S E L E C T I N G Y E A R L I N G R A H M A N I SHEEP
H. A. KARAM
College of Agriculture, Alexandria University, Egypt
HE most efficient method of selection is that which results in the maxiT mum
genetic improvement per unit of time and effort expended (Hazel
and Lush, 1942). They found that the total score method or index was
most efficient. Hazel (1943) outlined the method for constructing selection indexes. Hazel and Terrill (1946) presented an index for selecting
Rambouillet lambs. Karam et al. (1953) suggested an index for farm flock
lambs.
The purpose of the present study was to construct an index for selecting
yearling Rahmani sheep, rams and ewes, for replacements. At present, the
most important traits to consider are body weight, fleece weight and type
of birth (litter size). They all have a direct effect on income from the
sheep enterprise, i.e. meat and wool. There are other traits such as market
grade, staple length and wool grade but they are not expected to be of
economic importance until there is a demand for them by the consumer
and the industry.
There are several constants which must be obtained and used in constructing the index. They are the relative economic value for each trait,
the phenotypic and genetic variances, and the phenotypic and genetic covariances between traits.
Materials
The data used in the present study were taken from the records of
Rahmani sheep kept on the Experimental Farm of the College of Agriculture from 1943 to 1956. The sheep were fed on Berseem (TriJolium
Alexandrinum) from November until May. During the summer they grazed
in the fields on waste feeds and were given a concentrate supplement consisting of one part undecorticated cottonseed cake to one part rice bran.
Results
The Economic Values. The economic value of a trait is defined as the
relative effect of a unit change on net profit. Any increase in the cost of
production due to that extra unit should be considered. Karam (1957,
1959a) estimated the average lambing percentage among Rahmani sheep
as 124% and the average lamb mortality, from birth up to 4 months of
age, as 26.9%. Using an annual replacement rate of 20% one would
expect a flock of 100 ewes to wean about 72.5 lambs each year.
1452
RAHMANI
SHEEP
1453
The economic value of a unit change in yearling weight is a function of
market price and the cost of production. Table 1 presents the average
price of mutton over a period of 10 years from 1947 to 1956. Years prior
to 1947 were still affected by Second World War conditions. T h e average
price of 1 kilogram of mutton was 25 piasters. There is no information
available on live weight prices. On the basis of an estimate ( 4 4 % ) for the
dressing percentage of Rahmani sheep as reported by Badreldin (1951)
the average price of 1 kilogram of sheep would be about 11 piasters.
TABLE 1. THE AVERAGE PRICES OF WOOL AND MUTTON ACCORDING TO
CAIRO AND ALEXANDRIA MARKETS FOR THE YEARS 1947 TO 1956
(ANNUAL STATISTICS, MINISTRY OF FINANCE)
Piasters per pound for:
Year
Mutton
Brown and
black wool
1947
1948
1949
1050
1951
1952
1953
1954
1955
1956
Average
10.08
10.38
10.45
11.00
12.55
12.13
11.30
11.08
11.14
11.13
11.12
7.97
9.06
9.12
10.92
12.77
12.71
11.52
11.6o
13.17
12.25
11.11
Therefore the price of the extra kilograms of sheep sold each year would
be 7.98 pounds. I n E g y p t mutton prices are almost the same as lamb
prices and mutton conformation has little or no economic value. The cost
of maintaining an extra kilogram of body weight in a flock of 100 ewes
from the end of the Berseem season until the new Berseem is available
will not be more than 3.08 pounds. This was based on the assumption that
one animal consumes (per day) an amount of feed worth an average of
one piaster. Such feeds are usually given to sheep in the dry lot. The maintenance cost while on Berseem was negligible. There will be no extra
charge for shipping since it is usually done on a per head basis. Therefore
the economic value of a 1 kilogram change in yearling body weight is
about 4.90 pounds.
About 117 lambs would be weaned every year if each lambing ewe in
the flock had a twin. This means 44.5 more lambs. A twin lamb averages
about 35 kilograms by the time it is 12 months old (Karam, 1959b). On
the basis of the previous discussion its value will be 3.85 pounds and the
cost of maintenance about 2.41 pounds. Ten more piasters should be subtracted for handling and shipping charges, leaving 1.34 pounds as the
1454
KARAM
TABLE 2. P H E N O T Y P I C VARIANCES AND COVARIANCES
Item
Yearling weight
Fleece weight
Type of birth
Yearling weight"
Fleece weight ~
Type of birth
36.99
0.55
0.08
--.39
- - . 02
0.12
a Recorded in kilograms.
economic value of an extra twin born lamb or 60.73 pounds for the 44.5
lambs.
