The effect of improved reproductive performance on genetic gain and inbreeding in MOET breeding schemes for beef cattle

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The effect of improved reproductive performance on

genetic gain and inbreeding in MOET breeding schemes

for beef cattle

B Villanueva, Ja Woolliams, G Simm

To cite this version:

Original

article

The

effect of

_{improved reproductive}

performance

on

genetic gain

and

_inbreeding

in

MOET

_breeding

schemes for beef cattle

B

Villanueva

JA Woolliams

G Simm

1

Scottish

_{Agricultural College,}

West Mains

Road, Edinburgh,

EH9

,!JG;

2

Roslin

Institute

_(Edinburgh),

_{Roslin, Midlothian,}

EH25

9PS,

UK

(Received

9 November

1994; accepted

6 March

₁₉₉₅₎

Summary - The effect of

_{improved reproductive techniques}

on

_genetic

_progress and

inbreeding

was

_investigated

in MOET

_(multiple

ovulation and

embryo

transfer)

schemes for beef cattle. Stochastic simulation was used to model a closed scheme with

_overlapping

generations.

Selection was on a trait measured in both sexes, with

heritability

0.35, and was

carried out for 25 years. The number of

breeding

animals was 9 sires and 18 donors.

_Embryo

production

was modelled

_using

a Poisson distribution with the _parameter distributed

according

to a _gamma distribution. The mean number of transferable

embryos

_per flush

and per donor was

_5.0,

with a coefficient of variation of 1.28 and

_{repeatability}

between

flushes of 0.22. This model was

compared

with models used in

_previous

studies

(fixed

number of

_embryos

per flush or variable number of

embryos

but with zero

_{repeatability}

between

_flushes).

The coefficient of variation and the

repeatability

of

_{embryo yield}

influenced

_inbreeding

rates. The rate of

_inbreeding

was underestimated

_by

_{up to} 17% when

_variability

of

_{embryo production}

was

_ignored.

Without a constraint on the number

of calves born per year,

improved

success rates for

_embryo

collection and

_embryo

transfer

technologies

led to notable increases in

_genetic

_progress.

_However,

the rate of

_inbreeding

was also increased with

improved techniques.

When the number of calves born _{per year} was

_{fixed, genetic}

_progress was maintained but

inbreeding

rates were

_{substantially}

reduced

(by

up to

_11%)

with

_{improved techniques}

due to the

_opportunity

of

_{equalizing family}

sizes.

There was no benefit from sexed semen with constrained number of calves per year.

beef cattle

_/

MOET

_/

_embryo

_/

_{genetic gain}

_/

_inbreeding

Résumé - Effet de l’amélioration des _techniques de _reproduction sur le _progrès

génétique et sur la consanguinité dans des schémas MOET pour bovins à viande.

Notre

_{investigation}

avait _pour but d’étudier

l’effet

de

techniques

de

reproduction

améliorées

sur le

_{progrès génétique}

et sur la

consanguinité,

dans le cadre de schémas MOET

_(ovulation

multiple

et

_transfert

_d’embryon)

_pour les bovins à viande. Grâce à une simulation

stochas-tique, un schéma

_fermé

a été modélisé avec

_{générations}

chevauchantes. La sélection a été

l’héritabilité était de

0,35.

Les nombres de

reproducteurs

mâles et de donneuses étaient de 9 et 18 respectivement. La

_{production d’embryons}

a été modélisée en utilisant une dis-tribution de Poisson dont le

_paramètre

avait une distribution _{gamma. Le} nombre _moyen

d’embryons transférables

recueillis par collecte et par donneuse était de

5,0

avec un

coef-ficient

de variation de 1,28 et avec une

_{répétabilité}

de _0,22 entre collectes. Ce modèle a

été

_comparé

avec d’autres modèles utilisés dans des études antérieures

(qui

utilisaient un

nombre déterminé

_d’embryons

_par

_collecte,

ou un nombre variable

_d’embryons

mais avec une

_{répétabilité}

nulle entre

_collectes).

