A survey of the fertility of Icelandic stallions
M.C.G. Davies Morel
∗, V. Gunnarsson
1University of Wales, Welsh Institute of Rural Studies, Aberystwyth, Ceredigion SY24 5DP, UK
Received 15 December 1999; received in revised form 23 March 2000; accepted 18 July 2000
Abstract
Very limited information is available on the breeding performance of Icelandic stallions, let alone the effect that management practices may have had on such performance. As an extensively kept, largely genetically isolated breed of horse it provides a good model for the study of factors that affect reproductive performance without the additional complication of selective breeding, infectious infertility and breed effect. A survey was conducted using 27 Icelandic stallions covering 1590 mares within the normal Icelandic breeding system (May to September). During the season, stallions cover mares within three periods of time, each period being of a similar length (average 35.5 days). During period 1, mares are covered in hand and at pasture. During periods 2 and 3, all mares are covered at pasture. The overall fertility rate for Icelandic stallions was calculated. The effect of a range of variables on fertility was investigated statistically using a number of models in an attempt to minimise the effect of confounding factors. An overall adjusted fertility rate for Icelandic stallions of 67.7% was obtained. The following factors were shown to have a significant effect on fertility: age of mare (P <0.001), training level of stallion (P <0.05) and method of breeding (P <0.05). For some individual stallions reproductive status of the mare also had a significant (P <0.001) effect. Many of these factors have been observed to effect FR in other more intensively managed equine populations. However, the less dramatic detrimental effect of age and the lack of a significant effect of mare reproductive status in most stallions suggests that infertility problems are less evident in Icelandic mares, possibly due to less emphasis on selection for athletic performance and the accepted culling of subfertile stock. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Horse; Fertility; Breeding
1. Introduction
The Icelandic horse was introduced into Iceland, mainly from Norway, 9–11 centuries ago. It has subsequently remained largely purebred (Eldjarn, 1981; Adalsteinsson and
∗Corresponding author. Tel.:+44-1970-624471; fax:+44-1970-611264.
E-mail address: mid@aber.ac.uk (M.C.G. Davies Morel).
1Present address: Holar College, 551 Saudarkrokur, Iceland.
Thorkelsson, 1991; Palsson, 1996). The Icelandic horse stands 135 cm high with an es-timated worldwide population of 160,000 (80,000 within Iceland). The horses are largely managed extensively, stallions running out with their groups of mares under semi feral con-ditions during the summer. Some horses which are in training are kept in during the winter period and some of the more popular stallions cover mares in an in-hand mating system at the beginning of the season, before being turned out to pasture with mares. The breeding season of the Icelandic horse is from May to September with a peak from May to July (Dyrmundsson, 1994). Very limited information is available on the breeding performance of the Icelandic horse, let alone the effect that management practices may have. One re-port, however, suggests that fertility rates may be as high as 82.1% (Hugason et al., 1985), which compared to the fertility rates reported for other equine populations, for example, Thoroughbred 53–77% (Sullivan et al., 1975; Merkt et al., 1979; Bowen, 1990; McDowell et al., 1992), Ponies 35–82% (Day, 1939; Hugason et al., 1985; Bristol, 1987; Garrot and Taylor, 1990), Heavy horses 59% (Day, 1939) and Light horses 52% (Day, 1939) is very good. However, this figure arose from work carried out on a limited number of mares, and the results appear to exclude barren mares in some instances, hence, suggesting a possible over estimation of fertility (Hugason et al., 1985).
An official registration system for Icelandic horses has been in operation for many years but is limited in the information it records regarding reproductive performance. The fer-tility of Icelandic stallions, though believed to be high, is felt by many breeders to have declined in recent years with a greater incidence of subfertile stallions. This survey, there-fore, specifically aims to provide an overall figure for the fertility of Icelandic stallions. It is also hoped to provide information on the effect that various parameters may have on fertility rates. Only with such basic information is it possible to analyse the current fertility rates of Icelandic horses and to organise breeding management in a way best suited to maintaining good fertility. Such information will also create a starting point for further research and development work in the field. More generally this research, on an unimproved, genetically isolated group of ponies, rather than the more usually used horses, may also help eluci-date some of the differences that exist between horses and ponies; the differing effects of environment and the effects of man’s intensive management and selective breeding.
