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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
Effect
of fluid and treatment on pregnancy data and EED: Standardbred
Conclusions
on preliminary data: Fertility
statistics: Pregnancy
rate per cycle: The
higher pregnancy rate per cycle in the TB population compared to the SB (70.4 %
versus 62.0 % P < 0.0001) was not expected but likely reflects economics
rather than the population of mares. The differences become even clearer when
examining the average services per cycle, which was 1.04 for the TB population
(on average four mares got re-bred from every 100 cycles) and 2.21 for the SB.
Interesting enough, the SB seasonal pregnancy rate (number pregnant at 45 days) was slightly higher (88.6%) than
the corresponding TB figure (86.7%). Although
not examined at this time, we have long identified SB breeders as having less
regard for time of foaling than the TB breeder. Possible reasons for this are 1)
The SB breeder is more likely to retain the offspring to train themselves and
thus not as concerned as to time of foaling 2) Most TB breeding farms stop
breeding by Christmas (< 4 month breeding season), whereas SB farms continue
to breed on well into February and even March and 3) SB farms are less likely to
involve veterinarians in routine breeding farm management, due primarily to
economics. This is the reason that the average service per cycle for TB mares is
1.04 versus 2.21 for SB. So SB mares are bred multiply and can have increased
chances for intrauterine infections and are more likely to be bred at
inopportune times such as post ovulation, especially if teasing isn’t integral
to management. In addition to the above, the staff levels tend to be much higher
per number of horses on TB farms than the SB. The
fact that we have shown the SB seasonal pregnancy rates are identical to the TB
(despite a significant difference in fertility per cycle), clearly illustrates
that they breed longer into the breeding season. The SB regulatory authorities
should also be congratulated on delaying the date of first breeding (~October
1st to coincide with an official first foaling date of September 1st -
exactly one month behind the TB breeders) and clearly helpful in utilising the
effect on increasing cyclicity from increasing daylight and warmth). Twins and EED
Many
more twins were identified both per cycle and per pregnancy (9.7% and 13.8%) for
TB compared to the SB (2.2% and 3.5%) respectively (P <0.001) at the (~) day
15 pregnancy test. These differences between breeds have been reported before
(Ginther 1992)
and relate mostly to the frequency of ovulations between breeds.
Especially interesting was the outcome of twin pregnancies. The reduction of the
twins to singletons (success of veterinary intervention, i.e. to crush one) was
high in the TB population, as only 10 of 245 twins (4.0%) were not detected as
single pregnancies at day 25. During the same period 8 of 46 SB twin pregnancies
were both lost (17.4%), which is significantly more (P<0.006). We believe
that this difference is related to fundamental management practices on breeding
farms. Because the incidence of twins is recognised as low on SB farms, in
general the first pregnancy test is delayed until the mare should have returned
to oestrous. This delay until day 17-20 often interferes with our ability to
manipulate twins apart for a successful crush as fixation (the beginning of
implantation) has occurred and twins together can be difficult to move part for
crushing
(McKinnon and Rantanen 1998b)
. The
incidence of EED was similar between mare populations with more (P< 0.01)
pregnancies being lost in the SB group (7.1%) between day 15-25 than the TB
group (3.7%). It is interesting to speculate as to the differences being due to
a difference in the number of mares with post breeding uterine inflammation as a
result of less examination and treatments. Foal heat breeding Induction
of ovulation The
low number of mares treated with Ovuplant was only because the drug was not
commercially available at that time (1994-5) and we were doing some trial work
for the company. Data collected over the last few years has indicated closer to
50% of mare cycles are associated with Ovuplant administration. The low number
of Standardbred mares treated with ovulation induction drugs reflects the
economics of breeding as well as the reduced necessity to use them with AI
programs. Another
interesting finding was the increased fertility (P< 0.0001) (pregnancy rate
per cycle) of TB mares with induction of ovulation (1189/1651, 72.0%) versus not
treated (527/817, 64.5%). Effect
