1Units
are $Cdn for production, days for HL, linear score unit for SCS, cases for
mastitis, and minutes for milking time.
2Discounted
expressions per unit of genetic over 15 years at a rate of 5%.
3Marginal
value times number of expressions divided by genetic standard deviation and
expressed relative to 100 for production.
The TEV is
expressed in $ to increase its appeal to and use by breeders because it allows
them to compare bulls on a scale they easily understand.
In addition, the TEV can be used to compare bulls according to semen
price and expected profits. Dividing
a sire's TEV by the number of units of semen required to produce a milking
daughter (approximately 6 in most herds, depending on conception and survival
rates) yields the expected future profit from using a unit of semen.
Based on this simple calculation, a breeder can afford to spend about
$5 more per unit of semen for each additional $30 in TEV.
The current top Holstein bull in Canada has a TEV of $846.
2.2 Sub-indexes
The
decision to use a set of sub-indexes, rather than a single index with many
traits was based on the willingness to sacrifice some potential genetic
response by using the theoretically optimal index in exchange for simplicity
and, hopefully, a better understanding of the index by breeders.
According to Dekkers and Gibson (1998), "an index that is based on
sound scientific principles but is not accepted has much less impact on
selection for an overall breeding goal than does an index that may not be
optimal technically but that receives acceptance by the targeted users".
The sub-index procedure simplifies the development, interpretation, and
explanation of the overall index because each sub-index can be derived and
explained independently from the other sub-indexes (Dekkers and Gibson 1998).
This factor becomes more critical as an index becomes more complex and, at the
time of its introduction in 1996, the TEV was much more complex than were the
other indexes (such as the LPI) available to dairy breeders in Canada.
In addition
to the practical aspects related to simplicity and understanding by breeders,
the sub-index approach has several technical advantages over the traditional
single index approach that were summarized by Dekkers and Gibson (1998) and
include 1) traits within a sub-index are usually biologically related and
hence, genetic relationships among them are often known more precisely than
are the relationships among less biologically related traits that may be
included in a single overall index, 2) weights among traits within a sub-index
are relatively robust to estimation errors, due to the usually high genetic
relationships among traits in a sub-index, 3) because sub-indexes are often
lowly correlated, the weights on sub-indexes follow more closely the perceived
relative importance of the selection goals for each sub-index, and 4)
sub-indexes facilitate the a) combining EBV from separate genetic evaluations,
b) the adoption of customized selection indexes with varying emphases on
sub-indexes, and c) the future introduction of new sub-indexes.
A technical disadvantage to the sub-index approach is that it does not
account for genetic correlations among traits in separate sub-indexes, but
this factor is not likely to be of great importance in most common situations
(Kulak et al. 1996).
2.2.1. Production sub-index
The current
sub-index for production is
PROD = (9 ´ ProteinEBV + 2 ´
FatEBV) ¸ 11,
which places standardized weights of 9 on EBV for protein yield and 2 on
EBV for fat yield. (The result is divided by 11 for standardizing of the
sub-indexes in the TEV). The
weights were based on a study by Gibson et al. (1995) that considered future
demand for dairy products and changes in management.
Since the adoption of the current test-day genetic evaluation of dairy
cattle (Jamrozik et al. 1997) in February 1999, the EBV for protein and fat
have actually become sub-indexes themselves. These sub-indexes use EBV for
yield in lactations 1, 2, and 3 to maximize genetic progress for an aggregate
genotype that is based on production in lactations 1 to 8.
As documented in an unpublished technical report, Collard et al. (1999)
estimated the relative economic value for yield in each lactation based on the
expected number of discounted expressions of lactational production and
genetic correlations among lactations. They
confirmed that the optimal selection index was one that applied approximately
equal weighting upon standarized EBV for yield in each of the first three
lactations.
2.2.1. Longevity sub-index
The
sub-index for longevity is simply the published sire ETA for functional herd
life (then standardized). More detail about the Canadian evaluation for herd
life can be found in articles published by Jairath et al. (1998) and by
Boettcher et al. (1999). The
published ETA for herd life combine separate EBV for "direct" herd
life (DHL), which is based on actual records of daughter survival, and
"indirect" herd life (IHL),
which is based on sire ETA for conformation traits.
The EBV for
DHL are calculated with a three-trait animal model, for which the three traits
are cow survival in lactations 1, 2, and 3.
The most recent evaluation was based on records from approximately 1.7
million cows. Although survival
is recorded as a binary (0/1) trait, a linear model is used for the
evaluation. Fat and protein
production (deviation from herd average in first lactation) are included as
covariates in the analysis, so the resulting EBV are for functional herd life. The overall EBV for DHL is the average of EBV for survival in
lactations 1, 2, and 3.