2. Materials and methods
A survey was carried out in 1995 with the co-operation of Icelandic horse breeding associations and individual stallion owners, whose stallions were standing at stud throughout Iceland. In total, 27 stallions were used in the study, covering a total of 1590 mares. The stallions were selected according to the following criteria:
1. Those used by the local breed association. This allowed the use of routine paperwork and minimised any differences between mare groups, as all mares had been accepted for breeding by the breed associations.
2. Those aged between 5 and 18, to minimise any age effect.
4. Those used during all three breeding periods, to allow direct comparisons to be made between in hand and pasture mating and between periods.
No changes were made to the normal extensive management of the stallions during the period of the survey. A standard report form was completed for each stallion request-ing information on general stallion condition, environmental influences or changes that might have affected the stallion, weather conditions, feeding, exercise or training and pasture. Information concerning the mares covered was also requested which included mare age, colour, breeding status, body condition along with the result of pregnancy di-agnosis. Foaling rates were obtained in the spring of 1996 from the breeding association records.
Icelandic stallions are bred in three distinct periods during a breeding season. The exact start date and length of each period is determined by the local breed association and/or the stallion owner or leasee. In general, the periods are similar in length, on average 5 weeks. During period 1, stallions are used for covering in hand or run out at pasture with mares, during periods 2 and 3 all stallions are out at pasture with a different group of mares for each period. During periods of pasture breeding mares are all turned out together on the same day with the stallion being released immediately after. He remains with that group of mares until the end of the period. In all cases, each pasture is only used for one group each summer.
2.1. Statistical evaluation
Statistical evaluation was carried out on data for 27 Icelandic stallions. The calculation for fertility rate (FR) was based on the result of foal or no foal for each mare that was covered. For mares covered in hand the FR is measured per cycle, whereas for mares at pasture it is measured per period.
As illustrated in Table 2, the range of values for adjusted fertility rates lies between 41.5 and 88.0, therefore, ANOVA is appropriate for statistical analysis of the results. When comparison within a factor or an interaction was required, contrasts among means (least squares means) were tested, assuming a normal distribution of the trait in question (Snedecor and Cochran, 1980).
The following factors were taken into account when statistically evaluating FR: 1. Factors (number of observations):
(a) stallion (27); (b) period (3); (c) training level of stallion (3); (d) reproductive status of mare (RS) (3); (e) body condition of mare (3); (f) colour of mare (2).
2. Covariates:
(a) age of stallion; (b) age of mare; (c) group size (number of mares within a period); (d) length of a period (days).
Table 1
Results for analysis of variance by models 1–4 for overall fertility rate for 27 Icelandic stallions in the breeding season 1995
Model Total R2 Source/covariate d.f. P-value Significance level R2
(a) Significance of factors and covariates in the models
No. 1 14.9% Age of mare 1 0.000 ∗ ∗ ∗ <1.0%
(Age of mare)2 1 0.000 ∗ ∗ ∗ 1.0%
Stallion 26 0.000 ∗ ∗ ∗ 3.8%
Reproduction status 2 0.309 NS
Stallion×RS 52 0.000 ∗ ∗ ∗ 6.1%
Colour group 1 0.977 NS
Body condition 2 0.928 NS
No. 2 3.6% Age of stallion 1 0.720 NS
(Age of stallion)2 1 0.937 NS
Group size 1 0.005 ∗ ∗ <1.0%
Length of period 1 0.008 ∗ ∗ <1.0%
Level of training 2 0.043 ∗ <1.0%
Period 2 0.106 NS
No. 3 (relevant) Breeding method 1 0.046 ∗ <1.0%
No. 4 (relevant) Number of coverings 3 0.472 NS
(b) Covariate coefficients
Covariate coefficient S.D.