of intraluminal fluid and or treatment on fertility. As
previously reported
(McKinnon et al., 1987; Adams
et al., 1987)
intrauterine fluid post ovulation can have a negative effect
on pregnancy results. In the current study fluid post service significantly
effected (P<0.0001) pregnancy in TB mares (114/222, 51.4%) compared to
non-detection of intrauterine fluid (1602/2216, 72.3%). The same was true for
the SB mares (97/234, 41.6% compared to 1197/1852, 64.6%, P< 0.0001). In
the TB population treatments pre service or a single treatment post service was
associated with normal pregnancy rates, however more than one treatment post
service was associated with a reduction (not significant P<0.15) in pregnancy
rates (465/681, 68.3% compared 1251/1755, 71.3%). Decreased time to conception
For mares entering breeding farms, the time to the first breeding has a
big impact on when mares will foal. Early foals consistently occur from early
breeding. Many mares are not cycling naturally at the time of year when we wish
them to become pregnant (early September), this is especially true in the
southern latitudes. How can we get mares to cycle earlier? 1)
Provide an increased amount of light beginning around 60-90 days before
the first anticipated breed. 16 hours per day is necessary. French workers have
determined the amount of light is not as important as previously thought.
Another interesting finding from the same people was that the light regime does
not have to continue for the whole 90 days. It is just as beneficial to start
the program and then stop the lights after 35 days. Beware!!!, this does not
mean that the lights can be started late. All light does is send signals to the
brain (through a negative melatonin feedback). If the program starts late the
mares will cycle late. Another commonly forgotten part of the early cycling
equation is warmth and nutrition. Mares not fed well or stressed will not cycle
as early as well fed and non-stressed mares. This may mean that providing light
in large paddocks where mares are fed together is not as useful as previously
thought. 2)
Drugs.
Stimulatory drugs. These drugs start mares cycling when they do not
appear to have real ovarian activity. There is lots of new information coming
through on these types of drugs and unfortunately some of it is not as
conclusive as previously thought. Currently it appears that Dopamine antagonists
(Domperidone and Sulpiride) hold the most promise in shifting the balance on
ovarian activity. Please recognise that not all these drugs work all of the
time, in fact some work only 60% of the time. In Australia, Domperidone is
marketed for humans as ‘Motilium’.
Ovulatory drugs. Drugs that consistently ovulate follicles that
previously were thought to be too immature to respond are now being used
clinically. Our laboratory (Goulburn Valley Equine Hospital) was first to report
of the use on Ovuplant in treating mares to have them cycle early at a
predetermined time. A detailed discussion of research work supporting the
clinical use of Ovuplant is provided below. Breeding farms that have used
Ovuplant for forcing mares to cycle can attest to its success, but many forget
that the mare has to be close to ovulating anyway and we are merely advancing
her time of the first ovulation in a controlled manner. This drug will not work
when mares have little ovarian activity. Increase pregnancy rate per cycle
Decreasing the number of breeds per cycle
Decreasing number of breeds per cycle provides less contamination of the
reproductive tract and prevents depletion of sperm numbers from the stallion.
Our recommended breeding objectives are a breed per cycle figure of 1.05. In
this scenario every 100-mare cycles would account for 105 breeds. This has
important ramifications for management. To achieve this objective mares need to
be managed intensively. When a suitable sized follicle is detected then a drug
to induce ovulation is administered. Ovulation must be confirmed at 24 or 48 hr
after breeding because some stallions do not have sperm that lasts well in the
reproductive tract. It
has only been readily accepted by breeding farms relatively recently that this
practice will decrease labour and increase pregnancy rates per cycle and it has
become absolutely paramount for management of breeding stallions with large
books (>100 mares) or restricted services such as some shuttle stallions. One
of the most useful drugs for controlling ovulation time and thus keeping the
number of breeds per cycle down to ~1.05 is Ovuplant. The properties and results
of our experiments with this drug are listed in the short articles towards the
end of this paper.
