I'm not an expert but I do subscribe to a UK agricultural newsgroup.
A hefted sheep is one that stays on a piece of "home territory". This
became an issue during the foot and mooth disease outbreak among farm
animals in the UK in 2001. Sheep in the English Lake District are
hefted. They spend much of the year on the fells (hills) without being
physically kept in place. They are free to travel as far as they like -
tens of miles if they get the urge. However, each flock tends to have
its own general territory. The assumption was that hefting is partly a
matter of custom handed down by example from sheep to lamb over many
generations. The concern was that following a massive cull of sheep the
replacements would not have the same habit or custom.
The OED gives the noun:
heft, n.3
Etymology: Variant of haft n.2 2.
local.
(The sheep in) a settled or accustomed pasture-ground.
1960 K. Williamson & J. M. Boyd St. Kilda Summer 84 The Hirta
flock is divided into hefts, more or less discrete groups each
restricted to its own particular range.
1961 New Scientist 9 Nov. 341/2 The natural unit in hill sheep
farming is the heft—the group of sheep that habitually graze
within the confines of a particular area of hill ground.
From a letter to the Prime Minister from a man in the Lake District:
http://www.conservationwatcher.com/heftedflocks.html
12 Apr '01
Dear Sir.
Heafed or Hefted Flocks
The landscape of the British uplands that we like to regard as our
heritage, that tourists love to visit, written about by Wordsworth
and Sir Walter Scott, was shaped by men and their sheep. The Heafed
or Hefted flocks of the Welsh Mountains, the Lake District, and the
Pennines from High Peak to the Borders, the Cheviots, Galloway Hills
and the Scottish Highlands.
The Heaf or Heft is the territory occupied by a flock on open hill.
The individual sheep live and move within these boundaries according
to season and weather. Each lamb inherits its mother's patch and out
on the hill it is possible to pick out family groups made up of
different generations.
<much more detail about the life of sheep on the fells>
--
Peter Duncanson, UK
(in alt.usage.english)