NAPERVILLE, Illinois, June 5 (Reuters) - Parts of the U.S. Corn Belt have been historically dry over the last month, pressuring both corn and soybean health and offering an early challenge to the government's record yield forecasts.

Data from the U.S. Department of Agriculture on Monday afternoon showed 64% of the U.S. corn crop in good or excellent (GE) condition, down from 69% a week earlier and below the lowest trade guess of 65%.

That marked the largest weekly decline in U.S. corn health since mid-August 2020, and Crop Watch corn conditions earlier on Monday had also reflected the largest drop since August 2020.

U.S. corn conditions had not fallen 5 percentage points or more at this time of year since 2012, when severe drought eventually cut corn yields by a quarter. In fact, aside from that one week in August 2020, a fall of 5 or more points had not been observed in any other week since 2012.

Corn health in early June 2021 fell by 4 points in two consecutive weeks, followed by a 3-point drop in the month’s third week to 65% GE. Near-record low ratings in the drought-stricken Dakotas weighed, though corn health in top grower Iowa lost 14 points in June's second week due to extended dryness.

Timely rains and moderate temperatures in July and August 2021 allowed for a record national corn yield, so the 2023 crop is not yet out of the game. But it needs to rain soon, and that is unlikely at a widespread level for at least another week, so conditions may fall again before any recovery is possible.

Prior to 2012, U.S. corn condition declines of 5 points or more during June were uncommon. The last three instances occurred in 2007, 2001 and 1994, though the crop was initially rated higher in those years versus 2023.

Corn conditions debuted at 59% GE in early June 2019 due to a waterlogged start, but 64% GE is otherwise the lightest for early June since 63% in 2013 and below the year-ago 73%.

Corn health in No. 2 grower Illinois featured the most concerning trend in the latest week, down 19 points to 50% GE, the state's lowest for the week since 2002. The week's five-year average is 70%.

SOYBEANS

Initial U.S. soybean conditions came in at 62% GE Monday, below both the average trade guess of 65% and the five-year average of 67% for the season's first rating.

Soy conditions began at 54% GE in late June 2019, but 62% is otherwise the lowest initial soy rating since 57% in 2008. Last year's initial score of 70% was issued a week later than this year.

Top bean grower Illinois was 51% GE as of Sunday, most recently comparable with 50% one week later in 2012. Nebraska at 58% GE was 21 points below the same week a year ago, before drought severely cut the state's yields.

Similar to U.S. corn yields, soy yields are not typically best when early health conditions are low, but there is more variation and potential leeway.

The last two times U.S. soybean yield was above the long-term trend when initial conditions were 62% GE or worse was in 2005 and 1997, though the starting 51% in 1992 gave way to a record yield more than 7% above trend.

Soy conditions were 73% GE by mid-July 1992, though that was an unusual year. July 1992 across the Midwest was the second wettest and third coolest in the last 128 years, following the Midwest's fourth driest May-June period at just 56% of normal precipitation.

Many climatologists consider 1992 a "year without a summer" due to unusually cool temperatures across the Northern Hemisphere, crediting volcanic ash from the mid-1991 eruption of Mount Pinatubo in the Philippines as the possible culprit.

Karen Braun is a market analyst for Reuters. Views expressed above are her own.

(Editing by Sonali Paul)