Sales of medium- and heavy-duty trucks grew just 6.3% in the U.S. in June, the sector’s smallest monthly increase since December 2010, WardsAuto data shows.

Class 4’s loss of almost 50% was the biggest culprit in the June slowdown. Class 6 also fell below year-ago, while the remaining classes saw more modest increases than those of recent months.

Sales tumbled 47.5% in Class 4, with a 99.1% plunge at Ford. A 58.6% loss at Daimler was followed closely by the 52.5% drop in imported Isuzus.

Isuzu’s domestically built models posted Class 4’s biggest increase, up 306.4%.

Class 5 sales rose 18.6%, the best performance of any big-truck class last month, thanks to a whopping 776.8% jump at Freightliner, which in June sold 173 trucks vs. 19 in like-2011. Daimler also posted a triple-digit percentage increase, while PACCAR suffered the biggest decline, down 88.0%.

The 22.4% decrease in Class 6 sales was the second-biggest June loss in the big-truck sector, precipitated by falloffs at the volume manufacturers. Ford sales tumbled 79.5%, Daimler slid 20.6% and International, Class 6’s volume leader, slipped 3.6%.

PACCAR’s Kenworth surged 59.7%, the best performance in Class 6 last month.

Class 7 posted a 12.2% gain in sales, with most manufacturers seeing increases. However, declines at the high-volume manufacturers, such as Ford and International, tempered growth at PACCAR and Daimler, both up more than 30%.

Class 8 deliveries climbed 14.9% compared with year-ago, with the only decline recorded by International, down 12.2%. The 29.4% hike in Freightliner sales was Class 8’s biggest year-on-year increase, with Volvo Truck’s Mack Truck not far behind, up 27.5%.

Through the year’s first half, sales of medium- and heavy-duty trucks in the U.S. ran 25.4% ahead of like-2011, with 173,723 units.

Big-truck stocks rose in June. Class 8 had a 67 days’ supply at the end of the month, up from year-ago’s 49 days. Medium-duty days’ supply climbed to 87 from 60.

Both groups had more than 40,000 units in inventory at the end of last month, up from the high-20,000-unit range in like-2011.

In other big-truck news, Navistar’s board of directors adopts a Stockholder Rights Plan designed to prevent a takeover of the truck maker through “coercive tactics” such as accumulating shares in the open market or through private transactions. The plan also should prevent a person or entity from gaining control of Navistar without “offering a fair and adequate price” to its stockholders, the Illinois-based manufacturer says.

Navistar, which was targeted for takeover in the spring by Volkswagen, on July 11 and 12 saw its top shareholders, investment firm MHR and investor Carl Icahn, take larger stakes in the company, reports Reuters.

Icahn on July 12 increased his stake from 11.87% to 13.19%, a day after MHR boosted its holding from 13.6% to 14.95%. A 15% threshold was set in the Stockholder Rights Plan, beyond which any investor exceeding that level would see their shares weakened.

Navistar’s share price has declined recently, and it announced late last week it would stop pursuing exhaust-gas recirculation technology as the sole means of reducing its diesel engines’ emissions after failing to meet U.S. Environmental Protection Agency emissions standards for two years.

Navistar now says it will use EGR in concert with urea treatment, or selective catalytic reduction. The change of direction pushes the introduction of the company’s new diesel engines into 2013.