Le

_coefficient

de variation et la

_{répétabilité}

de la

production d’embryons infLuencent

le taux de

_{consanguinité.}

Si on ne _{tient pas compte}

de la variabilité de la

production d’embryons,

la sous-évaluation du taux de

consanguinité

peut atteindre 17%. Sans contrainte sur le nombre de naissances de veaux _{par an,} un

_plus

grand

pourcentage de réussite dans la collecte

_d’embryons

et l’amélioration des

technolo-gies

de

transfert

contribuent ensemble à augmenter considérablement le

progrès génétique.

Cependant,

l’amélioration des

_techniques

a aussi pour

effet d’augmenter

le taux de

consanguinité. Quand

le nombre de veaux _{nés par} an est

_,fixé,

le

_{progrès génétique}

_peut être maintenu, tout en réduisant le taux de

consanguinité

(jusqu’à

13%),

en

_employant

les

_{techniques améliorées,}

à cause de la

_{possibilité d’égaliser}

la taille des

_familles.

Il

_n’y

a aucun

_bénéfice

à utiliser du _sperme sexé

_quand

le nombre de veaux _par an est

_fixé.

bovin à viande

_/

schéma à ovulation _multiple et transfert _d’embryon

_{(MOET) /}

embryon

/

gain génétique

/

consanguinité

INTRODUCTION

The value of

_multiple

ovulation and

_embryo

transfer

_(MOET)

in

_breeding

schemes for

increasing genetic gain

has been

_widely

studied in

_dairy

cattle

_(see

review

by

Dekkers, 1992;

Ruane and

_Thompson,

₁₉₉₁₎

and to a lesser extent in beef cattle

_(Land

and

_Hill,

_1975;

Gearheart et

_{al, 1989;}

Keller et

_{al, 1990; Wray}

and

Simm,

1990)

and

_sheep

_(Smith,

_{1986; Wray}

and

_Goddard,

_1994).

_Early

studies concentrated on extra

_genetic

_progress

_expected

with MOET. More recent studies have also considered

_possible

risks associated with the use of MOET

techniques.

By greatly increasing

the numbers of _progeny to be

_{produced by individuals,}

genetic

progress can be

_improved

due to increased intensities of selection.

_However,

the extra

_gains

can be

accompanied by

increased

_inbreeding

since fewer

parents

contribute to the next

_generation.

The adverse effects of

_inbreeding

_(loss

of

_genetic

variability,

loss of

_{predictability}

of

_{genetic gain}

and

_inbreeding

_depression)

should be taken into account when

_optimum

schemes for

_{genetic improvement using}

reproductive

technologies

are

_{investigated.}

One of the main

_shortcomings

in earlier studies was the

_assumption

of constant

family

size,

or the

_assumption

of a variable

_family

_size,

but with no correlation

between the number of

_{embryos produced}

in successive recoveries. In fact there is

a wide _{range in} the size of families

following

MOET and

_analyses

of MOET data

have indicated a non-zero

_{repeatability}

of

_{embryo production}

(eg,

Lohuis et

_{al, 1993;}

Woolliams et

_al,

_1995).

The increase in the variance of

_embryo

yield

can lead to

increased rates of

_inbreeding

and reductions in

_{genetic gain. Recently,}

Woolliams

assumption

of zero correlation between flushes is

removed)

and describes _very

accurately

the number of

_embryos

obtained _per donor and _per flush. Villanueva

et al

₍₁₉₉₄₎

have used this model in a simulation

_study

to

_investigate

different

strategies

for

_controlling

rates of

_inbreeding

in MOET

_breeding

schemes for beef cattle. With current values of

_parameters

_describing

success rates of

_reproductive

technologies,

rates of

_inbreeding

were _very

_high

for schemes with 18 donors and 9

sires,

even when the most efficient

_strategies

for

controlling

inbreeding

were used

(factorial

mating designs

and selection on best linear unbiased

_prediction

(BLUP)

breeding

values

_assuming

an inflated

heritability).

In this _paper we

_investigate

rates

of _progress and

_inbreeding

obtained when different models for

simulating

embryo

production

are utilized.