No. 1 Age of mare 0.038s 0.011
(Age of mare)2 −0.0017 0.00042
No. 2 Age of mare 0.034 0.010
(Age of mare)2 −0.0016 0.00041
Age of stallion −0.0093 0.026
(Age of stallion)2 0.000096 0.0012
Group size of mare within period −0.0062 0.0022 Length (days) of period 0.0047 0.0018
was then performed, as appropriate, to obtain the adjusted % FR. Many more models could have been considered but for simplicity only those most likely to result in significance have been given. These two models were as follows:
Model 1:
Fertility rate=stallion×RS+colour of mare+body condition of mare+age of mare+ (age of mare)2.Stallion×RS refers to the interaction between these two variables.
Model 2:
Fertility rate=training level+period+age of mare+(age of mare)2+age of stallion+ (age of stallion)2+group size+length of period.
was made to minimise this effect of age by the selection of stallions in the age range 5–18 years, a limited effect was observed in this work (Section 3.1.7).
For the analysis of the effect the method of breeding had on stallion fertility two further models were used to specifically look at: the effect of the two different methods of covering (model 3) and the effect of the number of coverings (model 4).
To compare the two different methods of covering (in hand versus pasture) data was used from the first period only as no in hand mating was practised in periods 2 and 3. Data from all 27 stallions was available but only 416 mares were covered during the first period. The same statistical procedure was used as detailed above with the breeding method included as a factor in the following model:
Model 3:
Fertility rate = breeding method+training level of stallion+reproductive status of mare+body condition of mare+colour of mare+age of mare+(age of mare)2+age of stallion+(age of stallion)2+group size+length of period.
A further evaluation of the method of hand covering was carried out to see if the number of times that a mare was covered per cycle had an effect. Data from 17 stallions used to mate in hand 258 mares were available for this analysis. The same statistical procedure was used as detailed above with the number of coverings (1, 2, 3 or >3) included as a factor in the following model:
Model 4:
Fertility rate=number of coverings+training level of stallion+reproductive status of mare+body condition of mare+colour of mare+age of mare+(age of mare)2+age of stallion+(age of stallion)2+group size+length of period.
3. Results
In total, data from 27 stallions covering 1590 mares were collected, 16 (1%) mares were excluded as a result of, death before foaling (7), or incomplete statistics (9). These 16 mares were evenly distributed among the stallions.
Results for analysis of variance by models 1–4 for overall fertility rate for 27 Ice-landic stallions in the breeding season 1995 are given in Table 1. In the following re-sults adjusted means for stallion % FR are also given. Comparisons were made using the adjusted means so ensuring that the effects of the confounding factors, as given in the corresponding statistical model (1–4 detailed in Section 2), are taken into account.
3.1. Overall fertility rate (FR) of Icelandic stallions
Table 2
The fertility rates of 27 Icelandic stallion used in the 1995 breeding season
Stallion Number of mares Number of breeding days FR mean Adjusted FRa
Mean S.D.
aFertility rate has been adjusted in accordance with model 1.
3.1.1. The effect of mare reproductive status on Icelandic stallion fertility rate
Mares were categorised into three groups: (a) lactating mares; (b) barren mares that were intentionally not bred the previous year (1994); (c) barren mares that were mated in 1994 but did not foal in 1995. FR for mares within these three categories are given in Table 3. Adjusted FR is highest for intentionally barren mares and lowest for lactating mares, however, the difference is not statistically significant. In contrast, if the effect of interaction between reproductive status and individual stallions (Fig. 1) was tested statistical significance is achieved (P <0.001). This interaction accounted for the largest part of the total variance for stallion fertility rate of the individual factors given in model 1 (6.1%) (Table 1).
