NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
180 AN APPRAISAL OF FACTORS AFFECTING FERTILITY IN SHEEP [Vol. XXI, Part III,
year. This change of .1405 lamb in lambing average on account of the increase in
age is practically the same as the average difference of .1403 lamb between the
expected lambing averages from second to seventh year of age, according to the
curvilinear regression calculated from within ewes within line differences.
This age effect leads to the consideration that if the majority of the ewes in the
flock could be of age at which maximum lambing percentage is expected, the flock
would give the maximum economic returns. The relationship of the lambing
average and the flock average age from year to year is found to be .9958, indicating
that the lambing percentage is affected by the flock average age.
Weight of ewe. It is generally recognized that by having breeding ewes in
thrifty condition and gaining in weight (flushing) before breeding, the lamb crop
is increased. The data in this case did not lend themselves for this study.
Other factors. Other equally important factors which need careful considera-
tion to determine their relative importance for the effectiveness of selection for
fertility are: the influence of sire, the maternal age effect on the performance of
the daughters, the type of birth and its influence on the lambing average and lastly
the individual differences in fertility. For the study of these factors, the records
of 1,368 lambings from 420 ewes born from eight lines on the experiment stations
were classified into different sire groups. Each of these sire groups was further
classified into two main divisions. (a) The ewes born to mature dams and (6) those
born to young dams. The dams four years of age and above were considered as
mature and those that were two or three years of age were treated as young. Each
of these divisions was further subdivided into twin born and single born ewes.
Under each of these subdivisions the lambing records of each of the ewes were
arranged against their age. The analysis of variance of 1,368 lambings with their
test of significance is given in Table II.
TABLE II
Analysis of variance of lambings from ewes born on the station
Source of variance
Total
Lines
Within lines
Between sires within lines
Within sires within lines
Ewes from mature vs. young
dams within lines
Within ewes from mature vs.
* young dams within lines.
Twin vs. singls born within lines
Within twin vs. single born
within lines
Between ewes within lines
Within ewes within lines
Between ages of same ewe within
Hues
Within ages of same ewe within
lines
d.f.
1367
7
1360
56
1304
8
1296
8
1288
412
876
38
838
..
Sums of
squares
440.1550
6.1602
433.9948
20.2067
413.7881
16.0037
397.7844
6.7127
391.0717
166.9774
224.0943
28.1428
195.9515
..
Mean
squares
..
.8800**
.3191
.3608
.3173
2.0005**
.3069
.8391**
.3036
.4053**
.2559
.7406**
.2338
..
F
value
..
2.76
1.14
..
6.52
..
2.76
..
1.58
..
3.17
..
..
Proportion
within
line
..
..
..
.0466
..
.0369
..
.0155
..
.3847
..
.0648
.4515
1.0000
Proportion
to
total
.0140
.0459
.0364
..
.0153
..
.3794
..
.0639
.4452
1.0000
** Highly significant
September, 1951]
R.
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DESAI
181
This table also gives the proportions of variations due to different factors within
the lines. These variations were tested against their own within variations.
Line effects. There are highly significant differences between lines for the lamb-
ing averages. Some of these differences may be confounded with station differences
which could not be separated out. The sums of squares of deviations, 6.1602,
as shown in Table II, is 1.40 per cent of the total sums of squares.
Sire effects. The results of this study indicate that the differences due to the
sires within the line are not significant. This is mainly because of the fact that the
whole data are from the selected ewes. The ewes that fall within a certain standard
of selection are retained and those below that standard are culled out. Naturally
the deviation that would exist due to the sires has been removed by selection. The
sum of squares of deviations of 20.2067 due to 64 sires, contribute 4.66 per cent
of the within line differences or 4.59 per cent of the total variations.
Maternal age effect on the daughter's performance. The differences in lambing
percentage due to age, as previously shown, are highly significant. Possibly they
may be due to the changes in the endocrine functions which occur according to age.
Can such changes in a dam affect the performance of her daughter ! To study this
aspect, the lambing records were grouped, as already mentioned, on the basis of
whether they were born to dams when they were mature or young. The differences
between these two groups, due to the age of the dams are highly significant. The
sums of squares of deviations of 16.0037 from their mean are 3.69 per cent of the
total differences within lines, or 3.64 per cent of the total differences. The question
arises, naturally, as to the direction of these differences, whether they are in favour
of the mature dams or of the young dams. The data classified for this purpose by
age of dam and daughter are given in Table III. These differences show that the
young dams have some influence on their daughters in giving higher lambing per-
centage than those born to mature dams.