Management of mare cycles There
is no doubt the single biggest factor in improved breeding efficiency has been
the introduction of ultrasonography. This has made veterinary involvement
mandatory on large breeding farms and has dramatically improved all our
knowledge of the mares cycle and overall fertility, not to mention improvement
in the management of twins. Equine reproductive management can now be practiced
as a science instead of an art. Few
people predicted the impact that ultrasonography would have on equine
reproductive management and understanding of reproductive physiology. The
ability to examine a mare's reproductive tract non-invasively with
ultrasonography provided the opportunity to diagnose pregnancy earlier than by
rectal palpation, effectively manage twins and detect impending early embryonic
death (EED). However, ultrasonography should not be limited to these areas. It
can be used to diagnose uterine pathology, such as intrauterine fluid, air,
debris, cysts and occasionally abscessation and neoplasia. In addition,
ultrasonographic examination of the ovaries may aid in determining stage of
oestrous cycle, status of preovulatory follicles, development and morphologic
assessment of the corpus luteum (CL) and in interpreting ovarian irregularities,
such as anovulatory or haemorrhagic follicles, neoplasia and peri-ovarian cysts.
The costs of equipment initially resulted in a rather limited application of
reproductive ultrasonography. Clients enthusiastically support use of
ultrasonography to detect pregnancy. However, the same fee schedules for routine
examination before and/or after breeding are not as easily accepted. An approach
that allows us to scan multiply while still keeping clients and farm managers
happy is something all of us strive to organise each year. A more logical and
thus practical approach to diagnosis and treatment of physiological and
anatomical abnormalities of the mare's reproductive tract would be forthcoming
if we can continue to develop a means to use the equipment more routinely. In
addition, valuable information would be available from correlation of fertility
data with normal and abnormal ultrasonographic observations. Regardless,
informed clientele prefer routine ultrasonography and its use results in a more
interactive approach to farm management with an increased awareness of the
events associated with breeding, ovulation and early foetal development.
Stallion fertility and management The
stallion is forgotten in the fertility equation despite being responsible for
half of the probability outcome in each breeding. If a group of matings results
in an outcome of 80 % of mares becoming pregnant per cycle then the mare and
stallion fertility contributions are most likely 0.9 and 0.9. When the fertility
of stallion drops then the outcome is affected. If his fertility is halved then
only 40% of mares will go in foal per cycle. However because people have the
mistaken belief that there is nothing we do to influence stallion fertility,
they ignore his contribution. It is disappointing to recognise that within
Australia, only a few people are adequately trained to evaluate stallion
fertility parameters. Perhaps that is another reason why the stallion is too
often forgotten. Management
has a huge impact on fertility. Well-managed farms have good pregnancy and
foaling rates. However, when pregnancy rates per cycle on the same farm are
compared between stallions quite often there are major differences in the
figures. In those cases, providing the percentages of mares in the barren,
maiden and foaling groups are similar; the stallion is demonstrating his
“inherent” fertility. That is something he is born with. I am sure many of
us have seen the stallion that just seems to have to sniff his mares and they go
pregnant. Compare him with the stallion that takes two, three or even four
cycles to get mares pregnant. They can end up with the same number or percentage
of mares pregnant at the end of the breeding season, but it is a lot more work
with the less fertile stallion and his progeny may have an average foaling date
that is later than acceptable for commercial foal sales. Nobody knows what the
real differences in fertility are caused by. However, quite frequently we see a
difference in the characteristics of progressive motility and good tense sperm
producing testicles between the stallions. Despite this, there are sometimes no
obvious differences and the only explanation is that one stallion is able to
produce more fertile sperm than another. Mare fertility and management
Infectious fertility. Ability
to recognise that some mares have poor clearance of uterine fluids was proposed
many, many years ago, however only recently has a technique been able to measure
the lack of evacuation of uterine contents from some mares. This technique from
Florida has resulted in the knowledge that some treatments are extremely useful
such as oxytocin and certain forms of prostaglandin. In general ultrasonography
tells us how bad the fluid is (usually infection) and also how to treat it. I.e.
treat locally with antibiotics or treat with agents to evacuate the fluid from
the uterus or a combination.