Table 3
The effect of the reproductive status of the mare (lactating, intentionally barren and unintentionally barren) on Icelandic stallion fertility ratesa
Reproduction status Mares FR means FR adjustedb
n % Means S.D.
Lactating 893 56.7 65.6 65.5 3.3
Barren (planned) 467 29.7 68.1 70.0 3.6
Barren 214 13.6 68.2 67.5 4.2
Total and average 1574 66.7
aNo significant difference exists between groups. bFertility rate adjusted in accordance with model 1.
individual stallions was highly significant (P < 0.001) and explained 4.6% of the total variance as per model 1. For eight stallions, the difference in FR between these two groups of mares was significant (P <0.05). For four (numbers 10, 14, 22 and 25), the difference was negative (average difference−43.28%), non lactating mares > lactating mares; for the other four (numbers 2, 11, 26 and 27) the difference was positive (average difference
+25.73%), lactating mares>non lactating mares. The overall FR for these eight stallions was 63.8%, compared with 69.0% as the overall FR of the other 19 stallions. This difference was not significant.
3.1.2. The effect of mare body condition on Icelandic stallion fertility rate
Mares were categorised into three groups: (a) very thin/poor condition; (b) moderate/good condition; (c) very fat. FR for mares within these three categories are given in Table 4. FR is highest for very thin mares and lowest for very fat mares. However, possibly due to the small number in each category, the difference was not statistically significant. No significant
Table 4
The effect of mare body condition (very thin, moderate/good and very fat) on Icelandic stallion fertility rates (%)a
Body conditions Mares FR means FR adjustedb
n % Means S.D.
Very thin 21 1.3 71.4 71.1 10.3
Moderate/good 1497 95.1 66.7 67.7 1.9
Very fat 56 3.6 64.3 66.4 6.4
Total and average 1574 66.7
aNo significant difference exists between groups. bFertility rate adjusted in accordance with model 1.
difference was evident between mares in extreme condition (very fat or thin) and mares in moderate/good body condition.
3.1.3. The effect of the colour of the mare on Icelandic stallion fertility rate
Mares were categorised into two groups: (a) light coloured and (b) dark coloured. No significant difference in FR for mares within these two categories was evident, 64.6% (light coloured) and 67.1% (other colours). No significant interaction between individual stallions and mare colour was evident when tested in accordance with model 1.
3.1.4. The effect of mare age on Icelandic stallion fertility rate
The average age of mares was 11.2 years (S.D. = 5.0), with a range of 1–28 years. The age of the mares had a highly significant (P < 0.001) effect on stallion fertility rate, particularly evident in the low FR in young mares (Table 5). This factor, measured as, age of mare+(age of mare)2, explained 1.7% of the total variance (Table 1).
3.1.5. The effect of breeding period (periods, length of periods, group size within periods) on Icelandic stallion fertility rates
On average, the breeding season for stallions in the survey spread from 13 May to 5 September 1995. There was no definite start and finish date for the whole season or for the different periods. The earliest a stallion commenced period 1 was 22 April and the last stallion finished period 3 on 10 October 1995. No statistical difference in fertility rate between periods was evident. Neither the group size of mares within periods nor the length of period had an affect upon FR. Both group size and length of period accounted for<1% of the total variance in fertility according to model 2 (Table 6).
3.1.6. The effect of training level on Icelandic stallion fertility rate
Table 5
The effect of age of mare on the overall fertility rate of 27 Icelandic stallions
Age of mare (years) Number of mares FR of each age group
1 1 100.0
2 4 50.0
3 65 58.5
4 45 62.2
5 60 63.3
6 106 70.8
7 152 71.1
8 129 71.3
9 113 72.6
10 101 71.3
11 101 70.3
12 90 62.2
13 96 74.0
14 90 66.7
15 83 60.2
16 59 74.6
17 77 66.2
18 53 56.6
19 40 65.0
20 37 64.9
21 31 48.4
22 17 64.7
23 12 8.3
24 7 14.3
25 3 33.3
27 1 100.0
28 1 100.0
moderate training had significantly (P <0.05) lower fertility rates than either of the other two groups.
