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Reproductive
Efficiency of horses in Australia
Introduction. What is a feasible outcome of yearly
breeding of horses? Is it reasonable to expect a foal very year? To answer those questions we need to look at
some historical perspectives of breeding efficiency and some physiological
parameters of horses. Firstly, horses have adapted over millions and
millions of years to create a reproductive pattern that ensures their survival
in the wild. We have then domesticated them and imposed our own constraints on
their breeding performance. We artificially impose the time of the breeding
season wherein most Thoroughbred farms have stopped breeding at the time of
summer solstice (Dec 21 in the Southern Hemisphere) which happens to be the time
that the highest % of mares are cycling
(Osborne 1966)
.
We have no or very little selection pressure on fertility; rather primarily it
is the horses’ performance that dictates the breeding pressures. As well, the
high economic value of many horses dictates breeding with older animals that may
have accumulated a variety of negative influences on breeding potential. Secondly, the horses gestation is ~340 days.
This only gives us the ability to maintain a yearly inter-foaling when mares are
back in foal with a successful breeding occurring no later than 25 days after
foaling. If mares are not bred on the foal heat, then there is only one
opportunity to maintain a yearly inter-foaling interval. If the mares’
gestation was ~280 days as with most breeds of cattle, then there would be
plenty of time to breed her and maintain a yearly inter-foaling interval. Sheep
have a gestation of ~148 days, so there is even a chance for two pregnancies per
year in those breeds that cycle all year round (but who do you know that can
produce lambs twice a year?). It would be a much easier decision of whether or
not to try and maintain a yearly inter-foaling interval of one year if the
gestation of a horse was the same as a Rhinoceros (540 days), Giraffe (440 days)
or Elephant (22 months). What then should we be aiming at with
regard to annual foaling percentages? Hans Merkt and colleagues made an
interesting observation on records from German Hanoverian horses over a 158-year
period. ‘An evaluation of the foaling rate achieved in the Hanoverian breed
in Germany between 1815 and 1973 showed that during this period no improvement
of the reproductive rate was achieved. The decennial foaling percentage remained
within 50-60% except for those decades which included the two World Wars and
their aftermaths. The annual foaling percentage remained maximal until the
number of mares covered/stallion rose above 80 and it also remained high
throughout the reproductive life of the stallion. Only in the oldest stallion
(32 years) was there a significant lowering of fertility’
(Merkt
et al., 1979)
. A more recent report suggested that breeding
efficiency had improved in the UK
(Ricketts and Young 1990)
.
They reported seasonal pregnancy rates of 63% in 1971 and 72% in 1989. The live
foaling rates were 54% and 66% respectively. This improvement in number of mares
pregnant and number of foals born corresponds nicely to the gradual increase in
research interest and establishment of a number of laboratories devoted entirely
to the study of equine reproduction. However a survey of well (intensively)
managed Thoroughbred farms around Newmarket
(Morris and Allen 2001)
suggested that the main improvement in efficiency on those farms was an
improvement in the number of mares that did not loose their pregnancy compared
to a previous study by one of the authours
(Sanderson and Allen 1987)
. The number of mares foaling was 82.7% in 1998 and 77% in 1983.
Figure 1 shows that the number of stallions
peaked in 1985-6 and has been gradually dropping since.
When we look at the number of mares versus
stallions (at left) we can see that both mare and stallion numbers dropped off
quickly from the heights of the mid 1980’s however the mare numbers seem to
have stabilised somewhat.
When we compare the number of available mares
per stallion we can see that is steadily rising. A figure that ultimately should
put pressure on breeding efficiency. Figure 3. The number
of available mares per stallion
Perhaps
the most interesting statistic to be noted was the live foal per mare bred
percentages (at left). Back in 1981 we were only recording around 40% live foals
per mare covered. That figure was almost 76% by the end of 2000. Apart from the
afore mentioned UK experiences it would stand to reason that the improvement in
fertility could be related to a multi-factorial influence. Firstly, the value of
the individual has risen dramatically and coupled with that has been an
information explosion, together with an improvement in our equipment to manage
reproductive processes. This increased understanding has been passed on to all
levels of farm management. The net result has been a stabilising in the numbers
of foals born despite still steadily falling mare numbers. However, there are some other issues that should
be examined. Firstly, does the Thoroughbred breeding record presented
above reflect the other breed associations and secondly, is the live foal
rate consistent between farms or is it affected by management? Before looking at those issues in depth we
should try to standardize our terms of reference. Is
the number of foals born an accurate estimate of the real fertility of a breed
of horse? Probably not we think. The most accurate estimate of a stallions’
fertility data is the number of mares pregnant each cycle. At the end of the
season the number of mares pregnant may reflect the number of cycles that the
stallion had an opportunity to breed rather than the true fertility. For
instance, if one farm has a 75 % pregnancy rate per cycle then after two cycles
for each mare (eliminating mares pregnant), 93% of mares are pregnant. If you
have another farm that has a pregnancy rate per cycle of only 40%, then they can
achieve a 93% pregnancy rate after 5 cycles. The difference is huge in relation
to agistment and veterinary costs, not to mention lost opportunity costs because
the foals are born so much later. This later discussion may in part explain the
English report that quotes Weatherbys general Stud Book as reporting the
percentage of live foals to be 79% in 1998 compared to our Stud Book records
that officially list percentage of foaling mares as 74% in 1998 and 76% in 1999
and the year 2000. The more recent English report
(Morris and Allen 2001)
list almost 20% of breedings after the first 3.5 months of the breeding season.
