WASHINGTON » Monsanto Co. reported better-than-expected earnings results for the third quarter on Wednesday as executives of the huge agricultural business continued to make a case for a $45 billion takeover of Swiss competitor Syngenta AG.

St. Louis-based Monsanto reported earnings of $2.39 per share on stronger revenue from crop chemicals, compared with results of $1.62 per share in the prior-year period.

The average estimate of eight analysts surveyed by Zacks Investment Research was $2.05 per share.

But Edward Jones analyst Matt Arnold said the earnings gain was driven by a one-time licensing payment from Scotts Miracle Gro, which sells Monsanto’s signature weed killer Roundup to consumers.

“The company’s underlying operating performance was more mixed,” Arnold said. He noted that Monsanto maintained its full-year earnings guidance at the lower end of its range of $5.75 to $6.

Its shares dropped $6.46, or 5.7 percent, to finish at $106.32 in trading Wednesday.

Monsanto reiterated its goal to more than double its 2014 earnings per share by 2019, emphasizing the potential benefits of a combination with Swiss chemical maker Syngenta, which has three times rejected its unsolicited offers.

“Our proposal to combine with Syngenta is an exciting logical next step for our business, offering the opportunity to accelerate innovation and support a more diverse group of farmers around the world,” CEO Hugh Grant said in a statement.

Syngenta provided a more detailed explanation this week of its concerns with Monsanto’s takeover offer. In a video posted to its website, Chairman Michel Demari said the offer undervalues the pesticide maker’s business, which “they are trying to buy on the cheap.” He also reiterated concerns that Monsanto is underestimating the antitrust challenges.

Monsanto tried to sweeten the offer for shareholders earlier this month, adding a $2 billion guarantee should the proposal fall apart. Syngenta said the proposal remains “inadequate.”

Basel-based Syngenta is the world’s largest crop chemical producer and its acquisition would help Monsanto diversify its offerings beyond Roundup. Sales of that chemical have been hurt in recent months by concerns about its safety when used in large-scale industrial farming.

In March the International Agency for Research on Cancer labeled Roundup’s key ingredient, glyphosate, a “probable carcinogen.” The company has demanded a retraction from the group, a French research arm of the World Health Organization. The group has no regulatory powers and no commercial sanctions are expected as a result of the ruling.

The U.S. Environmental Protection Agency, which makes its own determinations, said it would consider the French agency’s evaluation.

For the period ended May 31, Monsanto said, revenue rose 8 percent to $4.58 billion, but missed Wall Street forecasts. Four analysts surveyed by Zacks expected $4.65 billion.

Sales were hurt by decreased demand for the company’s best-selling product, biotech corn seeds, as farmers turned to soybeans and other crops. More farmers are favoring soybeans because they cost less to grow and can withstand broader weather variations.

Monsanto’s biotech seeds have genetically engineered traits that help farmers increase their crop yield, despite their higher costs.

Weakness in seed sales was offset by growth in the company’s chemical business, including a $274 million payment from Scotts Miracle Gro.

Monsanto shares have decreased roughly 6 percent since the beginning of the year, while the Standard & Poor’s 500 index has increased 3 percent. The stock has dropped 7 percent in the past 12 months.