Foal Heat breeding. Many
of our clients wish to breed mares on foal heat. The reasons are generally
obvious. The gestation of the mare averages 340 days. However some are longer.
Most of us involved with commercial breeding farms recognise a financial penalty
for foals born later than the normal accepted times. Without breeding at foal
heat it may be hard to maintain a foaling interval of 365 days in some cases. In
addition clients with brood mares on farms sometimes are paying high agistment
costs and place pressure on managers to get their mares pregnant and send them
home as soon as possible. Sometimes farm mangers cannot resist the temptation to
breed at foal heat despite the potential problems, just because the mare is
showing heat and their knowledge that foal heat mares ovulate quite quickly. Put
simply the positive aspects of foal heat breeding is a decrease of the
inter-foaling interval and all the attendant benefits. The negative aspect that
many fail to recognise is the increase in early embryonic death. The interplay
of these opposing outcomes should be factored into any decision-making analysis. GVEH
Breeding approach 1)
All mares are examined at day 2-4 post foaling. By that time if problems
have occurred during foaling then they will be identified. We believe that this
exam is critical if we are to prevent mares become “problem broodmares”.
Each year we identify mares that if inappropriately treated will become
long-term disasters. 2)
All mares are treated with an infusion of Lactated Ringers Saline with
antibiotics. We believe that this helps only the occasional mare, but more
importantly we have invaded the mares’ reproductive tract and that may need to
be treated in case we have introduced something. 3)
Mares looking good are scheduled to be re-examined on Day 9-10 post
foaling. We will not breed mares any earlier than Day10. On day 9-10 if the mare
has ovulated she is scheduled for PGF2µ administration in 6 days. If she looks good (no fluid and uterine tone
is good) she is scheduled to breed if possible. If there is a question then she
is treated and or re-examined according to follicle size and presence or absence
of uterine fluid. 4)
Great care is taken to avoid breeding mares with intraluminal fluid
accumulations. Lastly
there are two more rules that we adhere to. A) Only breed mares that are young
and reproductively healthy (i.e. <12 YO) and B) Do not breed on the foal heat
if the mare has been boxed and not forcibly lunged. Lastly,
old mares that are bred on foal heat and become pregnant have such a high rate
of early embryonic death or abortion we strongly recommend against the practice. Decrease pregnancy losses Pregnancy
losses can broadly be divided into early embryonic death (EED) (< day 45 of
gestation) and abortion. There
are well-documented probabilities of foetal loss for different time periods of
pregnancy. The recognition that these losses have repeatable frequencies
reported in different studies should point to a common denominator that is
present in multiple populations of mares. My
view is that management is the most important factor affecting the well being of
horses. I would go one step further and say, “all disease is caused by management”. This is of course a dramatic
statement but many times true if you ponder the myriad of diseases seen on stud
farms. In particular, overcrowding, concentration of disease and the handling of
horses in systems designed for efficiency rather than safety are all factors in
breeding wastage. Below
are figures on the number of mares bred and the resultant number of foals born.
Clearly the fertility of horses is quite low when compared to other species.
However there is an encouraging trend in the overall fertility.
