The combination sent shares up $6.10, or 5.1 percent, to $126.73 in Wednesday trading.
The company raised the low end of its 2014 forecast to between $5.10 and $5.20 per share. Previously, the company’s estimate was for earnings of $5 to $5.20 per share.
Monsanto Co. said the $10 billion share buyback will take place over the next two years. The company has about $1.1 billion remaining under its previous share-buyback plan.
The St. Louis company said it earned $858 million, or $1.62 per share, in the three months ended May 31. That was down from $909 million, or $1.68 per share, a year ago. Revenue was virtually flat, at $4.25 billion.
Analysts polled by FactSet had expected Monsanto to report earnings of $1.54 per share on revenue of $4.39 billion for the quarter.
The company reported a 16 percent decline in sales of genetically engineered corn seeds, its best-selling product. Farmers in the U.S. and abroad have been switching from corn to more profitable soybeans.