Harley-Davidson said on Tuesday it will close a plant in Kansas City, Missouri as it consolidates manufacturing operations after its motorcycle shipments fell to their lowest level in six years, sending its shares down more than 8 percent.

The Milwaukee-based company forecast a drop in shipments to dealers this year as it expects retail sales in the United States - the company's biggest market - to dip.

Despite generally higher U.S. consumer spending, Harley is grappling with an aging customer base and younger, more price-sensitive buyers hesitant to embrace the iconic brand as previous generations have done.

Its shares fell 8.5 percent to $50.59 on the New York Stock Exchange. The stock fell nearly 13 percent in 2017.

Harley said it expects to ship 231,000 to 236,000 motorcycles this year after shipping 241,498 vehicles in 2017, the lowest number since 2011. That is at the low end of its previous forecast of 241,000 to 246,000 units.

In the December quarter, Harley's U.S. sales declined 11.1 percent from the year before and overseas sales dipped 7.7 percent. Overall sales in the quarter were down 9.6 percent.

As it adjusts to lower demand, Harley said it will consolidate work at its motorcycle assembly plant in Kansas City, Missouri, into the one in York, Pennsylvania, eliminating about 800 jobs at the Kansas City plant but adding 450 at the York facility.

Harley said the move would result in restructuring costs of $170 million to $200 million through 2019 but would save the company $65 million to $75 million a year after 2020.

It also announced the closure of its wheel operations in Adelaide, Australia, which will affect 100 employees.