Our experiences would indicate that in Victoria and NSW not many mares are bred
in December (the fourth month). The average pregnancy per cycle in the UK study
was 60%, which is lower than the 74% reported for intensively managed
Thoroughbred studs in Australia
(McKinnon 1998a)
. The goal of any breeding farm should be to get
mares in foal as soon as possible whilst maintaining their ability to deliver a
live foal. There is little point in having a mare become pregnant if she has a
high probability of loosing it. If we examine breeding efficiency (not
necessarily the same as fertility) it could probably be better defined as % of
foals born per service or per cycle bred. Well-managed Thoroughbred breeding
farms will maintain foaling % of around 70% per covering. Despite all the
figures and preceding discussion the goal of most breeding farms is actually to
maximise the number of foals born. Just because the efficiency figures look good
does not imply that the return from the farm is maximal. It stands to reason
that the farm has the same result financially when a foaling percentage of 70%
is obtained with 60 mares occurs versus a foaling percentage of 60% for 70
mares, when the service fees are identical. So the goals may be defined from the
point of farm management as ‘obtaining the highest number of live foals’. Current
RIRDC Study
In
1999 RIRDC funded a study on the breeding efficiency of horses in Australia. The
project aims are listed below. Appended
at the back of this article is the Access database that we have developed to
analyse breeding efficiency data. The
project is still years from completion however some very useful results have
already been forthcoming. Currently we are validating the database and running
trial ‘queries’, the term used in Access for asking questions of databases. Project Synopsis Outcomes
and deliverables of the proposed research Outcome: A thorough analysis of the efficiency of breeding practices in the
Thoroughbred and Standardbred industries in Australia. Objective
1): Analysis of breeding records via the Stud
Books. This is only expected to provide an overview of industry practices. For a
variety of reasons, records submitted to the Stud Book (both racing codes) are
only a superficial view of breeding practices, however they do represent the
true and relevant fertility’s of the different breeds and even different
stallions on the same breeding farms. Objective
2): Analysis of individual mare records from
selected breeding farms. This is an ambitious attempt to collate data
from a wide variety of sources. It is estimated that individual mare records of each breed from a variety of studs that have been chosen to represent three levels of management will be used to compare factors such as fertility per cycle, effect of fluid pre and post service, effect of scanning or not, rate and time period of early embryonic death, etc. Answers to these
questions may provide RIRDC with a balanced view of problems in this area of the
industry and areas that should be targeted for further research. For example we
estimate that early embryonic death accounts for as much as 15% of losses on
Thoroughbred farms, however this data comes from small populations that may be
skewed or not representative of the total population. Establishing an accurate
figure would be useful in targeting funding for further research. Background,
relevance and potential benefits Research into breeding practices is hampered by
lack of a large data set to draw conclusions from. For instance we published on
the incidence in Thoroughbred versus Standardbred mares of twins (15% versus 5%)
and yet few studies have been able to demonstrate these differences due to
inadequate sample size. Establishing a data base on the breeding
performance of the Thoroughbred and Standardbred horses in Australia is of
particular use due to identifying trends (i.e. have we improved efficiency on
well managed breeding farms?) and identifying areas that need research (i.e. how
much is lost to the industry each year from foals that are either lost as early
embryonic death or aborted and should those areas be targeted for serious and
expensive research?). Preliminary
results from data entered into the Access database. Some
specific factors that effect breeding efficiency have been recorded. Where
possible we have tried to contrast the differences according to breed
(Thoroughbred (TB) versus Standardbred (SB)). Numbers
of mares examined. The
database we have chosen to work with for this presentation has 1833 TB mares and
1330 Standardbred mares. The number of cycles examined was 2436 and 2086
respectively. All Thoroughbred mares were bred by natural service and all
Standardbred mares by artificial insemination (AI). The data was retrieved from
the years 1994-5. Some
summary data is listed below. Table
1: Fertility
statistics:
Table
2: Twins
and Early Embryonic Death (EED)
Table
3: Foal Heat
Table
4: Ovulation
induction: Thoroughbred
Ovulation
induction: Standardbred
Table
5 Effect
of fluid and treatment on pregnancy data and EED: Thoroughbred
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