Jan. 20 (Bloomberg) -- Southern Copper Corp. fell the most in three weeks as a slump in commodity prices dimmed the earnings prospects for Peru’s biggest producer of the metal.

Southern Copper, a unit of Grupo Mexico SAB, retreated 1.4 percent to $35.30 today in Lima, the biggest drop since Dec. 28. The stock gained 8.5 percent in the previous three days.

“Investors were buying ahead of next week’s earnings release as they’re betting on a strong quarter,” said Janet Chong, a trader at Lima-based brokerage Latinoamericana SAB. “The decline in copper prices reversed the trend.”

Copper fell as much as 1.8 percent in New York, trimming a second weekly advance, after a Chinese purchasing managers’ index released by HSBC Holdings Plc and Markit Economics showed a preliminary reading of 48.8 for January. The dividing line between contraction and expansion is 50.

Southern Copper probably had full-year profit of $2.39 billion, according to the average of six analyst estimates compiled by Bloomberg, from $1.55 billion the previous year.

Copper, poised for its best January since 2004, also dropped today as a stronger dollar reduced the metal’s appeal as an alternative asset and as some investors capitalized on copper’s advance to a four-month high.

--Editors: James Attwood, Brendan Walsh

To contact the reporter on this story: Alexander Emery in Lima at aemery1@bloomberg.net

To contact the editor responsible for this story: David Papadopoulos at papadopoulos@bloomberg.net