The Cass Freight Index measure of domestic shipping demand edged up a bare 0.6% in March from the month before, an unseasonable slowing of growth at the end of the quarter. Freight-payment company

Cass Information Systems Inc.

said in an analysis of its closely watched index that the freight market is clearly in a slowdown.

At the same time, freight rates appear to be pulling back from recent historic highs. Prices for last-minute spot loads in the truckload market, which make up a fraction of the market but tend to lead trends in contract rates, are sinking as shipping demand comes closer into balance with available truck capacity. Dry van truckload spot rates excluding fuel surcharges are down 37% since December and 27% in the past month, according to

Bank of America Corp.

analysts.

“I think it’s fair to say that the days of expecting rate increases are pretty much over,” said Avery Vise, a trucking analyst at FTR Transportation Intelligence. “It’s a question of just how quickly things are going to normalize. The idea that you can just sort of print money is over.”

Bank of America’s measure of trucking capacity available to shippers jumped last week to its highest level since June 2020, while its measure of shippers’ outlook for freight rates dropped sharply to the lowest level since July 2020.

Lowell, Ark.-based

J.B. Hunt Transport Services Inc.,

a bellwether for the U.S. freight market, will kick off the sector’s earnings next week with its first report since the carrier, which has a range of truckload, dedicated shipping services and intermodal operations, counted $760.8 million in net profit in 2021, including $242.2 million in earnings in a booming fourth quarter.

The country’s biggest truckload carrier, Knight-Swift Transportation Holdings Inc., which reports earnings April 20, saw revenue skyrocket 28.3% last year, to almost $6 billion, including a 42% gain in revenue in the fourth quarter that generated a $254.6 million net profit.

The profits capped a surge in gains for truckers during the pandemic, as a sharp increase in consumer demand and a rush by retailers to restock depleted store shelves jammed freight networks and sent prices spiraling upward.

Now, some shippers that had signed expensive contracts as they scrambled for scarce capacity are setting those deals aside to look for lower rates in a shifting market, said Jeff Tucker, chief executive of Haddonfield, N.J.-based freight broker Tucker Company Worldwide Inc.

Transport for trucking’s biggest operators remains in high demand, but “is it going to be as exciting as the last couple of years going forward? Maybe not,” said Mr. Tucker.

John Luciani, chief operating officer at privately held less-than-truckload carrier A. Duie Pyle, said inflation is cutting into the consumer trade that feeds a big share of trucking demand. The West Chester, Pa.-based carrier’s first quarter was “lackluster” compared with last year’s robust growth, he said, with volumes growing by single digits year-over-year rather than the double-digit rate the company expected.

Freight transportation stocks have been turning downward as signs of a slowdown emerge.

The Dow Jones Transportation Average, which tracks 20 large U.S. companies including trucking giants J.B. Hunt and Ryder System Inc., is down about 11% from a March 29 high, compared with a 4% decline in the S&P 500 over the same period.

Bank of America analysts recently downgraded transportation stocks including truckers

Werner Enterprises Inc.,

Schneider National Inc.,

Saia Inc.,

TFI International Inc.

and

ArcBest Corp.

citing “deteriorating demand outlooks and rapidly falling freight rates.”

Jason Seidl,

a transportation analyst at investment bank

Cowen Inc.,

said headwinds from the impact of inflation and of the Russia-Ukraine war will present challenges for carriers following a strong growth period. “I think the overall tone for earnings coming up on the trucking side is going to be a hefty dose of reality from the last month,” said Mr. Seidl.

Write to Lydia O’Neal at lydia.oneal@wsj.com