for Monsanto's long-term viability as a leading agricultural player," said the banker, who did not want to be quoted by name.

A merger is expected to face opposition from environmental groups as well as farm lobbies who worry that farmers could end up paying higher prices for agricultural inputs.

Monsanto officials say glyphosate remains a key part of the company's future, and that it has signed an expanded commercial licensing and technology agreement with Scotts Miracle-Gro Co to extend its Roundup brand herbicide in the lawn and garden industry, a deal that adds $300 million in gross profit contribution for Monsanto.