M.C.G. Davies Morel, V. Gunnarsson Animal Reproduction Science 64 2000 49–64 59
Table 9 Effect of number of coverings per mare on fertility rate of 17 stallions used in hand in period 1
a
Number of coverings Mares
FR means FR adjusted
b
Mean S.D.
Once 48
62.5 61.2
9.3 Twice
148 72.3
64.9 6.8
Three times 47
72.3 62.1
9.0 Four times or more
15 60.0
45.1 13.3
258 69.8
a
No significant difference exists between groups.
b
Fertility rates were adjusted in accordance with model 4.
3.1.9. The effect of the number of coverings on the fertility rate of stallions used in hand Mares that were bred in hand were covered on average 2.1 times, the number of coverings
did not significantly affect stallion FR Table 9.
4. Discussion
The overall adjusted FR of Icelandic stallions according to this survey is 67.7. This is somewhat lower than the previously report reported figure of 82.1 Hugason et al., 1985.
However, differences in management practices and the total number of mares used plus the apparent exclusion of some barren mares in Hugason’s work may go some way to explain the
difference. Factors such as intensity of use, preselection against low fertility, management, genetic similarity and mare infertility may also be argued to have affected the results. Such
factors were, however, considered and their effect minimised in the stallion population selected. The use of various statistical models ensured that other confounding factors were
taken into consideration. One factor that may have had an effect in this particular study is that of breeding management. It is conventional to report FR as the percentage of mares
conceiving to a single oestrous cycle Dowsett and Pattie, 1987. In this survey, however, such a figure was only possible for stallions bred in hand. When the stallion is covering
at pasture information on conception to a single cycle is harder to obtain. In this survey, mares were on average turned out at pasture with a stallion for 35.5 days which would
have allowed some mares to conceive to a second oestrus having failed at the first. The FR resulting from this work can only truly be considered to be the number of mares that foaled as
a percentage of the number of mares that were put to a stallion. This figure, however, would consider that mares which aborted had not conceived and this, in part, may compensate
for any over estimation of the figure due to the failure to detect conception per oestrous cycle.
In conclusion, the estimated adjusted FR, 67.7, arrived at in this study may be consid- ered an accurate estimate of the true fertility of Icelandic stallions. This figure compares
favourably with figures for other breeds of stallion Day, 1939; Sullivan et al., 1975; Merkt et al., 1979; Hugason et al., 1985; Bristol, 1987; Bowen, 1990; Garrot and Taylor, 1990;
McDowell et al., 1992.
60 M.C.G. Davies Morel, V. Gunnarsson Animal Reproduction Science 64 2000 49–64
The effect of reproductive status on FR was not significant when stallions were tested as a group. However, the figures suggest a lower FR for lactating mares and a higher FR for
mares intentionally barren. Reports for other breeds suggest the opposite; that conception rates are generally lower in barren mares Schideler, 1993. It is evident that if the incidence
of infertility problems is high within a population then the FR of barren mares would be depressed. The relatively similar FR for lactating and barren mares observed here would
suggest that the incidence of infertility problems within the Icelandic horse population is low. The highly significant effect of interaction between stallions and the reproductive sta-
tus of mares is interesting. This interaction indicates that for some stallions the effect of reproductive status of the mare does have a significant effect on FR. This is clearly demon-
strated by three stallions which showed the greatest difference between FR for lactating and barren mares 32.9, 51.6 and 56.0, in favour of barren mares. All three stallions
were sired by the same stallion who has a history of low FR when covering lactating mares Gunnarsson, 1997. This correlation may suggest that the effect of mare reproductive sta-
tus on stallion FR is in part genetically controlled. In addition, it is possible that sexual behaviour may have a bearing. In general, during pasture mating, it is the mares that initiate
courtship. Some mares show very active sexual behaviour towards a stallion, eliciting his attention and even defending the stallion from other mares Asa et al., 1979; Bristol, 1982;
Ginther, 1983; Daels and Hughes, 1993. It is apparent that barren mares tend to show this behaviour more than lactating mares, presumably due to the presence of the foal and lacta-
tionally depressed ovarian activity Daels and Hughes, 1993. It is possible, therefore, that stallions which show lower FR for lactating mares are slower breeders, exhibiting lower li-
bido and may need the strong foreplay of a mare in order to provoke sexual interest. As such the barren mares would have more control over the stallion which would predispose them
to a higher FR. Conversely, stallions that exhibit a higher FR with lactating mares, may be those with a very high, aggressive libido. Such stallions may object to the strong aggressive
sexual behaviour observed in some non lactating mares. In this survey, one such stallion violently attacked seven non lactating mares at pasture, though covered such mares in hand
with success. Such differences in behavioural patterns may be accounted for by differences in hormonal stimulation resulting from mare:stallion interaction, or the sociological status
of the stallion, both of which are known to affect circulating testosterone levels Irvine and Alexander, 1991; Pozor et al., 1991; McDonnell and Murray, 1995.