TABLE III
Lambing percentage of daughters by their age and age of dam
Mother
Mature dam
Young dam
Performance of daughters
Lambing
. per cent
Number . . .
Lambing
. per cent
Number . . .
Age of daughters in years
2
124
213
129
182
3
137
210
144
190
4
149
138
162
138
5
149
79
144
85
6
149
37
161
46
7
138
16
162
21
The further classification of these data by the type of the birth of the daughters
is given in Table IV. The effect of the young dams on the performance of their
182 AN APPRAISAL OF FACTORS AFFECTING FERTILITY IN SHEEP [Vol. XXI, Part III,
daughters is also consistently indicated when the twin born daughters from the
mature dams are compared with those from the young dams. The same is true in
the case of the single born daughters.
TABLE IV
Lambing percentage of daughters by type of birth and age
Dam
Mature dams
Young dams
Daughters
Type of birth
Twin born
Single born
Twin born
Single born
Performance
Lambing
Number
Lambing
Number
Lambing
Number
Lambing
Number
per cent
per cent
per cent
per cent
Age of daughters in years
2
128
159
115
54
134
82
125
100
3
132
154
150
56
151
82
139
108
4
149
103
151
35
174
65
152
73
5
150
66
146
13
151
39
137
46
6
153
34
100
3
164
25
157
21
7
138
16
..
. .
158
12
167
9
The records were further grouped by stations with a similar classification, as
in Table V, under each station. The same trend in favour of the young dams was
noticed for each of the stations. When the grouping of the data was based on
the different lines, it was clearly demonstrated in the lines where the number of
records was high.
To test the validity of this effect further, the lambing records of the daughters
born to the same dam at different ages were pooled together for every station.
The number of ewes having such daughters with records, went down very markedly.
Even then the lambing percentage of the daughters born to their mothers when
young was higher than that of those born to them when mature. It was also
apparent that most of the daughters from the young dams only came into selection
Thus according to these data, there appears to be a trend indicating that ewes
born to young dams have given higher percentage of lamb crop. On the average,
the daughters from the young dams have given according to the data .0933 lambs
more than those by daughters from mature mothers
Effect of type of birth. When 1,613 lambings were classified by type of birth
and by age, the twin born ewes consistently showed higher percentage of lamb crop
at every year of age. The lambing percentages of these two groups are given in
Table VI by age.
September, 1951]
R.
N.
DESAI
183
TABLE V
Lambing percentage of daughters by type of birth, age, and station
Station
Dam
S.E.
Mature
Young
W.C.
Mature
Young
N.W.
Mature
Daughters
Type-birth
Twin
Single
Twin
Single
Twin
Single
Twin
Single
Twin
Single
Lambing
per cent
L. per cent
Number
L. per cent
Number
L. per cent
Number
L. per cent
Number
L. per cent
Number
L. per cent
Number
L. per cent
Number
L. per cent
Number
L. per cent
Number
L. per cent
Number
Age of daughters in years
2
118
17
100
4
120
5
114
14
117
42
118
11
154
24
127
15
142
48
117
18
3
147
15
125
4
100
4
136
14
133
39
155
11
154
26
157
16
143
49
168
19
4
138
13
150
2
167
3
144
9
145
22
162
8
181
21
137
8
158
33
158
12
5
129
7
100
2
133
3
133
6
118
11
100
1
131
13
125
4
150
26
100
2
6
167
3
..
..
100
1
..
167
3
..
..
140
5
100
1
150
18
150
2
7
150
2
..
..
..
..
..
..
..
..
..
..
150
2
..
..
136
11
..
..
184 AN APPRAISAL OF FACTORS AFFECTING FERTILITY IN SHEEP [Vol. XXI, Part III,
TABLE V—contd.
Lambing percentage of daughters by type of birth, age, and station—contd.
Station
Dam
Young
N.C.
Maturo
Young
Daughters
Type-birth
Twin
Single
Twin
Single
Twin
Single
Lambing
per cent
L. per cent
Number
L. per cent
Number
L. per cent
Number
L. per cent
Number
L. per cent
Number
L. per cent
Number
Age of daughters in years
2
137
27
145
20
127
52
114
21
115
26
120
51
3
154
28
135
23
118
51
136
22
154
24
136
55
4
174
23
152
21
146
35
138
13
167
18
157
35
5
158
12
133
12
173
22
175
8
173
11
142
24
6
188
9
157
7
150
10
..
