Putting maidens to work:– can this work for Merinos?
P. R. Kenyon
Sheep Research Centre, Institute of Veterinary, Animal and Biomedical Sciences
Massey University, Private Bag 11 – 222, Palmerston North, 4442, New Zealand
Introduction
Breeding of ewe lambs (maidens, 8 – 9 months of age) is not common in New
Zealand with only 30% being bred with a average lambing percentage of just 60
(range 20-120, Kenyon et al 2004b). The percentage of farmers breeding ewe lambs
has increased over the last 10 years and is seen by many in the industry as the way
forward.
Potential advantages from successfully breeding ewe lambs include:
 the production of a lamb within the first year of life
 more efficient use of herbage in spring
 more lambs produced on farm within a given year
 higher nett profit
 an early selection/screening tool
 an increase in lifetime performance, if ewe breeding does not reduce lifetime
within the flock
 more progeny born on farm therefore potentially more select pressure
 potential reduction in the generation interval if progeny born to ewe lambs are
selected as replacements.
However there are a number of disadvantages or potential limitations to this
management option (Kenyon et al 2004a) which has limited its uptake by farmers.
These include:
 low and variable reproductive performance
 increased feed requirements during their first year of life
 the need for adequately sized ewe lambs at 8 months of age
 if the ewe lamb experiences ‘hardship’ during her first pregnancy and
lactation there is the potential for reduced 2-year-old live weight and
reproductive performance and decreased lifetime reproductive performance
 progeny born to ewe lambs are often smaller at weaning and of lower value
 ewe lamb breeding is often associated with extra costs – such as the
requirement for more rams
 increased workload.
Best practise guidelines for breeding ewe lambs
Prior to breeding
Ewe lambs should also be given appropriate vaccinations pre-breeding to protect
them against abortion diseases.
Under New Zealand conditions in comparison to the Romney it has been found that
Finn and East-Friesian ewe lambs, and composites including these breed types have
higher lambing percentages for a given live weight (Kenyon et al 2004b). The
Coopworth breed is the next most productive. All other breed types did not differ from
the Romney although numbers were low in most of the other breeds making
comparison difficult.
Live weights and conditions scores at breeding and changes in live weight prebreeding have the biggest impact on ewe lamb reproductive performance, other than
breed (Moore et al. 1978, Meyer and French 1979, Moore and Smeaton 1980,
Dyrmundsson 1981; McMillan and Moore 1983, Kenyon et al. 2005, 2006a, 2009,
2010). Unfasted minimum live weight targets in the range of 38 – 40 kg (mature
weight 60 – 65 kg) are required to maximise the chance of the ewe lamb conceiving
early in the breeding period. Condition scores should be a minimum of 2.0. Further
advantages of a relatively high breeding live weight include: heavier ewe lambs and
their progeny at weaning (Schreurs et al. 2010), reduced likelihood of birthing
difficulties and heavier ewes at their second breeding.
Ewe lambs should be exposed to vasectomised rams 17 days prior to breeding. This
exposure can increase the overall numbers pregnant and can advance the mean
breeding date (Kenyon et al 2005, 2006a 2006b). Vasectomised rams should be
used at ratios of 1:70-100 (Kenyon et al 2007a). Alternatives to vasectomised rams
examined to date have included short-scrotum ram lambs and exposures to entire
rams for just a few days but, both of these are less effective (Kenyon et al 2008a;
2008c).
Shearing ewe lambs at least one month prior to breeding has been shown to increase
the lambing percentage (Kenyon et al 2004b). The mechanism for this response is
not known and is not driven by a change in live weight.
The breeding period
Ewe lambs are often bred 1 to 1.5 months after the mature ewe flock in New Zealand
allowing them extra time to achieve puberty, and to ensure that the late pregnancy
and lactation period coincides with spring pasture growth.
Ewe lambs are ‘shy’ breeders, they are less likely to seek the ram and stand for him
(Dyrmundsson 1981). Therefore farmers need to maximise the chance the young
ewe comes into contact with the ram during the appropriate period. To achieve this, it
is suggested that the breeding of ewe lambs occurs in smaller, flatter paddocks and
not in the same flock as the mature ewes.
The optimal ewe lamb to mature ram ratio is likely to be 1:75-100 (Kenyon 2004b
2010). Well grown ram lambs (7 – 9 months of age) can be bred with ewe lambs
however this will require lower ratios if high performance rates are required (Kenyon
et al. 2007b). It may be advisable if young rams are to be used to utilise them for only
17 days followed by mature rams.
The use of mature rams with ewe lambs directly after they have been used with the
mature ewe flock, within the same season, is a way of reducing total ram costs on
farm, without negatively affecting pregnancy rates (Kenyon 2007b; 2009).
When selecting rams for breeding with ewe lambs, the size of the ram and the shape
of his shoulders should be considered in relation to potential birthing difficulties,
although there is little scientific information to support this.
The pregnancy period
Pregnancy and lactation in a ewe lamb has the potential to negatively affect her
breeding live weight and performance at her next breeding (Kenyon et al 2008d).