3.1.7. The effect of age on Icelandic stallion fertility rate
Stallion’s average age was 9.7 years (±3.7) (Fig. 2). Age did not have a significant effect on FR over the whole breeding season. However, if analysis was carried out for period 1
Table 6
The effect of period on Icelandic stallion fertility rate (%)a
Period Mares Mares/stallion Length of period FR mean FR adjustedb
Mean S.D.
1 395 14.6 36.2 71.2 69.4 2.55
2 594 22.0 29.8 67.7 71.7 2.25
3 585 21.7 40.4 65.1 64.8 2.13
Total and average 1574 19.4 35.5 66.7
Table 7
The effect of training (intensive, moderate, or no training) on Icelandic stallion fertility rates (%)a
Stallions Mares/stallion FR mean FR adjustedb
Mean S.D.
Intensive 5 64.2 74.1 73.6 3.19
Moderate 14 58.6 63.7 63.7 1.87
No training 8 54.0 66.9 67.7 2.62
Total and average 27 58.3 66.7
aSignificant differences (P < 0.05) were evident between moderate training and all others and between
intensive training and all other groups.
bFertility rate adjusted in accordance with model 2.
Fig. 2. Age distribution of 27 Icelandic stallions in the breeding season 1995 in Iceland.
only a significant (P < 0.05) effect of age was apparent, but accounted for<1% of the total variance (Table 1).
3.1.8. The effect of the method of breeding on stallion fertility rate
The method of breeding was categorised during period 1 as: (a) in hand and (b) at pasture. The data available for the comparison of these two methods is limited to period 1, 17 stallions used in hand and 10 at pasture. The mares:stallion ratio and length of period were similar between the two groups. The method of breeding had a significant (P <0.05), effect on FR but accounted for only 0.9% of the total variance in fertility according to model 3 (Table 1). Stallions used at pasture had 11.6% higher adjusted FR than stallions used in hand (Table 8).
Table 8
The effect of method of breeding in period 1 on Icelandic stallion fertility rates (%)a
Method Stallions Mares Mares/stallion Period length FR mean FR adjustedb
Mean S.D.
In hand 7 258 18.3 36.9 69.8 66.7 5.1
At pasture 10 158 18.8 37.8 72.2 78.3 6.1
Total and average 27 416 18.5 37.2 70.7
Table 9
Effect of number of coverings per mare on fertility rate (%) of 17 stallions used in hand in period 1a
Number of coverings Mares FR means FR adjustedb
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
aNo significant difference exists between groups. bFertility 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.
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).
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.
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
In conclusion it is apparent that the calculated adjusted FR for Icelandic stallions of 67.7% maybe considered an accurate estimation.
possibly due to their better reproductive performance and the accepted culling of subfertile stock.
The significant difference in FR observed in favour of pasture bred mares is interesting and leads to the conclusion that mares kept in a more natural, less stressful, environment benefit, at least as far as FR is concerned.
Having provided an accurate estimate for the FR in Icelandic stallions this figure can be used as a baseline for future comparisons and investigations. This estimate, plus the elucidation of the main factors that affect FR in Icelandic horses should allow the future management of Icelandic stock to be manipulated accordingly and so capitalise on the beneficial aspects indicated. Lessons may also be learnt from these results with regard to the management of other equine populations.
Acknowledgements
The Agricultural College of Holar, the Icelandic Agricultural Productivity Fund, the Local Horse Breeding Association of Iceland, the Farmers Association of Iceland, Dr. Agust Sigurdsson, and Silvia Lutkins for statistical advise.
References
Adalsteinsson, S., Thorkelsson, F., 1991. Isenski hesturinn, litaafbrigdi. Islandsmyndir, Reykjavik.
Amann, R.P., 1993. Physiology and endocrinology. In: McKinnon, A.O., Voss, J.L. (Eds.), Equine Reproduction. Lea and Febiger, Philadelphia, London, pp. 658–685.