..
160
11
162
13
7
180
5
167
3
133
3
..
..
140
5
166
6
S. E.-South East; W. C.-West Central; N. W.-North West; N. C.-North Central.
The number of records of the twin born ewes, on which the percentage is worked
out, is consistently higher than that of the single born ewes. This is mainly because
of the selection for twin born lambs for replacing the flock. When the above data
are plotted against age of ewe, as in Plate XIII, fig. (C), it shows that the twin born
ewes (TB) consistently keep themselves at a higher level in lambing average than
the single born ewes (SB). Another peculiarity that can be noticed in this figure
is that the differences between the averages of the two groups remain unchanged
until three years of age, then they widen very much at the fourth year and continue
to remain so up to the sixth year. Can this be interpreted that as the preproductive
September, 1951]
R.
N.
DESAI
185
functions come to perfection at maturity, the differences between these two groups
are clearly defined in Plate XIII. The mean difference between them is .12 lambs.
The behaviour of these two groups after the sixth-year of age, as in Plate XIII,
fig. (C), is not conclusive because of the small numbers of records at this age.
TABLE VI
Lambing percentage of twin and single born ewes
Type of birth
Twin born
Single born
Performance
Lambing per cent
Number
Lambing per cent
Number
Age of ewes in years
2
123
346
113
240
3
134
239
128
172
4
158
166
127
119
5
151
122
124
62
6
152
71
136
33
7
161
34
182
9
The differences in lambing percentages of the two groups are highly significant
when tested by the analysis of variance method and are only significant at 5 per cent
level when tested by x 2 test. The sums of squares of deviations of 6.7127 due to the
type of birth is 1.55 per cent of the within line differences, or 1.53 per cent of the
total. The study of the performances of the twin and single born ewes indicate
that the former not only give higher percentage of lamb crop but also the higher
percentage of triplets than the single born ewes. The differences in triplets produc-
tions, though not statistically significant, are close to the probability at 5 per cent
level. At the same time, it was found that the number of still births and deaths at
birth are significantly more in the case of twin born ewes than in the single born
ones. These results agree very well with those of other workers like Kiser and
Christagau [1940].
Every ewe, except in the case of identical twins, is a different genotype and
would react differently with the environment. Similarly fertility would be affected
at various stages by many physiological factors. It is, therefore, expected that
individual ewe would be a source of large variations. As the data did not allow for
the analysis to study the individual variation within sire groups, these were
estimated within lines. The sums of squares of 166.9774 between ewes within lines
is 38.47 per cent of the within line variations. This shows that the selection of the
ewes must be very carefully made on individual basis in order to increase the lambing
percentage of the flock.
Age effect
The effect of age was also analysed by the method of analysis of variance. The
sums of squares of deviations of 28.1426 due to age of ewes is 6.48 per cent of the
within line differences, or 6.39 per cent of the total. This is very close to the per-
centage of 6.21 shown by the regression method when the effect of year is removed.
186 AN APPRAISAL OF FACTORS AFFECTING FERTILITY IN SHEEP [Vol. XXI, Part III,
Effect of inbreeding
Many workers have investigated the effects of inbreeding of dam on fertility
in many of the smaller species of animals. Usually there is not much inbreeding
practiced in sheep and cattle. According to Winters et al. [1947] the average in
breeding coefficient of sheep at the Minnesota Experiment Station may not
have been raised more than 4 per cent as a result of inbreeding from 1936 to 1941.
Prediction of lambings
In order to find out if prediction of future lambings could be made from the
first or first and second lambings combined, two correlation coefficients were
calculated. One between the second year lambings vs. third, fourth and fifth year
lambings combined, and the other between the second and third year lambings
together vs. fourth and fifth year lambings.
Both of the correlations are highly significant, but the difference between
the two correlations is not significant. Therefore, it can be concluded that the
predictions can be made from the second year lambing as well as from the
second and third year lambings. However, it would be safer to predict from the
latter than from the former, even though the difference between the two correla
tions is not significant.
DISCUSSION
The effects due to various factors may be true but the adjustment that may be
made for them should account for a substantial portion of the total variation to
effectively increase the genetic gain. The expected gain for one cycle of selection
is directly proportional to √1-a [Cockerham, 1949] where 'a' is the fraction of the
total variation adjusted for. According to this study the expected gains by adjusting
for the factors influencing lambing average are given in Table VII in the order of
their importance.
