American businesses stepped up their inventory rebuilding again during July in the face of burgeoning sales, providing still another sign that the recession probably has run its course, the government reported yesterday.

New figures compiled by the Commerce Department showed business inventories up 0.8 percent, or $3.64 billion, over the month to a new level of $450.67 billion. Inventories rose by 0.3 percent in June.

For all the rush to rebuild stocks, the increase in inventories didn't keep pace with the sharp rebound in sales levels, resulting in a further paring down of the excess stocks that had worried analysts in previous months.

The closely watched inventories-to-sales ratio edged down to 1.49 months' supply in July, compared to 1.52 months in June, making it likely that further stockbuilding -- and increased production levels -- will be needed in the future.

The figures came as, separately, the Agriculture Department reported that the damage to the nation's corn crop is not quite as severe as some analysts had expected -- indicating food prices may not rise as rapidly as feared.

In new estimates published yesterday, the agency forecast a corn harvest of 6.53 billion bushels this year -- below the record 7.76-billion-bushel crop of 1979, but still beyond the 6.4 billion-bushel level some had expected.

At the same time, however, the agency boosted its estimate of the United States wheat crop this year beyond the record levels predicted previously. The decline in the corn yeild is the first since the weather damage of 1974.

The report of stepped-up inventory building in July tended to bolster other recent economic indicators in suggesting that the recession may be at or near its end. Sales levels generally are up and joblessness is on the way down.

Although officials have been somewhat circumspect about declaring the downturn over, both governments and private economists have hinted strongly in recent weeks they believe the slide has about come to an end.

However, most analysts still are forecasting a sluggish recovery, with the jobless rate remaining high through most of 1981. At the same time, many economists see inflation back on the rise. The outlook now is still uncertain.

The figures on inventories showed retailers rushing to rebuild their stocks. Inventory levels in that sector rose 1.2 percent in July after falling in June by 0.1 percent.

Manufacturers' inventories rose 0.4 percent in July after rising 0.1 percent in June. And wholesalers' stocks surged 1.6 percent, following a 1.2 percent increase in June.

The report showed overall busines sales jumped by 2.9 percent, or $8.5 billion, in July, to a seasonally adjusted $302.71 billion. Sales rose by 0.6 percent in June, following declines during the bulk of the year.

Crop damage in the farm belt has been spotty, although the impact has been severe where the drought has hit hardest. Agricultural economists have warned that food prices are likely to rise sharply in coming months.

Soaring farm prices already have pushed wholesale food prices up sharply, in a jump analysts say soon will be passed on to retail shoppers. Meat prices also have begun to rebound, following months of depression.

The Agriculture Department's estimate of the likely corn harvest was 2 percent below the forecast of 6.65 billion bushels that the agency issued on August 1.

Officials also trimmed their forecast for the soybean yield, reducing it to 1.83 billion bushels, down 3 percent from the 1.88-billion-bushel forecast issued in August.

The department also raised slightly its forecast for the nation's wheat crop. The 1980 harvest now is estimated at a record 2.35 billion bushels, up 10 percent from 1979 production levels.

Department economists said the drought that plagued much of the farm belt earlier this summer now appears to have eased in most farming areas, but grain prices still are rising as a result of the widespread crop damage.

The forecasts the agency issued yesterday take into account all the damage done to crops since the last estimate in early August.