Asa, C.S., Goldfoot, D.A., Ginther, O.J., 1979. Sociosexual behaviour and the ovulatory cycle of ponies (Equus caballus) observed in harem groups. Hormones Behav. 13, 49–65.
Asbury, A.C., Lyle, S.K., 1993. Infectious causes of infertility. In: McKinnon, A.O., Voss, J.L. (Eds.), Equine Reproduction. Lea and Febiger, Philadelphia, pp. 381–391.
Arnason, Th., Bjarnason, Th., 1993. Growth and size of Icelandic toelter horses. In: Proceedings of the Horse Breeding and Production in Cold Climatic Regions, Reykjavik, 11–13 August.
Ball, B.A., 1993. Embryonic death in mares. In: McKinnon, A.O., Voss, J.L. (Eds.), Equine Reproduction. Lea and Febiger, Philadelphia, London, pp. 517–531
Bowen, E.L., 1990. Breeding records. Blood Horse 114, 4341.
Bowen, J.M., Wigington, S., Bergeron, H., 1983. Advancing the equine breeding season naturally. In: Proceedings of the 8th Equine Nutrition and Physiology Symposium, Kentucky, pp. 90–95.
Brinsko, S.P., Ball, B.A., Ellington, J.E., 1995. In vitro maturation of equine oocytes obtained from different age groups of sexually mature mares. Theriogenology 44, 461–469.
Bristol, F., 1982. Breeding behaviour of a stallion at pasture with 20 mares in synchronised oestrus. J. Reprod. Fertil. 32 (Suppl.), 71–77.
Bristol, F., 1987. Fertility of pasture bred mares in synchronised oestrus. J. Reprod. Fertil. 35 (Suppl.), 39–43. Carnevele, E.M., Ginther, O.J., 1992. Relationships of age to uterine function and reproductive efficiency in the
mare. Theriogenology 37, 1101–1115.
Carnevele, E.M., Griffin, P.G., Ginther, O.J., 1993. Age associated subfertility before entry of embryos into the uterus in mares. Equine Vet. J. 15 (Suppl.), 31–35.
Carnevale, E.M., Bergfelt, D.R., Ginther, O.J., 1994. Follicular activity and concentrations of FSH and LH associated with senescence in mares. Anim. Reprod. Sci. 35, 231–246.
Daels, P.F., Hughes, J.P., 1993. The abnormal estrous cycles. In: McKinnon, A.O., Voss, J.L. (Eds.), Equine Reproduction. Lea and Febiger, Philadelphia, pp. 144–160.
Day, F.T., 1939. Some observations on the causes of infertility in horse breeding. Vet. Rec. 51, 581.
Dowsett, K.F., Knott, L.M., 1996. The influence of age and breed on stallion semen. Theriogenology 46 (3), 397–412.
Dowsett, K.F., Pattie, W.A., 1987. Variation in characteristics of stallion semen caused by breed, age, season of year and service frequency. J. Reprod. Fertil. 35 (Suppl.), 645–647.
Dyrmundsson, O.R., 1994. Reproduction of Icelandic horses with special reference to seasonal sexual activity. Icelandic Agric. Sci. (Buvisindi) 8, 51–57.
Easley, J., 1993. External perineal conformation. In: McKinnon, A.O., Voss, J.L. (Eds.), Equine Reproduction. Lea and Febiger, Philadelphia, pp. 20–26.
Eldjarn, K., 1981. Ord I belg um islenska hestsinn og uppruna hans. Eidfaxi 4, 4–6.
Garrot, R.A., Taylor, L., 1990. Dynamics of a feral horse population in Montana. J. Wildl. Manage. 54, 603–612. Ginther, O.J., 1983. Sexual behaviour following introduction of a stallion into a group of mares. Theriogenology
19, 877–886.
Gunnarsson, V., 1997. A survey of fertility of Icelandic stallions. MSc thesis, University of Wales, Aberystwyth, UK.
