He says the reputation of the hardy high-country cross-bred has been lost in recent years as farmers rush to adopt other cross-breeds to lift lamb production.
But Mr Duncan said he had noticed a swing back to the breed as high-country farmers realised its dual value - a low-maintenance, fertile breed producing prime lambs and a fleece that meets the specifications for United States-based sock producer SmartWool.
The Duncan family, John and Judy, Johnny and Geraldine, have been breeding half-breds since the late 1980s, initially at Wedderburn but since 1992 across the Maniototo Plains at Craigneuk, Gimmerburn.
Craigneuk covers 5200ha over two properties that are run as one, with about two-thirds hill and one-third flats, of which 800ha is irrigated.
Commercial merino and half-bred ewes spend all but their last year on the hills, while Romney ewes and young stock are farmed on the flats.
In response to the introduction of SmartWool contracts, the Duncans have fined up their half-breds from producing an average 26.5 micron fleece to an average of 23 microns.
Mid-micron breeders are now part of the New Zealand Merino marketing group, which has developed direct supply contracts with many wool users, a move Mr Duncan supports.
"New Zealand merino are good to work with," he said.
All his half-bred hoggets and 85% of his ewes produced wool that met the contract specifications.
While the contract price this year appears to be lower than that achieved at auction, Mr Duncan said it had previously always been greater, giving suppliers guaranteed income at a premium price.
"Over time, we have been a long way ahead," he said.
A ram-buying client, John Hore, agrees.
He and wife Jenny own the 4000ha Ida-Vale Station at Kyeburn, running 13,000 half-bred sheep, and he said all their wool went into a SmartWool contract at a guaranteed price.
Mr Duncan said that unlike some half-bred ram breeders, they crossed merino rams over Romney ewes.
Some Canterbury breeders cross merinos with English Leicesters, but Mr Duncan has found his breeding programme means less culling and the wool is soft handling.
Supplying SmartWool has put them in the public eye.
They have hosted several visits from US retailers and Mr Duncan said some of their comments had provided valuable insights into the thinking of US consumers.
One visitor thought sheep had to be killed to provide their fleece, while others had never seen, let alone touched, wool on a live sheep.
To fine up the fleece on their half-breds, Mr Duncan chose finer-wooled merinos and Romneys, but not at the expense of other productive traits.
The productivity of the Duncan flock showed half-breds were competitive with other breeds, he said.
Mixed-age ewes lamb between 145% and 160% on hill country and old ewes, which are mated to a Dorset Down ram, between 165% and 170% on flat country.
They clip 5.5kg of wool.
Halfbred-Dorset Down lambs can grow 500g a day and at 65-70 days old, kill out at 18.5-19kg.
Mr Duncan said he kept them a bit longer this year and they were killed at 21.4kg.
"This is their potential. Most lambs we aim to keep to over 20kg and we're doing that comfortably."
The Dorset Down cross lambs are also yielding 1% to 2% higher than Romney-Dorset Down lambs.
At shearing time, Mr Duncan does not need to employ a classer, as his contract only requires the fleece to be shaken then put straight into the press.
He checks the wool on each sheep before it goes into the woolshed to identify those with strong fleece.
Mr Duncan said because half-breds browsed paddocks rather than grazed them, they complemented cattle and did not eat out blocks as some breeds did.
Tupping starts in early May, with merino rams put over 1200 Romney ewes in tupping flocks ranging from 50 to 400.
Commercial half-bred ewes are mated to a Dorset Down.
Replacement ewe hoggets are supplied from the half-bred ram breeding business.
The half-breds start winter at the back of the farm but are gradually moved forward, spending the main part of winter on the front country, where they can be fed hay and baleage.
The Romney ewes are kept on the flats and follow a conventional wintering system.
Ewes are shorn in early September with the old ewes lambing in early October and the hill ewes later that month.
Mr Duncan said half-breds were easier to work with in the yards than some other breeds, and John Hore agreed.
"They are a kind sheep with a good nature," he said.
Mr Hore said he was achieving 150% lambing out of his half-breds on paddock country and 120% on hill and selling lambs at 34-35kg liveweight.
"They are the cheapest sheep to run on the place," he said.
Mr Duncan said half-breds were "the forgotten animal. But if you sit down and work out their lambing percentage, wool weights and lamb weights and compared that with sheep on the same type of country, they wouldn't be beaten."
Craigneuk stud
• John and Judy, Johnny and Geraldine Duncan.
• Gimmerburn, Maniototo.
• Craigneuk 2600ha, Awatea 2800ha, run as one.
• 6500 half-bred and merino ewes, 2500 Romney ewes and 700 Dorset Down ewes.
• 50 cows and finish 250 to 300 steers.
• 100 hinds and 100 velveting stags.
• Sell 350 to 360 half-bred rams and 200 terminal sire rams a year.
• Staff: Charlie Kennedy, Scott Watson and Brooke Elliot.