The EBV for
IHL are calculated based on an index of sire ETA for mammary system, feet and
legs, frame and capacity, and rump. Respective
weights for these 4 components are 8, 4, 1, and 1.
The EBV for
direct and for indirect herd life are then combined with procedures similar to
the method used to calculate international sire EBV (Schaeffer 1994). The resulting EBV for combined herd life has a correlation of
approximately 0.80 with the ETA for both DHL and IHL. The combined ETA for herd life are calculated and published
for approximately 6000 sires with progeny test information for both DHL and
conformation. The ETA are
expressed in terms of the average life expectancy (measured in lactations) of
daughters of a given sire, set to a base of 3 lactations.
Cows do not currently receive ETA for herd life, due to the low
heritability and late expression of the trait.
2.2.3. Udder health sub-index
The
sub-index for udder health is
UDDER = (-13´SCS + 3´MSpeed + 6´Udepth) ¸ 17
where SCS, MSpeed, and UDepth are standardized EBV or ETA for somatic
cell score, milking speed and udder depth.
The values -13, 3, and 6 are respective weights. (The index is divided
by 17 for standardization in the TEV). Details
about the development of the udder health index have been published by
Boettcher et al. (1997 and 1998). The
selection goal for the udder health index is decreased SCS and incidence of
clinical mastitis and decreased milking time.
No genetic evaluation for clinical mastitis is available in Canada, so
the other traits are used as indicators for mastitis resistance.
Udder depth has no direct economic value but is included in the index
for its relatively high genetic relationship with SCS and milking time.
Sire EBV for the udder health index are not published, only its
individual components.
2.2.3.1 Somatic cell score
The ETA for
somatic cell score are now calculated as part of the Canadian test-day genetic
evaluation (Jamrozik et al. 1997). The
evaluation is a multiple trait procedure with SCS and milk, fat and protein
yield for first, second, and third lactations.
The published ETA for SCS is a sub-index of SCS in each lactation,
standardized to an average of 3.00. Weights
for SCS across lactations are 0.25, 0.65, and 0.10 for lactations 1, 2, and 3,
respectively, based on work recorded by Dekkers (1995).
In contrast to the weights for the components of the production
sub-index, the weights for SCS vary across lactations because SCS and
resistance to mastitis have greater standardized economic values in later
lactations. This difference occurs because mastitis occurs more frequently in
later lactations and causes more losses in terms of discarded milk and other
factors (Kolstad and Dekkers, 1994, unpublished).
The weight on second lactation is much greater than on third lactation
because these weights were derived by assuming that SCS and resistance to
clinical mastitis were the same traits genetically in all lactations >1.
Only the combined EBV is generally published for bulls in Canada, but sire EBV
for the individual lactations are available over the internet (http://www.cdn.ca).
Like EBV for herd life, EBV for SCS are not officially published for
cows.
2.2.3.2 Milking speed
Details
about the development of the national genetic evaluation for MS in Canada are
given by Banos and Burnside (1992). Farmers
provide to their milk recording agency a subjective appraisal of MS for each
first lactation cow during the first few months in milk. Each cow is evaluated
on a 1 to 5 scale as either very slow, slow, average, fast or very fast in
relative total milking time independent of milk yield. The data is then
normalized using the Snell (1964) transformation and evaluated with an animal
model. Since February 1997, the
sire ETA for MS have been expressed as the percentage of future first
lactation daughters that are expected to be average or fast milkers.
These bull ratings range from about 55% to 80% with the average 69%
Average or Fast. No ETA for MS
are published for cows.
2.2.3.3 Udder conformation
Sires in
Canada receive EBV for 12 udder conformation traits, nine of these traits are
specific descriptive traits such as fore udder attachment, front teat
placement, and udder texture, and three are the composite traits mammary
system, fore udder, and rear udder, which are based on combinations of the
various descriptive traits. Cows
are assigned a linear score on a 1 to 9 scale for the descriptive traits and
on a 1 to 18 scale for the composite traits.
Genetic evaluation of all conformation traits is with a single trait
animal model. The resulting EBV
are expressed on a standardized scale with an average of 0 and standard
deviation of 5.0. The EBV for 29
conformation traits (for the Holstein breed) are officially published for both
bulls and cows.
Udder depth
is the only conformation trait included in the sub-index for udder health
because it had the greatest relationship with SCS and MS (Boettcher et al.
