When Indianapolis Colts owner Robert Irsay loaded his National Football League team into vans last March and slipped out of Baltimore under cover of darkness, he left a $36 million gap in the Maryland economy.

The economic impact of the loss of the Baltimore Colts might even be as much as $40 million, according to a draft report on the importance of professional sports to the state's economy. A copy of the report, prepared for a special advisory commission appointed by Maryland Gov. Harry Hughes, was made available to The Washington Post late last week. The report used the most recent data available, some from 1983 and some from 1984.

Professional football's popularity notwithstanding, a decline in the horse-racing industry would have a more serious effect on Maryland's economy. The report estimates the total economic impact of the horse-racing and horse-breeding industries at $905.6 million, making it the No. 1 money generator among the state's sports.

According to the report, an estimated 3.5 million fans attended horse-racing events in Maryland in 1983. The sport provided $19.7 million in revenue to the state, an increase of nearly 6 percent over 1982. The total economic impact, however, is estimated at $407.5 million, while the impact of horse breeding is estimated at $498.1 million.

The report's findings tend to support estimates of the value of the horse industry made by backers of current legislative efforts to bolster the state's horse-racing industry.

Using what it termed "conservative estimates" in developing the report for the commission, the Maryland Department of Economic and Community Development's research division said the impact of all professional sports on the state's economy is $1.12 billion annually, and would have been $1.16 billion if the Colts had remained in the state.

The report calculated the impact of professional sports activities on the basis of income, employment, expenditures (travel, hotel/motel bookings, restaurant patronage) and state and local tax revenue. Researchers measured economic impact by totaling the value of all direct and indirect spending. They estimated the indirect and total economic impact of each sporting activity by using a multiplier of 2.5.

Researchers noted that there is a "great deal of interdependence between professional sporting events and related economic activities that are induced by these sporting events."

That finding is a key consideration in the current political and economic debates over proposals to build a new stadium in or near Baltimore.

The former Philadelphia Stars of the U.S. Football League will play at the University of Maryland's Byrd Stadium in College Park next season and may move to Baltimore for the 1986 season. It is too soon to estimate what economic impact the Stars will have, but a successful professional football team is capable of generating $35 million or more for Maryland's economy, the report said.

Professional baseball had an even greater impact on Maryland's economy in 1984. According to the report, which was delivered to the commission last week, the Baltimore Orioles' franchise generated an estimated $93.7 million for Maryland's economy in 1984. The Hagerstown Suns, a minor league baseball franchise, added another $3.5 million.

Direct and indirect spending at Washington Bullets basketball games during the 1983-1984 season generated $18.5 million, the report said. The National Hockey League's Washington Capitals, who share the Capital Centre at Landover with the Bullets, generated $28.9 million in total spending.

According to the draft report, auto racing added $29.85 million to the state's economy in; boxing, $18.46 million; soccer, $13.57 million; golf, $13.28 million, and wrestling, $6.85 million.

Besides conducting a broad study of the importance of professional sports to the economy, the commission will make recommendations on attracting and keeping sports franchises in Maryland. The panel had been expected to make its initial report some time next fall, but may complete the first phase of its work much earlier.