First-half copper production was 5% lower than last year, while cobalt output rose 28%. Zinc and coal output rose 8% and 10% respectively and nickel dropped 11% versus the same time last year because of maintenance.
London-listed Glencore's exposure to risk in Democratic Republic of Congo and Zambia has weighed on the company's share price, which has fallen while those of its diversified mining peers have risen.
In a statement, CEO Ivan Glasenberg said Glencore was addressing the challenges at its Katanga Mining unit in Congo with management changes and an operational review while in Zambia it was near the end of multi-year improvements.
The Zambian government's move to seize assets of fellow miner Vedanta have alarmed the industry, while in Democratic Republic of Congo, Glencore has faced a series of problems, including the death of artisanal miners who invaded its concession.
After the commodity price collapse of 2015-16, Glencore rebounded when the market predicted its willingness to operate in difficult jurisdictions gave it prime access to the minerals needed for the shift to an electrified economy.
Investors this year have become focused on avoiding risk following fatal accidents across the industry, as well as concerns about climate change.
AFRICAN POTENTIAL
Glasenberg said Glencore's African copper retained significant potential and would "play a key role in the transition to a low carbon economy".
He said he would explain turnaround plans when half-year results are announced next week, along with revised copper and cobalt guidance for Katanga.
Glencore said it would separate its African copper business from copper operations in less risky regions, including the Americas and Australia, during an overhaul of up to two years.
On Wednesday, Glencore's share price eased 1.3% to just under 270 pence by 1014 GMT. It is around 7% lower since the start of the year.
Analysts said it had been a difficult first-half for Glencore.
BMO Capital Markets, which rates Glencore "market perform" said Glencore would have to improve its second-half output significantly -- between 20% and 40% across nearly all of its commodities to achieve its full-year targets.
Glencore's full-year output guidance for copper, excluding Africa, is just over one million tonnes and roughly 1.45 million tonnes including Africa.
Cobalt output rose following the resumption of exports at the Kamoto Project in Congo in April, which had been suspended because of higher than permitted uranium content.
The resumption has coincided with a falling cobalt price, which has halved from above $60,000 per tonne in late 2018 to below $30,000.
Glencore said it faced a mark-to-market loss on around 10,000 tonnes of cobalt as its marketing business has not yet sold the cobalt it effectively bought from its mining business.
The $350 million EBIT (earnings before interest and tax) cobalt loss is mostly non-cash as the position was funded in 2018, Glencore said.