TABLE VII
The expected gains by adjusting for factors related to fertility
Variation due to
Ewes within lines
Age of ewe alone
Age and year
Sires
Age of dam
Type of brith
Percentage of total
variation
adjusted for
'a' × (100)
37.64
6.21
3.70
4.59
3.64
1.53
Percentage of in-
crease expected in
genetic gain at
each selection
1
1-a-1
100
26.94
3.26
1.90
2.38
1.87
.77
188 AN APPRAISAL OF FACTORS AFFECTING FERTILITY IN SHEEP [Vol. XXI, Part III.
The lambing percentage rises as the age increases up to the fifth year of age
and then gradually declines at an average rate of .1403 lamb, when the year effect is
ignored.
The sires within the line did not show significant differences in the lambing
average of their selected daughters. Still the variation that they have caused is
quite high as compared with that due to other factors.
It was found that the daughters born to dams when young, produced .0933
more lambs than those born to mothers when mature. The reasons for such an
effect are not known. However, it may be suspected to be due to some physiological
functions of dams having influence on the growing young one either in the intra-
uterine stage or during nursing. There are highly significant differences in the
lambing averages of the ewes within the lines, emphasizing the importance of the
individual ewe selection.
The twin born ewes give birth, on the average, to .12 more lambs than the
single born ewes. They also show more consistency in their higher performance
throughout their productive life. The frequency of triplets production is also higher
in the twin born group than in the single born
The prediction of the future lambings can be done from the first two lambings
at the second and third year of age.
REFERENCES
AsDell, S. A. and J. P. Wilman (1941). The causes of still birth in swine and an attempt to
control it. J. Agric. Res. 63, 345-353
—,
R. Bogart and G. Sperling (1941). The influence of age and rate of breeding upon
the ability of the female rat to reproduce and raise young. Cornell Univ. Memoir 238, 28
Bonnevie, Kristue (1919). Heredity of twins. Eugenical News, 4, 10
Busch, Chapmannu (1932). Twinning, sex ratio and genetic variability in birth weight in sheep
Jour. Heredity 23 (11)
Carlyle, W. L. and T. F. McConnell (1902). Some observations on sheep breeding from the
experimental station flock records. Wisc. Agr. Expt. Sta. Bull. 95
Clark, R. T. (1934). Studies on the physiology of reproduction in sheep. Anat. Rec. 60 (2)
125-134
Cockerham, Clark (1949). Heritabilities and repeatabilities in sheep. M. S. Thesis. Univ.
North Carolina
Gunn, R. N. C. (1942). Studies in fertility in sheep. Australian Vet. Jour. 18,3-15
Hammond, John (1914). On some factors controlling fertility in domesticated animals. J.
Agric. Sci. 6, 263-277
—(1914).
Fertility in mammals and birds. Biol. Rev. 16, 165-190
—(1944).
On the breeding season in sheep. J. Agric. Sci. 34(2), 97-105
Heape, (1899). Note on the fertility of different breeds of sheep with remarks on the prevalence
of abortion and barrenness therein. Proc. Royal Soc. B. 65 ; 65-111
Johanson, Ivar. (1932). Multiple births in sheep. Proc. Am. Soc. Ani. Prod. 285-291
Kiser, O. M. and R. J. Christagau. (1940). Breeding for single and twin lamb production. Proc.
Am. Soc. Ani. Prod. 182-184
Marshall, E. R. and C. G. Potts (1921). Flushing and other means of increasing lamb crop.
U. S. D. A. Bull. 996
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Nichols, J. E. (1925). Fertility in Southdown sheep. J. Agric. Sci. 16(3) 365
—(1926).
Fertility if sheep. J. Ministry of Agriculture B. 33(1) 218
Roberts, Elmer. Fertility in Shropshire sheep. J. Agric. Res. 22, 231
Snedecor, G. W. (1946). Statistical methods applied to experiments in agriculture and biology
3rd ed. Iowa State College Press. Ames. Iowa
Terril, C. E. and J. A. Stoehr (1939). Reproduction in range sheep. 32nd Annual Proc. Am.
Soc. Ani. Prod.
Winters, L. M., Comstock, R. E., Hodgson, R. E., Kiser, O. M., Jordan, P. S, and
Dailey, D. L. (1947). Experiments with inbreeding swine and sheep. Mem. Agr. Expt.
