VANCOUVER, British Columbia, Sept 24 (Reuters) - Slocan Forest Products (Toronto:SFF.TO - news) said on Monday it will double its planned lumber production cutback to 60 million board feet in the fourth quarter because of weak market conditions. The Canadian timber products company, which has an annual lumber production capacity of 1.5 billion board feet, said it will reduce production at its sawmills by 30 million in the first week of October. Slocan had already announced plans to reduce production by 30 million board feet at the end of December. Canadian lumber producers have been warning they face weaker demand, because of the slowing economy and the 19.3 percent duty on Canadian softwood lumber shipments imposed by the United States in August in a trade fight.