By Jacob Bunge And Chelsey Dulaney 

Monsanto Co.'s fiscal first-quarter earnings topped expectations on strength in its soybean-seed segment, but the company warned that declines in global corn acreage would pinch profits in its current quarter.

The world's biggest seed company by sales warned that farmers are changing their planting intentions and tightening spending on farm supplies amid the weakest grain prices in years, translating to lower-than-expected profits in Monsanto's core corn-seed business.

"The issue that we're facing really is reduced [corn] acres," Monsanto Chief Executive Hugh Grant told analysts on a conference call Wednesday.

The St. Louis company gave a disappointing outlook for its second quarter, saying it expects earnings to fall 5% to 10% as farmers plant fewer acres of corn, the company's largest source of seed sales, and plant crops such as soybeans instead. Analysts polled by Thomson Reuters had expected earnings to edge up 1%.

Monsanto shares rose 1.6% to $117.39 in midday trading.

Monsanto said net income in the quarter ended Nov. 30 dropped to $243 million, or 50 cents a share, from $368 million, or 69 cents a share a year earlier. Sales fell 8.7% to $2.87 billion.

Analysts polled by Thomson Reuters expected a per-share profit of 34 cents on revenue of $2.8 billion.

Some farmers in North and South America are shifting acreage from corn to soybeans--a far-smaller business for Monsanto--after a roughly 43% decline in corn prices over the past two years. Though soybean prices also have declined, soybean remains a more-profitable crop than corn for many growers.

Monsanto warned the shift will trim anticipated profits in its seeds-and-genomics division this year, with growth now expected to be in the "high single digits," versus double digits previously.

In the first quarter, Monsanto's corn seed and trait sales fell 12% to $928 million compared with the year-ago quarter.

Sales in Monsanto's soybean seed and traits segment grew 48% to $396 million.

Sales in the agricultural productivity business, which consists of crop-protection products and herbicides, fell 15% to $1.25 billion.

The company in October had forecast earnings about half of what it posted a year earlier as market conditions resulted in lower planted acres for cotton and corn.

Mr. Grant said Monsanto believes corn prices will rebound over time as demand from China for feed grains, as well as continued consumption by ethanol producers, chips away at the massive stockpiles produced by back-to-back bumper U.S. crops.

"I continue to be bullish on corn," Mr. Grant said. "The next season or two, we think that there's going to be a continued acceleration on demand."

Write to Jacob Bunge at jacob.bunge@wsj.com and Chelsey Dulaney at Chelsey.Dulaney@wsj.com

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