The average price of 1 kilogram of brown wool (since all Rahmani
wool is brown) was 25 piasters (table 1). An extra kilogram of fleece will
not increase shearing costs since it is done on a per head basis. The cost
of maintaining an extra kilogram of fleece was assumed to be similar to
that of maintaining a kilogram of mutton. Therefore; the net profit of a
unit increase in fleece weight in a flock of 100 ewes was 15.06 pounds.
Phenotypic Variances and Covariances. The phenotypic variances and
covariances required for the index are presented in table 2. The variances
were obtained from earlier work published by the author (Karam, 1957,
1959b, 1959c). The covariances were calculated from the relationship
Sxi xj--rxi xj Sx~Sxj. The correlations were estimated within year and season
as presented in table 3. Separate estimates are given for each sex. The
correlations between body weight and fleece weight of rams and ewes were
similar and therefore were pooled (0.32). This was used in obtaining the
covariance. Morley (1950) found, in Merino sheep, that the correlation
between grease fleece weight and body weight was 0.30. Terrill et al. (1950)
reported an estimate of 0.54 for the correlation of the same traits in range
Rambouillet rams.
Twin born lambs are usually lighter in weight and shear lighter fleeces
than singles. This was true of the correlations obtained for ewes. Those of
rams were small and insignificant. One was --.10 and the other -[-.10.
The extra care and feed which ram lambs usually receive might be the
cause of such low estimates. Therefore, only the correlations obtained for
ewes were used in calculating the covariances needed.
TABLE 3. P H E N O T Y P I C C O R R E L A T I O N S A M O N G T R A I T S
Traits correlated
Body wt. and fleece wt. of rams
Body wt. and fleece wt. of ewes
Body wt. and type of birth of rams
t~o~dy wt. and type of birth of ewes
Fleece wt. and type of birth of rams
Fleece wt. and type of birth of ewes
Body wt. and type of birth produced by ewe
Fleece wt. and type of birth produced by ewe
N u m b e r of animals
166
190
238
193"
218
170
193
170
r
0.34___ .07
0.28-t-.07
- - . 10___+.06
- - . 15___. 07
0.10-+-. 07
- - . 14___. 08
0.23___+.07
0.05-4-.08
R A H M A N I SHEEP
1455
Table 3 also presents the correlations of body weight and fleece weight
with type of birth produced by the ewe. They were 0.23 and 0.05, respectively. They were used in developing an index for ewes with one
lambing record. Terrill and Stoehr (1942) found that ewes which were
heavier as yearlings tended to wean more pounds of lamb per ewe year
during their lifetime. The major part of the difference was due to more
ewes lambing per ewe year and more lambs born per ewe lambing. The
regression of pounds of lamb weaned per ewe year on pounds increase of
fall yearling weight of the ewes was 0.49, 0.48 and 0.63 for Columbia,
Corriedale and Rambouillet ewes, respectively. They also found that there
was a slight advantage in lifetime average fleece weight in favor of heavier
yearling ewes.
T A B L E 4. G E N E T I C V A R I A N C E S A N D C O V A R I A N C E S
Item
Yearling weight
Fleece weight
T y p e of birth
Yearling weight ~
7.03
Fleece weight ~
0.17
0.03
Type of birth
0.00
0.00
0.01
Recorded in kilograms.
Genetic Variances and Covariances. Table 4 presents the genetic variances
and covariances. The variances were estimated from the relationship so.2-h2sx2, where h 2 is the heritability of the trait. Heritability estimates of
yearling weight, fleece weight and type of birth were 0.19, 0.41 and 0.08,
respectively (Karam, 1957, 1959b, 1959c).
The present set of data was not large enough to obtain reliable estimates
of genetic covariances needed for the index. The genetic covariance between
fleece weight and yearling weight was obtained from data presented by
Morley (1.950) on Merino sheep. The genetic covariance between fleece
weight and type of birth was considered to be zero on the basis of the
small and insignificant phenotypic correlation between the two traits
(table 3). Also a value of zero was assigned to the covariance between body
weight and type of birth. It is believed that the phenotypic correlation
between them is mainly environmental.
