Reuters - Commodities had the best quarter in 18 months as trading for March ended on Monday, with weather and economic growth likely to determine if gas, gold, grains, lean hogs and coffee continue going higher and draw money into the space.

Many energy, metals and agricultural markets started the year with a dismal outlook after a tumultuous 2013 when institutional cash left the sector to chase a rally in equities.

Weeks into the first quarter, however, natural gas and arabica coffee hit multi-year highs from a brutal U.S. winter and worsening Brazil drought, respectively. Gold later surged to a six-month peak on geopolitical fears over Ukraine and hog prices hit record highs as an American pig virus turned deadly.

The Thomson Reuters/Core Commodity CRB Index ended up 8.7 percent for the first three months combined, making it the best quarter since September 2012 for the 19-commodity index. The S&P 500, the Wall Street index which tracks the top 500 U.S. companies, rose only 1.3 percent. (Global asset performance, based in dollars http://link.reuters.com/pat75v)

Analysts think the commodity rallies that have been in force since the start of the year could fade as weather patterns change and markets start trending more modestly with improving global growth.

"We remain neutral on our commodities allocation," said Rob Haworth, senior investment analyst in Seattle for U.S. Bank Wealth Management, which oversees some $115 billion in assets.

"Agricultural commodity prices will likely weaken as current strong prices encourage plantings leading to lower prices," said Haworth. He also expected weaker price trends for precious metals, although cyclical commodities, such as oil and base metals, may turn positive.

Those familiar with how institutional investors allocate to commodities do not think the just-ended quarter had made a big difference to sentiment, or helped much in restoring the $50 billion in total commodity product outflows estimated by Citigroup for last year.

"A lot of the investors who were in commodities were very disappointed with the performance of the last two years and the fact that the risk of inflation has declined so much," said Don Steinbrugge of Agecroft Partners, a Richmond, Virginia-based firm that picks hedge funds in commodities and other asset classes for institutional investors.

"I think we're going to need to see more improvement in prices, a pick up in inflation and a longer positive experience before they increase their allocations," Steinbrugge said.

Arabica coffee rose 61 percent to be the quarter's best performer. Most of its gains were during Brazil's unprecedented dry weather between January and February, which led many analysts to mark down their production estimates from the world's No. 1 coffee grower.

The front-month May arabica coffee contract traded in New York ended at $1.7790 per lb, versus the fourth quarter close of $1.1070.

Arabica, however, has also pared gains lately as some rain finally reached Brazil's coffee belt. In Monday's session, the market was down 1.5 percent from the previous close and traders expect more volative prices ahead of the second quarter harvest.

Lean hog futures rose 48 percent to take the second spot in the quarterly performance.

Hog prices have traded at or near all-time highs since mid-February on worries about longer-term damage from the Porcine Epidemic Diarrhea virus. A U.S. Department of Agriculture report from last Friday said the hog herd as of March 1 stood at 97 percent from a year-ago, showing limited impact from the virus.

Lean hogs' most-active June futures contract opened limit down on Monday, reacting to the report, and finished almost 2 percent down at $1.27175 a lb.

Natural gas and gold, two of the most promising performers at the start of the year, have pared gains too.

The front-month contract for U.S. natural gas finished the quarter just 4 percent higher at $4.369 per million British thermal units after surging 65 percent in late February at the winter peak. The spot price of gold finished up 6.5 percent at above $1,283 an ounce, off the six-month high of $1,391 in mid-March.

The three main U.S. agricultural markets posted double-digit gains for the quarter, with corn up 19 percent, wheat 15 percent higher and soybeans up 11 percent.

Agriculture and precious metals prices should fall in coming months on diminishing concerns over crop supplies and the fiscal situation "as the weather improves and cooler heads prevail", said Sameer Samana, international investment strategist at Wells Fargo Advisors, which oversees $1.3 trillion in assets.