Henneke, D.R., Potter, G.D., Kreider, J.L., 1984. Body condition during pregnancy and lactation and reproductive efficiency of mares. Theriogenology 21 (6), 897–909.
Hintz, H.F., 1993. Nutrition of broodmares. In: McKinnon, A.O., Voss, J.L. (Eds.), Equine Reproduction. Lea and Febiger, Philadelphia, pp. 631–639.
Hugason, K.M., Arnason, Th., Jonmundsson, J.V., 1985. A note on the fertility and some demographical parameters of Icelandic toelter horses. Livestock Production Science 12, 161–167.
Irvine, C.H.G., Alexander, S.L., 1991. Effects of sexual arousal on gonadotrophin-releasing hormone, luteinising hormone and follicle stimulating hormone secretion in the stallion. J. Reprod. Fertil. 44 (Suppl.), 135–143. Kubiac, J.R., Crawford, B.H., Squires, E.L., Wrigley, R.H., Ward, G.M., 1987. The influence of energy intake and
percentage of body fat on the reproductive performance of non pregnant mares. Theriogenology 28, 587–598. Lange, J., Matheja, S., Klug, E., Aurich, C., Aurich, J.E., 1997. Influence of training and competition on endocrine regulation of testicular function and on semen parameters in the stallion. Reprod. Domest. Anim. 32 (6), 297–302.
McDonnell, S.M., Murray, S.C., 1995. Bachelor and harem stallion behaviour and endocrinology. Biol. Reprod. Monogr. Ser. 1, 577–590.
McDowell, K.J., Powell, D.G., Baker, C.B., 1992. Effect of book size and age of mare and stallion on foaling rates in thoroughbred horses. J. Equine Vet. Sci. 12, 364–367.
Merkt, H., Jacobs, K.O., Klug, E., Aukes, E., 1979. An analysis of stallion fertility (foals born alive) from the breeding documents of the Landgestut cells over 158-year period. J. Reprod. Fertil. 27 (Suppl.), 73–77. Palsson, P.A., 1996. Er islenski hesturin hreinraektadur I 1000 ar. Eidfaxi 2, 18–19.
Pickerel, T.M., Cromwell-Davies, S.L., Caudle, A.B., Estep, D.Q., 1993. Sexual preference of mares for individual stallions. Appl. Anim. Behav. Sci. 38, 1–13.
Pozor, M.A., McDonnell, S.M., Kenney, R.M., Tischner, M., 1991. GnRH facilitates copulatory behaviour in geldings treated with testosterone. J. Reprod. Fertil. 44, 666–667.
Schideler, R.K., 1993. History. In: McKinnon, A.O., Voss, J.L. (Eds.), Equine Reproduction. Lea and Febiger, Philadelphia, pp. 196–203.
Snedecor, G.W., Cochran, W.G., 1980. Statistical Methods (7th ed.). The Iowa State University Press, USA, pp. 352.
Sullivan, J.J., Turner, P.C., Self, L.C., Gutteridge, H.B., Bartlett, D.E., 1975. Survey of reproductive efficiency in Quarter Horse and Thoroughbred. J. Reprod. Fertil. 23 (Suppl.), 315–318.
Toledo, L.R.A.DE., Oliveira, F.R.A.P.D.E., Agusto, C., Esper, C.R., Leite, F.G., Viera, J.M., Leao, J.F.D.E.S., Santos, G.D.E.F., 1991. Influencia de exercisio controlado na eficience reproductive de geranhoes (Effect of controlled exercise on reproduction in stallions) Boletim de Industria Animal 45 (1) (1988) 175–184 (Animal Breeding Abstracts 59 (1991))
Umphenour, N.W., Sprinkle, T.A., Murphy, H.Q., 1993. Natural service. In: McKinnon, A.O., Voss, J.L. (Eds.), Equine Reproduction. Lea and Febiger, Philadelphia, pp. 798–808.