The Index. Figure 1 is a path diagram illustrating the relationships
among the traits considered in the present study. The aggregate genic
value of an individual was defined as H~alGI-]-a.,G._,+a:~G:~ where G1,
G2 and G:~ are the genic values for yearling body weight, fleece weight and
type of birth, respectively, and a~, a._, and a~ are the relative economic
values for these traits. Their phenotypes are included in the index which
was defined by Henderson (1951 ) as:
I----alIl-+- . . . .
+anIn
=ClXa+ . . . .
--[-CnXn.
11,
.
.
.
.
, In
are indexes for estimating G1, . . . . , Gn utilizing
information on the phenotypes of all traits xl , . . . .
, Xn. The advan-
1456
KARAM
tages of using this method of calculating the index were outlined by
Karam et al. (1953).
The following three indexes were calculated and compared using the
correlation between each of them and the aggregate genic value (H). The
relative economic values used were 1.0, 3.1 and 12.3 for body weight, fleece
weight and type of birth, respectively.
Figure 1. Path diagram illustrating relationships among traits considered to have a
direct effect on income in a Rahmani flock.
Xrw, X~w and X ~ are phenotypic expressions
for yearling weight, fleece weight and type of
birth (singles or twins). G's are genic values.
E's are non-additive genetic effects plus environmental effects. H is aggregate genie
value of the individual and I the selection
index. Straight lines represent path coefficients and curved lines correlation coefficients.
(1) includes body weight (xl), fleece weight (x~) and type of birth (x3).
I---alI1-4-a212-~a313
I1-----. 182 xl-~-i. 064 x~@. 770 x~
I2~.000 xlq- .395 x ~ - . 0 6 3 x:~
I 3 ~ . 000 xl--~- 9009 x,9-J-.044 x3
~x1@13.1 x2@8.2 xg
rm~---. 456
(2) Includes body weight (xl) and type of birth (x:0.
I--'~alI1 @a212-~-a313
I 1 = . 197 XI-@ 9 640 x3
I 2 = .005 xi-J-.O16 x3
I3--'. 000 xi@. 043 x:,
--xi-~- 5.6 x3
r m - - . 418
RAHMANI SHEEP
1457
(3) Includes body weight alone.
I - - a l 11-t- a,,I2 +a~I.~
I1~0.190 xl
12~0.005 x2
Iaz0.000 x3
~X 1
ri~i--0.398
The x's are in terms of deviations from the means. If all lambs being
indexed have the same amount of information on each of the characters
studied the absolute values can be used rather than the deviations, xl and
x2 are measured in kilograms and x:~ will have a value of one for singles
and two for twins. Comparison of animals by index should be done within
year, season and sex. Adjustments should be made for the effect of age of
dam.
Discussion. Any selection index should include only heritable traits with
economic importance. The index provides the most efficient method of
giving the right amount of emphasis to the traits considered. The resulting
genetic improvement will depend on the number of traits considered, the
heritability of each trait, the genetic and phenotypic correlations between
traits and the intensity of selection.
Rahmani lambs are slow growers and they do not reach market weight
until they are close, to one year of age. Selection of ram and ewe lambs
for replacements is partly done at weaning and the rest of it is done when
they are 12 months old. This is the most important and intensive selection
during the lifetime of the sheep. The characters thought to be of most
economic importance, at that age, were body weight, fleece weight and
type of birth.
The considerations previously given in estimating the economic values
of each trait might be given different emphasis, depending on the location
of the farm, type of farming and market prices and costs in different
localities. Accurate and efficient estimates of the economic values requires
several things. They are the lambing percentage, replacement rate, average
age of the flock and the level beyond which a change in the character does
not have further effect on income. Also required are the market price of
one unit of a character and the cost of maintaining, producing and marketing the products which result from a unit change in a character. Cooperation of those concerned with sheep breeding and agricultural economists
is highly recommended for solving such problems.
Data available for the present index were not extensive enough to
allow a realistic estimate of the genetic covariances. When these estimates
are available they should be used to improve the index.
The correlations between each of the three indexes developed and the
aggregate genic value show that the first index, including body weight,
fleece weight and type of birth, should provide the greatest gain. The
1458
KARAM
second index, including body weight and type of birth, is more efficient
than the third index which includes body weight alone. The correlation
between the index and the aggregate genic value is a measure of the
genetic gain expected when the index is used compared to that expected
if the genotypes of the animals were known.