1997 and 1998). Addition of other
udder traits to the sub-index improved the accuracy of the index by only a
relatively small amount (Boettcher et al. 1997 and 1998) and resulted in
essentially no difference in sire ranking for the udder health index and TEV.
3. LPI: Lifetime Profit Index
3.1.
Background and Overview
The LPI (Dekkers 1992). was the first official selection index value
introduced in Canada in 1990. From
the very beginning it was designed to evaluate proven bulls and cows as well
as young sires, embryos and even pregnancies based on a combination of
production and conformation traits. Following
the basic Canadian philosophy of "balanced breeding" the LPI places
a relative emphasis on its two sub-indexes, namely production (60%) and
conformation (40%):
LPI
= (6 x PROD + 4 x TYPE) x 8
where
PROD is exactly the same sub-index as described for the TEV index and TYPE is
a sub-index that includes conformation traits that contribute to overall
longevity. As with all other
sub-indexes used in Canada, each of these are standardized so the weights of 6
and 4 represent the standardized index weights for each sub-index.
The multiplicative factor of 8 simply expands the range of the scale
such that the top Holstein bulls have LPI slightly over 2000 points.
The desire to have a selection tool which can be easily understood, can
be applied to all groups of animals since EBV for traits involved are
available for all animals or their parents and reflects the Canadian breeding
philosophy has resulted in relatively heavy use of the LPI in Canada.
Although the TEV reflects true economic weights, the fact that it
includes functional traits, such as herd life, SCS and milking speed, for
which official EBV for cows are not currently available, has resulted in a
more limited use in Canada. Continued
development and extension of the TEV is ongoing, which will undoubtably lead
to greater usage of TEV compared to LPI (Van Doormaal, 1999c).
The maintenance of these two indexes, however, helps serve the
interests and breeding objectives of the diverse pool of producers and
breeders in Canada so both will likely remain for many years to come.
Sivanadian et al. (1998) demonstrated that a commerical producer could
expect to increase the genetic potential of his herd for profitability by
selecting from among the top bulls for either the TEV or LPI.
3.2.
Type sub-index
Because the production sub-index of the LPI formula has previously been
described, the focus here is on the sub-index for conformation traits, as a
measure of longevity:
TYPE
= (5xMAMM+4xFL+CAP+CONF) x CAF
This sub-index includes EBV for mammary system (MAMM), feet and legs
(FL), capacity (CAP) and overall conformation (CONF) with relative weights of
5, 4, 1 and 1, respectively. Due
to the fact that the production sub-index includes EBV for two highly
correlated traits, namely protein and fat, whereas the type sub-index includes
EBV for a combination of four traits which are less genetically correlated,
the type sub-index also includes a correlation adjustment factor (CAF) which
is approximately 1.200 for the Holstein breed.
In this way, the desire of the industry to have, on average, 60% of the
LPI points for bulls coming from the production sub-index and 40% from the
type sub-index is achieved (Van Doormaal 1999a).
The EBV for each of the conformation traits included in the type
sub-index are calculated from a single trait animal model genetic evaluation
system, based on only first lactation first classifications, as briefly
described earlier related to udder conformation.
Although the four conformation traits in the LPI are often considered
general traits rather than descriptive traits, the EBV are computed based on
actual classification scores for each trait not as composites of EBV for
descriptive linear traits.
4. Other Functional Traits in Canada
Canadian Dairy Network (CDN) is responsible for the calculation and
publication of all dairy cattle genetic evaluations in Canada. In
addition to the traits already discussed, namely production, conformation,
herd life, somatic cell score and milking speed, bulls in Canada also receive
an evaluation for calving ease, lactation persistency and non-return rate with
ETA for milking temperament and health traits expected in the future. Research is ongoing to determine how these traits can be
included in the existing TEV and LPI formulae with the appropriate emphasis in
conjunction with the Canadian breeding goal; maximizing on-farm profits.
4.1.
Calving ease
Bull evaluations for calving ease, both direct and maternal have been
available in Canada for several years. The
published bull ETA are calculated using a multiple trait (direct calving ease
and maternal calving ease) animal model based on a subjective appraisal of all
calvings on the farm provided by the herdsman.
Each calving is scored on a 4-point scale as unobserved or unassisted,
easy pull, hard pull, and surgery required.
Stillbirths and multiple birth calvings are excluded.
The actual scores are transformed to Snell (1964) scores
prior to analysis with a linear model. Knowledge of the service sire and the
sire of the female giving birth as well as all pedigree data allows for the
determination of bull evaluations for direct calving ease and maternal calving
ease, respectively.