Advances in

_embryo

_{manipulation techniques}

have been

_rapid

in the

_past

few

years and these are

_likely

to continue. One of the main

problems

in the

practical

application

of

_embryo

transfer in

_breeding

_programmes

_{using superovulation}

is the

high variability

among donors in the number of

embryos

collected. This

produces

a

high

variance in

family

size which in turn leads to a

_high

variance in the numbers selected from each

_family

_(and,

_therefore,

_high

_inbreeding).

Research is

being

addressed at

_reducing

this

_variability

and

_increasing

the mean number of

embryos

per collection.

Embryo

survival rates and

_frequency

of collection are also

likely

to be

_improved.

Luo et al

₍₁₉₉₄₎

have

_given

both

_pessimistic

and

optimistic

predictions

for future success rates of

_{embryological}

techniques.

The effect that

improved

future success rates for

_embryo

_recovery and

_embryo

transfer could have

on rates of response and

inbreeding

is

_investigated

in this paper.

Also,

the

techniques

for

_sexing

of

_embryos

or semen are

_already

used on a small scale. Semen and

_embryo

sexing

may become commercial in the near future and so the value of

_sexing

of semen

to increase

_genetic

_progress is also examined.

_Hence,

the results are

_expected

to be useful in

_identifying

those advances in

_{reproductive technologies}

which are

_likely

to

be of most value in

_breeding

schemes.

METHODS

Description

of simulations

The stochastic model to simulate a MOET nucleus scheme for beef cattle has been

described in detail

_by

Villanueva et al

_(1994).

The trait under selection was assumed to be recorded in both sexes and around 400 d of _age

_(between

385 and 415

_d),

at the end of a

_performance

test. The trait was simulated

_assuming

and additive

infinitesimal model with an initial

_heritability

of 0.35. The nucleus was established

with 9 males and 18 females of

_2,

3 and 4 years of age. The number of animals in each age group was

_{approximately}

the same. These unrelated individuals constituted the

base

population.

True

_breeding

values of base

_population

animals were obtained from a normal distribution with mean zero and variance

₍

_QA

₎

0.35

_(different

_age

groups had the same

genetic

mean).

Phenotypic

values were obtained

by adding

a

normally

distributed environmental

_component

with mean zero and variance 0.65.

Selection was carried out for 25 years. The number of

breeding

males and

females was constant over _years and

_equal

to the number of base males and females

period

= 6

months).

Estimated

breeding

values

_(EBVs)

were obtained

_using

an

individual animal model BLUP. The overall mean was the

_only

fixed effect included

in the model. Males and females with the

_highest

EBVs were selected and

randomly

mated

_according

to a nested

_design.

Each sire was used the same number of times

in 1 evaluation

_period.

Animals were selected

_irrespective

of whether

_they

had been selected in

_previous

_periods

and animals not selected were culled from the herd.

True

_breeding

values of the

_offspring

born _{every year,} were

_generated

as

where

_TBVI,

_TBV,

and

_TBV

_d

are the true

_breeding

values of the individual

i,

its sire and its dam

_{respectively,}

and mi is the Mendelian

sampling

term. The Mendelian term was obtained from a normal distribution with mean zero and

variance

_{(1/2)(1 -}

_(F

_i

+

_!d)/2]cr!,

where

F

s

and

F

d

are the

_inbreeding

coefficients

of the sire and

_{dam, respectively.}

The

_inbreeding

coefficients of the animals were

obtained from the additive

_relationship

matrix.

Values for

_reproductive

success rates

_(parameters

of

_{embryo yield, frequency}

of

embryo

collection and survival rate of transferred

_embryos)

were varied in different

schemes. The number of transferable

_embryos

collected _per flush and _per cow was

obtained from a Poisson distribution whose

_parameter

was distributed

_according

to a _gamma distribution

_(Woolliams

et

_al,

_1995).

This model is described in the next

section

_{(Model 1).}

Different values for the mean number of transferable

_embryos

_per

flush and _per

_donor,

the coefficient of variation and

_{repeatability}

of

_{embryo yield,}

the

_frequency

of

_flushing

and the

_embryo

survival rate were considered. Current

values were obtained from

_analyses

of extensive data on

_embryo

_recovery

(Woolliams

et

_al,

_1995).

