* Coal output up 4 pct, might be reduced later this year
* Koniambo issues slow down its ramp up
* Shares down 1.2 pct, more than wider sector (Adds details on zinc, analyst's comment)
By Silvia Antonioli
LONDON, May 5 (Reuters) - Miner and commodity trader Glencore reported weaker than expected first quarter output at some of its mining assets, with production of its top earner, copper, down 9 percent due to lower grades at two South American mines.
Glencore has a bigger exposure to base metals than iron ore compared with its large rivals. The company has a large commodity trading division, in addition to its mining and oil assets.
Bernstein analyst Paul Gait called the production figures "disappointing", with base metals and coal lagging expectations.
Copper output was 350,700 tonnes in the first quarter, below most analysts forecasts. The fall was due to lower grades at the Alumbrera mine in Argentina and the Antamina mine in Peru, and to a maintenance shutdown at Collahuasi, in Chile.
Coal production rose 4 percent in the first quarter to 35.6 million tonnes, thanks to the commissioning of two new thermal coal projects in South Africa. But a potential output cut at Glencore's Optimum coal operation in South Africa is expected to have an impact later in 2015, the company said.
Glencore reported a 7 percent increase in nickel to 23,800 tonnes, thanks to rising production at Koniambo.
The Koniambo nickel mine in the South Pacific territory of New Caledonia that Glencore inherited from its takeover of Xstrata in 2013 has suffered technical problems hampering its production.
In zinc, first quarter output at 356,200 was 16 percent up from with the same period last year, due to the expansion at Lady Loretta and McArthur River operations in Australia, but was down 8 percent from the previous quarter and at the lower end of analysts' forecasts.
Shares in Glencore were down 1.2 percent by 0816 GMT, underperforming a 0.8 fall in the UK-listed mining sector. (Editing by Jason Neely and Jane Merriman)