The effect of mare age, in particular old age, on FR has been reviewed extensively, generally it is associated with ovarian failure, reduced ova viability, uterine degeneration and
greater incidence of reproductive tract infections due to prolonged environmental exposure Carnevale et al., 1993; Schideler, 1993; Carnevale et al., 1994; Brinsko et al., 1995. The
lower FR observed in this work for 3–5 year old mares agrees with the literature for other breeds McDowell et al., 1992; Schideler, 1993. This is a reflection of the submaturity in
reproductive ability of such animals, which is unsurprising as Icelandic mares are relatively late maturing, not reaching mature size and conformation until 5–6 years of age Arnason and
Bjarnason, 1993. FR remained relatively unchanged until 22 years of age. This relatively high FR in older Icelandic mares may in part be due to the culling of infertile animals and
their removal from the calculations. This system of culling is not practised in other equine populations, for example, the Thoroughbred. In Thoroughbreds, FR declines and embryo
mortality increases in older age Carnevale and Ginther, 1992; Ball, 1993; Carnevale et al.,
M.C.G. Davies Morel, V. Gunnarsson Animal Reproduction Science 64 2000 49–64 61
1993 largely due to uterine infection arising from typically poor perineal conformation and resulting uterine incompetence Asbury and Lyle, 1993. Such conformational defects are
not observed in Icelandic ponies Easley, 1993. Limited information is available on the effect of training on FR, that available would
suggest a positive effect of moderate training and a negative effect of intensive training Toledo et al., 1991; Lange et al., 1997. The results of the current study demonstrated a
significant P 0.05, positive effect of intensive training and, negative effect of moderate training, which contradicts previous indications. However, little work has been done in
this area, it is hard to define what consists no, moderate or intensive training for different populations of horses and when, in relation to breeding, the training occurred. In this work
only five stallions were classified as being in intensive training compared to 14 in moderate training, experimental numbers are, therefore, very low. In this case, it is likely that those
stallions classified as not being in training, were in effect being “trained” when compared to other equine populations, as these stallions were all kept out over the winter time ensuring
they received some work from everyday activities, and hence, a basic level of fitness. It is also worth noting that stallions classified as in training were in fact only trained until late
in the first period and it is logical to assume that any effect would be most apparent during or very soon after training.
The current work supports previous studies Hugason et al., 1985; Bristol, 1987 in demonstrating that stallions at pasture had significantly P 0.05 higher FR compared to
stallions used in hand. This difference may in part be accounted for by the opportunity of some mares kept at pasture to express two oestrus periods during their time with the stallion,
and therefore, be covered on two cycles. In fact it is evident that some synchronisation of mares does occur in the presence of a stallion Bowen et al., 1983; Schultz, personal
communication, 1996 and as such several mares may have had the opportunity to exhibit two oestrus cycles per period 36.2 days. However, as discussed previously this may in
part be compensated for by the likely underestimation of FR due to aborting mares. This opportunity to be covered on two successive oestruses was not afforded to mares bred
in hand. Higher FR in pasture bred stallions may also be the result of a more sexually stimulating environment. This is supported by the apparent higher mating capacity observed
in stallions at pasture Bristol, 1982. It is known that the presence of an oestrus mare will effect endogenous hormone levels, in particular GnRH and testosterone Irvine and
Alexander, 1991, based on this knowledge it is now accepted that prolonged teasing with in hand covering is beneficial. Stallion management and the management of oestrus mares
may also have a bearing on the FR of the two different methods of breeding. All the stallions used in hand normally covered one mare per day with the rare exception of two per day.
Most mares were covered two or three times per oestrus at 48 h intervals. Mares were detected in oestrus by teasing and generally not by ultrasonic scanning. However, there is
still the possibility that inappropriate timing of covering may have inadvertently occurred, so detrimentally affecting FR.
The current study indicated that mare body condition, mare colour, breeding period period, length of period and group size, stallion age, and number of coverings did not
significantly affect FR. Previous research, however, would not necessarily support this. Though much of the work is conflicting, in general it is considered that severe obesity has a
detrimental effect on FR a widely held belief among Icelandic breeders and that mares in
62 M.C.G. Davies Morel, V. Gunnarsson Animal Reproduction Science 64 2000 49–64
moderate condition are more likely to conceive than mares that are thin Zimmerman and Green, 1978; Henneke et al., 1984; Kubiac et al., 1987; Hintz, 1993; Gunnarsson, 1997.
The current study would tend to agree in part with this, very fat mares having a slightly lower FR than average; though very thin mares appeared to have a slightly higher FR.
Lack of significance may in part be due to the low mare numbers. There has long been speculation that some stallions prefer mares of certain colours Pickerel et al., 1993; Davies
Morel, 1999. However, research in this area is very limited and the current study fails to demonstrate stallion preference. The lack of significant difference in FR between periods
is not unexpected as stallions were not used at either extreme of the breeding season. It is known that the performance of stallions at pasture is affected by both the weather and the
quality of grazing. In this study, no important or unusual differences in weather conditions were evident, between periods, or between the year in question and other years, however,
other years may prove different for the same stallions. No significant effect of the length of period or number of mares per group on FR was evident, though the longest period was
associated with the lowest FR. The range in these parameters was, however, only small. If the range had been greater a positive correlation between period length and FR and
a negative correlation between group size and FR might have been expected. The effect of stallion age in this study was, as would be expected, minimal. Stallions were selected
within the age range 5–18 years in order to minimise an age effect. Despite this, a significant P 0.05 association between age and FR was observed in period 1. However, even during
this period age had a very small influence on the total variance 1. If stallions outside the age limit of 5–18 years had been included, previous work on other equine populations
would suggest a significant age effect on FR would have been observed Amann, 1993; Dowsett and Knott, 1996. Finally, during in hand covering the number of matings per
oestrus also had no significant effect on FR, though the best FR were apparently obtained in mares that were covered two or three times per oestrus. Most other work has indicated
a significant effect Umphenour et al., 1993; Watson and Nikolakopoulos, 1996, lack of significance in the current study may be accounted for by the minimal human intervention
typical of Icelandic in hand breeding. Such a system allows a more natural, and arguable more accurate, determination of true oestrus and ovulation, in a less stressful environment
than is normal practice in other intensive equine breeding management systems.
5. Conclusions
