"We've had to downgrade the crop forecast by 18 million tonnes, from 56 to 38 (million tonnes) in the space of six months - it's just going in one direction," Cooper told a UN Commodities Conference.

Cooper said the lower forecast for the 2017/18 crop would be included in the IGC's monthly report, which is due to be published on Thursday.

The forecast is broadly in line with one issued by Argentina's agriculture ministry last week of 37.6 million tonnes, following a prolonged drought.

The IGC had forecast Argentina would produce 43 million tonnes in its last report on March 22, while the US Department of Agriculture's most recent forecast is 40 million.

"Interestingly just next door in Brazil it's going in entirely the other direction. We're seeing record crop there for soybeans," Cooper added.

Last month the IGC put Brazil's 2017/18 soybean crop at 114.0 million tonnes, marginally below the prior season's 114.1 million but the USDA has more recently put the crop at a record 115 million.

Cooper said a record soybean harvested area was expected in 2018/19 with broad-based gains in South America, China and Canada and some marginal gains in the United States.

World wheat area for the 2018/19 season was expected to rise by two percent boosted by an expected increase in US spring wheat sowings, Cooper said.

"The US is a major exporter, not the biggest these days - it's obviously the Black Sea countries, particularly Russia where exports have just ballooned, so Russia is I would say the price setter," he added.

Cooper said global supplies of maize (corn) were tightening.

"It's about potentially a smaller area in South America and the US, and the crop won't be as big as last year's record," Cooper said.

The IGC last month put the global maize crop in 2018/19 at 1.052 billion tonnes, below consumption at 1.094 billion, leading to a drawdown in stocks.