M.C.G. Davies Morel, V. Gunnarsson Animal Reproduction Science 64 2000 49–64 53
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 The overall and individual FR of the 27 stallions are given in Table 2. The average FR for
individual stallions was 68.0 S.D. = 11.1 per period. The overall mean FR, adjusted in accordance with model 1 was 67.7. The difference in fertility rate among the stallions is
highly significant P 0.001 and explains 3.8 of the total variance in the data according to model 1 Table 1. The range of values for adjusted FR 41.4–88.0 underlines this
large variance Table 2.
54 M.C.G. Davies Morel, V. Gunnarsson Animal Reproduction Science 64 2000 49–64
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 FR
a
Mean S.D.
1 50
129 48.0
56.2 8.6
2 63
85 66.7
59.3 10.1
3 58
101 74.1
73.6 7.6
4 79
104 41.8
41.4 6.5
5 70
96 65.7
57.3 7.9
6 41
91 75.6
69.3 10.3
7 43
120 79.1
78.6 7.7
8 35
84 77.1
65.5 10.3
9 46
109 63.0
72.7 8.8
10 72
105 41.7
62.1 7.8
11 65
122 72.3
72.4 6.6
12 55
114 85.7
84.6 6.8
13 45
107 82.2
80.8 7.6
14 66
108 57.6
63.3 7.9
15 58
115 74.1
74.7 7.5
16 64
109 84.4
88.0 8.9
17 81
106 71.6
73.3 6.6
18 65
115 73.9
74.3 7.0
19 53
104 66.0
66.8 8.3
20 71
83 74.7
74.3 6.4
21 75
123 60.0
60.9 6.0
22 36
104 47.2
62.2 8.9
23 52
112 73.1
68.1 9.5
24 65
94 70.8
72.4 6.4
25 53
112 41.5
43.6 7.1
26 64
120 68.8
67.3 7.3
27 48
100 79.2
63.8 9.6
Average 58.3
106.4 68.0
67.7 S.D.
12.7 12.1
11.1 10.7
Maximum 81
129 85.7
88.0 Minimum
35 83
41.5 41.4
a
Fertility 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.
Comparing all non lactating mares with lactating mares a similar non significant result is obtained. Again, however, the effect of the interaction between reproductive status and
M.C.G. Davies Morel, V. Gunnarsson Animal Reproduction Science 64 2000 49–64 55
Table 3 The effect of the reproductive status of the mare lactating, intentionally barren and unintentionally barren on
Icelandic stallion fertility rates
a
Reproduction status Mares
FR means FR adjusted
b
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
a
No significant difference exists between groups.
b
Fertility 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 thinpoor condition; b moderategood 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
Fig. 1. The effect of the interaction between stallion and mare reproductive status lactating vs. intentionally barren vs. unintentionally barren on individual stallion fertility rates. No significant interaction exists.
56 M.C.G. Davies Morel, V. Gunnarsson Animal Reproduction Science 64 2000 49–64
Table 4 The effect of mare body condition very thin, moderategood and very fat on Icelandic stallion fertility rates
a
Body conditions Mares
FR means FR adjusted
b
n Means
S.D. Very thin
21 1.3
71.4 71.1
10.3 Moderategood
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
a
No significant difference exists between groups.
b
Fertility rate adjusted in accordance with model 1.
difference was evident between mares in extreme condition very fat or thin and mares in moderategood 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 Based upon their training from January until early in the breeding season, stallions were
categorised into three groups: a intensive training; b moderate training; c no training. FR for stallions within these three categories are given in Table 7. Training level proved
to have a significant P 0.05 effect on stallion FR, but accounted for 1 of the total variance in fertility according to model 2 Table 1. Stallions in intensive training had
significantly P 0.05 higher fertility rate than the other two groups together. Stallions in
M.C.G. Davies Morel, V. Gunnarsson Animal Reproduction Science 64 2000 49–64 57
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
Maresstallion Length of period
FR mean FR adjusted
b
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
a
No significant difference exists between groups.
b
Fertility rate adjusted in accordance with model 2.
58 M.C.G. Davies Morel, V. Gunnarsson Animal Reproduction Science 64 2000 49–64
Table 7 The effect of training intensive, moderate, or no training on Icelandic stallion fertility rates
a
Stallions Maresstallion
FR mean FR adjusted
b
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
a
Significant differences P 0.05 were evident between moderate training and all others and between intensive training and all other groups.
b
Fertility 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 Maresstallion
Period length FR mean
FR adjusted
b
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
a
Significant difference P 0.05 was evident between in hand and pasture mating.
b
Fertility rate adjusted in accordance with model 3.
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