Sta. Bull., 364
THE INHERITANCE OF FERTILITY IN SHEEP
By R. N. DESAI, Assistant Professor, Animal Husbandry and Dairying*
Banaras Hindu University and L. M. Winters, Professor of Animal Husbandry,
University of Minnesota.
(Received for publication on January 1951)
S OMETIMES it may be possible to increase the production or performance
of an animal by partially controlling the environment but to do so in its entirety
is practically impossible. On the contrary the improvement in heredity is generally
a permanent investment and mainly depends upon the genetic variability.
The phenotypic variance is due to both genotypic and environmental differences
or, in other words, heritable and non-heritable variations. Heritability may be defined
as the heritable fraction of the total variance i.e., σ 2
H / σ 2
H + σ 2
E . Generally
it is expressed as the percentage of the total phenotypic variations. So also it is a
description of a particular characteristic in a certain population at some definite
moment. It can be raised or lowered by any breeding system or by changing the
environment. The methods of estimating it are based upon the degree to which the
closely related animals resemble each other more than the less closely related
animals do.
A dependable estimate of heritability and repeatability of fertility is useful in
deciding upon the breeding plans which are most likely to bring about the genetic
improvement. The repeatability is the coefficient of correlation between the
succeeding records made by the same animal in the same herd.
Generally it is believed that heritability of fertility within a breed or line is very
small [Wright, 1922]. Very little work is done on it in sheep. Recently Cockerham
[1949] estimated it to be .10±.05, and repeatability .06±.03. Joseph [1932] found
correlations of .21 and .31 in two groups between the twinning tendency of the ewes
and their daughters. The findings of Wentworth [1913] and Kiser and Christagau
[1940] also indicate similar limits of its estimates.
The purpose of this study was to estimate the heritability and repeatability of
fertility in sheep on the University of Minnesota Experiment Station so that they
could be used for planning a future breeding programme to improve the twining
tendency in the flocks.
MATERIALS AND METHODS
The data used for this study were taken from the records of the University of
Minnesota Experiment Station. The management and the feeding, etc., of the
flocks were more or less similar. The estimates of heritability for fertility were
made by intra-sire regression of lambing averages of offsprings on those of the dams.
Repeatability was also estimated by the intra-class correlation of lambing averages
by the method as outlined by Snedecor [1946].
*Paper no. 2689 Scientific Journal Series of the Minnesota Agricultural Experiment Sta
tions. This paper is condensed from Part II of a thesis submitted by the Senior author to
the Graduate Faculty of the University of Minnesota in partial fulfilment of the requirements
for the degree of doctor of Philosophy.
191
192
THE INHERITANCE OF FERTILITY IN SHEEP [Vol. XXI, Part III,
ESTIMATES OF HERITABILITY
Estimates of heritability for lambing averages were made by two procedures
for using the data under the method of intra-sire regression of offspring on dam.
In one procedure, the lambings of dam and daughters were tallied within a sire
group. For example, if daughter has only first and second lambings, then only
first and second lambings of dam were taken for forming a pair. If another daughter
from the same dam and sire has only second and third lambings, then only second
and third lambings of dam were taken for another pair. This, naturally, gave as
many pairs as the number of the daughters under each sire group. Of course, in
many cases, some of the data had to be dropped out simply because the lambings
could not be paired. The records from 48 ewes were left out in this way.
According to the second procedure, the different lambings of dam and the
lambings of all the daughters from the same dam and sire were averaged to form one
pair. When a dam had daughters from more than one sire, the dam's average was
used in forming a separate pair for each of the sires. In this case, the number of
pairs did not exceed the number of dams. This procedure has an advantage of
using the all data for the computation of regressions.
The intra-sire regressions were calculated from within sire groups sums of
squares and products. The dams and daughters with their lambing averages by
lines are given in Table 1.
TABLE 1
Numbers and lambing averages of dams and daughters in intra-sire regression seperated
by lines
—
S. E. Shrop
S. E. Crossed
W. C. Columbia
W. C. Shrop
N. W. Shrop
N. W. Crossed
N. W. Hamp
N. C. Shrop
N. C. Crossed
Over all
Number
of
sires
8
4
7
5
11
4
4
14
17
74
Number
39
25
98
25
121
23
33
70
108
542
Dams
Lambing
average
1.41
1.23
1.66
1.45
1.65
1.45
1.52
1.62
1.72
1.52
Number
54
25
125
28
139
25
44
86
113
639
Daughters
lambing
average
1.24
1.32
1.45
1.55
1.49
1.42
1.42
1.24
1.38
1.39