found the bit on use of chains -
"Use of chains during births:
In a situation where a calf is born to a
nulliparous cow, one that has not previously witnessed a birth to another cow,
one that has a history of aggression towards her calves or dystocia, or if it
known in advance that the calf is undersized or there are any other health
concerns, it might be a good idea to plan to remove the calf immediately after
birth for a rapid clinical examination and drying off, so it can stand without
slipping as soon as possible, and any necessary treatment. If the calves are
removed, it should occur within sight, sound and smell of the cow but safely
out of her reach. Staff should all be familiar to the cow and kept to an absolute
minimum. They should work as carefully, quietly and quickly as possible to
ensure early return to the cow and to allow bonding to begin at once.
This management can only be achieved without compromising the safety of
the keeping staff if the cow is adequately restrained. Chains (of varying
number and attachment) make this possible. If a cow is made familiar with
their use in the months preceding a birth, they do not appear to produce any
additional stress at the time of parturition. They can be kept under no tension,
unless the cow moves to the extremity of their length, and winched tighter
only if necessary to assist with the separation of a neonatal calf, should the
cow become aggressive. A recent birth at Twycross Zoo (2009) would have
resulted in a very badly injured calf without the mother being in chains (loose
then tightened) enabling the removal of the calf (N. Masters pers. comm.).
The decision to use chains should not be taken lightly as evidence continues to
be gathered that restraint free births are less stressful and have a higher
success rate than those chained. Each case is different and a cow with an
apparently healthy gestation, in the right social environment, with birthing
experience (hers or others in the group) and housing should be left to get on
with parturition without restraint. See also the discussion on infanticide
below."