Therefore the success of ewe lamb breeding, from a whole flock’s perspective, not
only hinges on the ability to ensure the ewe lamb is fed to meet the requirements of
both the pregnancy and lactation but also that she is fed enough to allow her to grow.
Target total liveweight changes of around 150 g/d throughout the entire pregnancy
period are required if the performance of the ewe lamb and her lambs are not to be
negatively affected (Morris et al 2005; Kenyon et al 2008b; Mulvaney et al 2008).
During the first two thirds of pregnancy much of this gain in total live weight will be
partitioned towards the growth of the young ewe herself. While in the last third of
pregnancy most of this change will be associated with the growing fetus(es). A well
grown ewe lamb will be structurally larger and therefore less susceptible to birthing
difficulties. Whereas a ewe lamb that does not grow in early pregnancy, through poor
nutrition, and is then fed well in late pregnancy, will have increased risk of birthing
difficulties. It is recommended that farmers weigh their pregnant ewe lambs regularly
to monitor their performance.
Ewe lambs should be pregnancy diagnosed via ultrasound to determine their
pregnancy status. Identification of non-pregnant ewe lambs allows their nutrition to
be reduced saving herbage for pregnant ewe lambs. In flocks which are known to
have a percentage of twin-bearing ewe lambs, identification and management of
these as a sperate flock in late pregnancy has been shown to increase the
percentage of lambs weaned (Kenyon et al 2004b).
Shearing ewe lambs during mid-pregnancy, between days 50 and 100 of pregnancy,
has been shown to increase lamb birth weight (Kenyon et al 2006c). An increase in
lamb birth weight from mid-pregnancy shearing of mature ewes is known to increase
lamb survival and weights of lambs at weaning (Kenyon et al 2003). Therefore, midpregnancy shearing of ewe lambs is likely to be most effective in environments where
poor weather during the lambing period is common.
In late pregnancy ewe lambs should be vaccinated against clostridial diseases, if this
is routine management for the mature ewe flock.
The lactation period
As with mature ewes, suitable paddocks should be selected for lambing. Twinbearing ewe lambs should be offered the most sheltered paddocks and managed at a
lower stocking rate during the lactation period (Kenyon et al 2004b). Herbage
allowances during lactation should ensure that live weight of both the young dam and
her offspring are not limited. To achieve this the young dam and her offspring need to
be offered high quality green herbage.
Due to the later breeding of ewe lambs, the time between the weaning of their lambs
and their next breeding is shorter than that of the mature flock. Therefore, it is
prudent to consider a relatively early weaning date to increase the length of time the
young ewe has to recover before her next breeding (Mulvaney et al 2009).
Post weaning to the next re-breeding
Post weaning the young dam should be weighed regularly and fed appropriately to
ensure she achieves a suitable re-breeding weight.
Does breeding the ewe lamb negatively affect her future performance?
An often stated concern of ewe lamb breeding is the potential for negative effects on
the young ewes future live weight and reproductive performance. Breeding ewe
lambs has been shown to slightly reduce two-year-old ewe breeding live weight
(Keane, 1974; Baker et al 1981; McMillan and McDonald, 1983; Kenyon et al 2008d).
But this effect has either had only a minor negative effect (Kenyon et al 2008d), no
effect (Suiter and Croker, 1970; More et al 1983; Akcapinar et al 2005) or a positive
effect (McCall and Hight, 1981; Moore and Miller, 1983; Craig, 1982; McMillan and
McDonald, 1983) on two-year-old fertility and fecundity and their lamb’s survival.
However, any reduction in liveweight has been found to disappear by the weaning of
their second set of lambs. Further recent data indicates there are no negative effects
on performance as a 3, 4 or 5 year-old-ewe or on ewe wastage (Kenyon
unpublished). A further potential advantage of ewe lamb breeding is that those that
fail to breed at 8 – 9 months, but are of suitable live weight, can be culled (Kenyon et
al 2008d). Studies are currently under way at Massey University to determine if
progeny born to either mature ewes or ewe lambs differ in terms of their lifetime
performance. Some New Zealand farmers are considering selecting progeny born to
ewe lambs as replacements as a means of increasing genetic gain.
Can the Merino be successfully bred as a ewe lamb?
There is no reason why the Merino cannot be bred successfully as a ewe lamb. Most
of the management interventions used to maximise breeding performance of ewe
lambs are irrespective of breed. However the environmental conditions under which
the Merino is often farmed are likely to limit the success of ewe lamb breeding. To
be successful with ewe lamb breeding, conditions must allow for relatively high
growth rates post her weaning (around 100 g/d) until she is bred at 8 to 9 months of
age. Conditions must then allow for the young ewe to continue to grow (around 150
g/d) during the first 100 days of pregnancy. If herbage conditions do not allow these
growth rates supplements could be used however, the cost effectiveness of these
would require investigation.
Conclusion
Breeding ewe lambs at 8 - 9 months of age has a number of potential advantages.
However, unless these young ewes are adequately managed, there is the potential
for negative effects on their future performance. This paper outlines management
practices that will allow farmers to achieve success with ewe lamb breeding.
Appropriate live weights and nutrition are the two factors which have the biggest
impact on the success of ewe lamb breeding.
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