The second index can be used if the fleece weights are not available.
Its efficiency is close to that of the first. This is primarily due to the relatively small variability of fleece weights.
Another selection problem, although less important than that of yearling
sheep, is that of ewes having one lambing record. The general practice is
to add to the flock more ewes than are needed for final replacement. This
is because some of them may not turn cut to be good mothers and hence
leave the flock. An index developed for ewes having one lambing record
is as follows:
I--aJl+a,.,I2+aaI:~
I 1 - - . 187 xl-J-.943 x2-~.843 x.~
I 2 = .000 xl-~ .384 x2--.029 xa
I3--" .001 x l - - . 0 0 3 x 2 + .088 xa
---xi-[-I 1 x2-~-4 xa
rm--" .473.
where xl, x~ and x3 are the yearling body weight, yearling fleece weight
and type of birth produced by ewe, respectively. The aggregate genic value
is the same as that defined earlier. The phenotypic covariances of body
weight and fleece weight with type of birth were 0.49 and 0.01 respectively,
as obtained from the corresponding correlations given in table 3. The other
variances and covariances needed were taken from tables 2 and 4.
Comparing the correlations between each index and the aggregate genic
value showed that this index is more efficient than any of the three estimated for yearlings. This was expected since type of birth is a measure of
the animal's own performance and not of its dam as used in the other
indexes.
Culling should not be based on index alone. Defects must be selected
against and other information considered important must be taken into
account in particular cases. The applicability of the index for different
kinds of flocks and localities will depend on the amount of data available
and on the magnitude of the differences in the composition of the flock and
management and feeding practices.
Summary
In the present study three indexes for selecting yearling Rahmani sheep
were developed. The traits selected for were: body weight, fleece weight
and type of birth. The relative economic values, the phenotypic and genetic
variances and covariances required for the indexes were obtained and discussed. The indexes are as follows:
RAHMANI SHEEP
1459
(1) Includes body weight ( x l ) , fleece weight (x2) and type of birth (x3).
I=xl+13.1
x 2 + 8 . 2 x~
(2) Includes body weight (xl) and type of birth (x3).
I=x~+5.6
xz
(3) Includes body weight alone.
I---x1
The correlations between each index and the aggregate genic value (H)
were obtained and used in comparing the indexes. The most efficient was
index ( 1 ), the least, index (3).
An index for a ewe with a lambing record was also constructed. I t contained body weight and fleece weight as a yearling and her own lamb production.
Literature Cited
Badreldin, A. B. 1951. Growth and carcass percentage in Ossimi and Rahmani sheep.
Faculty of Agriculture, Giza, Egypt. Bul. No. 3.
Hazel, L. N. 1943. The genetic basis for constructing selection indexes. Genetics 28:476.
Hazel, L. N. and J. L. Lush. 1942. The efficiency of three methods of selection.
J. Heredity 33:393.
Hazel, L. N. and C. E. Terrill. 1946. The construction and use of a selection index
for range Rambouillet lambs. J. Animal Sci. 5:412 (Abstracts).
Henderson, C. R. 1951. Cornell University, Ithaca, N. Y. (Mimeo.).
Karam, H. A. 1957. Multiple birth and sex ratio in Rahmani sheep. J. Animal Sci.
16:990.
Karam, H. A. 1959a. Some factors affecting lamb mortality in Rahmani sheep. Empire
J. Exp. Agr. 27:133.
Karam, H. A. 1959b. Effect of some environmental factors on weaning and yearling
weights of Rahmani sheep. Empire J. Exp. Agr. (In press).
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Rahmani sheep and correlations among these weights. Empire J. Exp. Agr. (In
press).
Karam, H. A., A. B. Chapman and A. L. Pope. 1953. Selecting lambs under farm
flock conditions. J. Animal Sci. 12 : 148.
Morley, F. H. W. 1950. Selection for economic characters in Merino sheep. Ph.D.
Thesis, Ames, Iowa. Iowa State College Library.
Terrill, C. E., W. H. Kyle and L. N. Hazel. 1950. Correlations between traits of range
Rambouillet rams. J. Animal Sci. 9:540 (Abstracts).
Terrill, C. E. and John A. Stoehr. 1942. The importance of body weight in selection
of range ewes. J. Animal Sci. 1:221.