The hotel and the general store at Mangamahu closed in the early 1970s due to improved roads, and a decline in wool and meat prices.

Kohanga and Manumanu were destroyed during the musket wars in 1840 and 1843, and most of the surviving inhabitants of the upper valley moved west to Parikino on the Whanganui River.

It gave settlers access to the nearer forest-covered hills of the area, (now Ruakiwi, Inzevar, Aranui and Mt View farms) and enabled packhorses to bring wool from sheep grazing on the high back-country tussock lands at Ngamatea and Waiouru.

From 1894 to 1908, Mangamahu village was a busy district supply centre with wagons moving up the Ridge Road to farms and railway construction sites in the central high country as far away as Ngamatapouri and the Waimarino, and with travellers going across the island from Wanganui to Napier.

Then in 1908 the Main Trunk Railway line through the centre of the island was completed and Mangamahu's importance as a transport hub greatly shrunk.

Merv's 1991 autobiography Home from the Hill (in NZ public libraries and also online) has detailed anecdotes about his childhood and farm-working days in Mangamahu between 1905 and 1930.

The topdressing enabled much more wool and sheep-meat to be produced, and Mangamahu bard John Archer describes the unexpected social consequences.

The valley grew so wealthy, from the super-pilots' loads The farmers all bought big new motor cars, and they tar-sealed our back-country roads.

In 1973 the price of oil skyrocketed, and this was followed by a reform government that abolished farm subsidies that had encouraged farmers to produce more than the market could absorb.

Sheep farming became uneconomic on the more marginal dissected slip-prone hills in the middle of the valley, and "Queen Street farmers", investors from Auckland, bought up thousands of hectares to plant pine plantations.

[2][3] Massey University lecturer Paul Kaplan headed a team of six researchers who interviewed nearly every adult in Mangamahu to find out how social conditions affected farm production.

He discovered that when Mangamahu farmers reached their early fifties, many did things like upgrading their farmhouse kitchens, while decreasing active involvement in managing their farms, instead of getting out onto the hills and increasing production.

[5] After the downturn in 1973, former sheep paddocks began to be planted in pines, and a corresponding decline in the output of fat lambs, wethers, wool and cattle from the valley.

But Pinus radiata takes only 25 years to grow to a loggable size in New Zealand, and by 2009 thousands of tonnes of pine awaited removal on the valley's sub-standard roads.

Andy Collins, a leading farmer at Mangamahu, stated that 15 to 20 million dollars of agricultural product were produced in the valley each year.