Effect of ewe live weight
Live weight has two effects:
(1) The static (live weight) effect
·
Heavy ewes have higher ovulation rates (and more lambs)
than light ewes.
·
Twinning (percentage of twin births to total
births) increases by about 6% per 4.5 kg increase in ewe live weight — i.e.
1.3% per kg increase.
·
This effect operates up to at least 70 kg live
weight and there is no evidence of a decline at the top end of this range.
Barrenness increases markedly under average weight of 40–45 kg for Mutton-based
breeds and 35–40 kg in Merinos.
High ewe live weight
and live weight gain during mating causes high O/R - for each extra kg of ewe
weight there will be 1.3% higher lambing percentage.
·
Comparisons of poorly reared and well-reared ewes
show that while the poorly reared ewes have fewer multiple ovulations; this is
probably a function of their lower adult live weight.
(2) The dynamic (live weight gain or
"flushing") effect
·
Some trials have shown ewes gaining weight quickly
(e.g. 0.5–1.0 kg per week) just before mating had higher ovulation rates than
ewes of similar weight with low or no weight gain.
·
However some results showed that ovulation rate was
more dependent on ewe live weight at oestrus than on previous changes.
#Seasonality of ovulation rate
·
Most sheep breeds are seasonal breeders and fewer
eggs are shed in the first cycles of the breeding season in early autumn.
·
The first cycle of each season features a
"silent heat" — i.e. ewes ovulate but do not show oestrus.
·
Highest lambing percentage coincides with mating
mid-season depending on location. Higher
numbers of abnormal eggs may be shed near the beginning and end of the breeding
season.
Each successive ewe
oestrus during the breeding season has about 0.15 more eggs shed.
#Synchronisation
· Synchronisation may be used to get ewes ovulating
simultaneously (e.g. to facilitate timing of artificial insemination) and/or to
induce ewes to cycle out of season.
·
The two ways of doing this are by using rams or
hormonal treatment.
Use of #rams ("ram effect")
* The introduction of rams early in the breeding
season stimulates ewes to ovulate within 3–6 days (without showing oestrus if
this is the first ovulation of the breeding season) and show oestrus about 17
days later.
·
Ewe flocks stimulated by the ram effect are thus
likely to be synchronised.
·
Rams introduced several weeks before normal onset
of oestrus may have no effect and late introduction will only stimulate those
few ewes, which have not begun cycling.
Synchronisation can
be used to get ewes ovulating simultaneously by using hormones or rams.
·
Rams used for synchronisation may be entire or
vasectomised. Some poor results with rams vasectomised for one year or longer
have been reported and this may be due to reduced libido in these rams.
·
High libido vasectomised rams are most effective
and should be introduced up to a week before normal if using as teasers (i.e.
to stimulate ovulation and oestrus).
Rams introduced early
in the breeding season, either vasectomised or entire, will stimulate ewes to
ovulate within 3-6 days.
·
Some breeds are more effective, with indigenous
rams are usually superior to Merino’s.
Hormonal synchronisation can be used to:
·
stimulate first oestrus (e.g. first oestrus
in young ewes or an early first oestrus for older ewes)
· synchronise ewes to show oestrus at the same
time (e.g. to condense lambing or for artificial insemination) during the
normal breeding season
·
Controlled internal drug sponges containing
progestagens (synthetic analogues of progesterone) are most commonly used.
·
Results are more heavily influenced by operator
skill and timing of insemination than product type.
·
The sponge is inserted in the vagina and withdrawn
after several days.
·
The ewe typically shows oestrus within three days
after withdrawal within the normal breeding season.
·
This time varies depending on dose level of
progestagen, type of device or sponge used and whether pregnant mare's serum
gonadotropin (PMSG) is also used to stimulate greater ovulation rate.
·
Ewes not fertilised at this oestrus will return to
oestrus about 16–17 days later and remain generally synchronised. This is
useful when planning a return visit for an AI technician.
Synchronisation for
out of season breeding usually requires PMSG also to stimulate ovulation.
·
Prostaglandin injections can also be used for
synchronisation but are not common.
·
Ask a vet about synchronisation or hormonally
induced oestrus if planning to use it for the first time.
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