Fertility
of mares bred in Australia (1992-1996). Why
are these figures so low? To start with there is no selection for fertility at
all. We are selecting for performance (mostly through economics) and the
conformation associated with good performance seems to favour a long sloping
vagina, which is definitely a poor conformation for breeding. Can we defend the
lack of selection for fertility? Most think so, as surely our aim is to produce
the ultimate athlete rather than a series of fertile but slow or similarly
performing animals. Given the lack of selection for fertility it seems difficult
to defend the ban on AI on technological grounds. A restriction on the number of
mares bred and their overall fertility makes sense on commercial grounds and is
more than likely driving the current sentiments on retaining a ban on AI. Why
the difference in Thoroughbred versus Standardbred fertility? The Standardbreds
used artificial insemination (AI) and that should have the helped the pregnancy
rate. When we looked at the number of cycles and the time of breeding cessation
we found that Standardbred breeders bred longer and for more cycles and thus the
overall fertility was not different. The reason for the differences in pregnancy
rate per cycle became more apparent when we examined the number of veterinary
examinations per mare. Significantly more examinations were performed in
Thoroughbred mares versus Standardbred mares. Why? Economics!!! Many of the
Standardbred mares were bred on multiple occasions per cycle and probably with
little or no relation to ovulation, but certainly with no data on fluid
accumulation in the uterus (infection) that could only be obtained with
ultrasonography. The Standardbred mares had a much higher percentage of
non-cycling mares, which was actually due to failure to identify teasing mares
rather than not cycling. Mostly, the lack of veterinary management was false
economics because at the end of the season there were fewer mares in foal and
the dates of conception were much later in the Standardbreds. My
point in the previous discussion is that it is the improvement in management
(both veterinary and farm) that results in more mares in foal at the end of the
season, but that because there are no selection procedures for fertility perhaps
we are getting mares in foal that normally would not have. In this scenario
there are mares with uterine abnormalities that need treatment and that with
therapy they become pregnant and then later have problems, as they were unable
to sustain the good uterine environment that is necessary to maintain pregnancy.
A classic example of this is the older mare bred on foal heat. As many as 35% of
mares over 15 years of age can be expected to loss their pregnancy by 45 days
(after being positively diagnosed as pregnant on day 15). We
are pushing these mares and they are not always standing up to the pressure. In
the early 1990’s we reviewed the veterinary literature on pregnancy loss and
found that most losses were associated with infection and that most were
bacterial in origin. Many people are aware that infectious viral abortion caused
by a Herpes virus can cause devastating outbreaks of abortion in intensively
managed horses (management again), yet this is insignificant as a cause of
wastage compared to the losses of early embryonic death and routine abortion
losses. Another interesting observation we noted was that most mares undergoing
early embryonic death had uterine fluid (inflammation) present when they
returned to heat (yet it wasn’t present when losses were first diagnosed) and
the cells types in the uterus demonstrated by biopsy were of the chronic
inflammatory type (lymphocytes). These findings led us to believe that probably
the most important cause of pregnancy losses was associated with uterine
inflammation/infection and that the reason theses mares were susceptible was
related to our pushing them to maintain an inter-foaling interval of one year.
In other words we are pushing mares too hard to get pregnant early. It
was these philosophies that led us to the thoughts on antibiotics during
pregnancy that are outlined below. We
decided to perform a study and treat potential cases of chronic uterine
infection with antibiotics, monthly, during all of thepregnancy. Because not all
EED and abortion is caused by bacterial infection/inflammation it was not
expected that all mares would maintain pregnancy. The population we chose to
treat had all undergone either EED or abortion at least once in the last season
prior to entering the trial. At
the conclusion of the study 43 mares had entered the program, they had a mean
age of 14.2 years and had EED or abortion an average of 2.2 times in the
preceding years. The
results of the study were that 36 mares became pregnant (84%). Of the 36
pregnant mares 3 had EED (8.3%), 4 aborted (11.1%) and 29 had foals (80.6%). A
study such as this, although encouraging, must be interpreted with caution as
there are no controls (i.e. a similar population of 43 mares that had no
treatment). Despite this we are confidant that we are beginning to make some
headway into preventing pregnancy losses. Another
area of recent improvement is recognition and management of the high-risk
pregnancy. These mares may have placental problems and present with premature
lactation or problems may be inferred from mares with stresses such as colic
surgery. Commitment to research has resulted in many
exciting discoveries that should continue and help us get more live foals each
year and in addition more foals from difficult breeders. |