Since 1997, calving ease ETA, which are only available for
bulls in the Holstein breed, have been expressed on a positive
descriptive scale which is closely related to the scale used to report the
calving ease appraisal on the farm. This
approach helps in the proper interpretation of published calving ease
evaluations (Van Doormaal, 1997). For
example, direct calving ease ETA range from 75% to 90%, with an average of
85%, and represent the percentage of calvings from virgin heifers which are
expected to be scored either Easy, or Unassisted or Unobserved.
Calving ease ETA are mainly used by producers to avoid matings which
can result in problems (i.e. to avoid mating bulls with poor calving ease ETA
to virgin heifers or small cows) rather than explicitly selecting for improved
direct calving ease on a population level.
The combined information on direct and maternal calving ease is useful
for identifying bulls that do not have the typical negative genetic
relationship between these two components of calving ease.
4.2.
Lactation persistency
Lactation persistency describes the average lactation curve of a bull’s
daughters, at a genetic level, over the first three lactations.
Sölkner and Fuchs (1987) stated that cows with flatter lactation
curves are easier to feed, subject to less physiological strain, and can be
fed diets with a greater proportion of roughage.
Dekkers et al. (1998) reported that the economic benefits of increased
persistency were particularly important within herds with longer than average
lactations. Bull ETA for each
lactation are a by-product of the multiple trait, random regression test-day
animal model used in Canada since February 1999.
The ETA for lactation persistency are based on milk yield (rather than
fat or protein and each bull’s ETA for 24-hour milk production on day 280 of
lactation is expressed as a percentage of their ETA for the same trait on day
60 of lactation. This ratio is
calculated for each of the three lactations and the resulting ratios are
combined into one overall lactation persistency rating, or sub-index, using
relative weights of 50%, 25% and 25% on lactations 1, 2 and 3, respectively.
Published ETA for combined lactation persistency have varying averages
and ranges across breeds (Van Doormaal, 1999b) but for Holsteins the average
ETA is 63% with a range of ±10% points.
4.3.
Non-return rate (NRR)
Bulls available through artificial insemination (A.I.) receive a linear
model fertility evaluation measured in terms of 56-day NRR.
First inseminations performed by A.I. technicians during the most
recent rolling 12-month period are analysed using a mixed linear model which
includes the effects of month of first insemination, age of cow at
insemination, semen price, breed of service sire, A.I. technician, herd,
service sire and residual error. Details
of the data, model and results are described by Van Doormaal (1993).
Bull evaluations for 56-day NRR are provided to A.I. centres and breed
associations in Canada for all bulls but basically only those for actively
marketed sires are available to producers.
Average bull ratings vary across breeds and is 71% for Holsteins.
Low fertility bulls seldom get totally excluded from use but bulls with
above average ratings do get used more frequently, especially on poorer
reproductive cows.
4.4.
Future functional traits
For
several years now, Canadian milk recording agencies have been collecting a
subjective measure of milking temperament within the first 120 days of the
first lactation. Researchers are
currently in the process of collating this data into one database for purposes
of genetic evaluation. Canadian
producers have expressed a desire for genetic information on this trait since
it affects their culling decisions, and therefore profitability.
An area of growing interest in Canada are health traits.
Work has been initiated for identifying important measures related to
health and desease as well as establishing standards for data collection.
Similarly, body condition scoring and mobility have received recent
attention (vanDorp et al. 1998). which will likely lead to genetic evaluations
in the future.
5. Summary
The Total Economic Value (TEV) is a sire selection tool designed for
commercial dairymen interested in selecting for increased profits via
production and functional traits including herd life, somatic cell score,
milking speed and udder depth. The development of the TEV in Canada has been based on
establishment of sub-indexes for production, longevity and udder health so
that a clear uderstanding of the relative economic importance of each
component is easily identified. The
TEV was originally designed as an index for sire selection but a factor
currently limiting the extensive use of the TEV is the fact that it is only
available for proven bulls and, therefore, females and young bulls and heifers
do not have any published values either directly or calculated as a parent
average.
Lifetime Profit Index (LPI) has seen extensive use in Canada since its
introduction in 1990, resulting in a significantly increased selection
intensity and genetic gain. Although
its development was less scientifically oriented compared to the TEV, it has
received major acceptance by breeders who sell stock nationally and
internationally.
Canadian Dairy Network calculates and publishes genetic evaluations for
an array of traits, some of which are included, either directly or indirectly
through the use of sub-indexes, in the TEV and LPI selection indexes.
Additional functional traits such as lactation persistency and milking
temperament will no doubt also eventually be added into the TEV formula, as
will future genetic evaluations for important health traits.
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