Potential future values were obtained from a _survey of international

experts

in

_reproductive

_technologies

_(Luo

et

_al,

_1994).

All calves were born from

embryo

transfer,

ie there were no calves from natural

_matings.

The survival to

birth of a transferred

_embryo

was

_assigned

at random with different

probabilities

in different schemes

_(0.55,

0.65 or

0.75).

The sex of the

_embryos

was also

_assigned

at

random with

_probability

_1/2

of

_obtaining

a male

(expected

sex ratio

d’/Q

=

1:1)

for most schemes. In order to evaluate the

_possible

benefit of

_using

sexed _semen, the

ratio was

_changed

to 1:2 and 1:3. In these _cases, the

_probability

of

_obtaining

a male was

1/3

and

1/4,

_{respectively.}

Males were assumed

_capable

of

_breeding

after

_being

performance

tested. The minimum _age of donors was 15 months. At all ages after

birth,

individuals were

subject

to a

_mortality

rate that varied with _age. Survival

probabilities

from birth to 3

_weeks,

6 months and

_{2, 5,}

10 or 15 _years were

_0.98,

0.97, 0.96, 0.93,

0.86 and

_{0.00, respectively. Thus,}

the maximum age of the animals

was 15 _years. Survival rates were assumed to be the same for both sexes.

Models for

_{embryo production}

In the

_present

_study,

the number of

_embryos

_produced

_per flush and per donor

was

_{generated using}

the model

_{proposed by}

Woolliams et al

_(1995).

In order to

investigate

the effect of

_including

extra variation in

_{embryo production}

on rates of

response and

inbreeding

this model was

compared

with models used in

_previous

collection but with zero

_{repeatability}

between

flushes).

Four different models were

analysed.

Model 1

The model

_{proposed by}

Woolliams et al

₍₁₉₉₅₎

_generates

the number of

_embryos

produced

from a

_negative

binomial distribution

(Poisson

distribution whose

pa-rameter is distributed

_according

to a _gamma

_{distribution).}

The number of

_embryos

collected from the ith donor in the

_jth

flush was

sampled

from a Poisson

distri-bution whose

_parameter

_.!2!

was

_sampled

from a _gamma distribution with

_shape

parameter

/!2

and scale

_parameter

v. In that _way a correlation between the number

of

_embryos

_produced

in successive flushes of a cow is included in the model. The

natural

_logarithm

of

_#i

_(parameter

_specific

for each

_donor)

was

_sampled

from a

normal distribution with mean 1L and variance

Q

2

.

The

logarithm of !3i

is taken to

avoid

_negative

numbers. The maximum value of

_A

_ij

was set to 30.

_{Let y}

₂

be the

number of transferable

_embryos

obtained at the

_jth

collection. Then the

_expected

value and variance

_of

_Yi

are

E(y

2

)

_

!2v

and

Var(y

i

) =

{3

i

v(1

+ (3’f),

_respectively

(Woolliams

et

al,

1995).

In order to

_explore

the effect of

_changing

these

_key

pa-rameters,

a simulation _program was written to simulate

_embryo

_{production using}

this model. The number of donors simulated was 100 000 and the number of flushes was 3 for each donor. The

_{repeatability}

was calculated as R =

u2/(a2

₊

Q

w),

where

_QB

is

the variance in

_{embryo production}

_among donors and

_a#

is the

vari-ance _among flushes

_{(within donors).}

The coefficient of variation was calculated as

CV =

(or2

/2 IM

AN

E

1

)

2

+

where MEAN is the overall _mean of

embryos

collected

per flush and per donor. The estimates of

Q

B

and

o-w

were obtained from an

anal-ysis

of variance of simulated data. Current values for

embryo production

(Luo

et

al,

1994)

correspond

to the

_following

_parameter

_{values: >}

=

1.46,

a

2

= 0.4 and v = 1.0. These values led _to a mean number of transferable

_embryos

_per flush and

per donor of

5.0,

with a coefficient of variation of 1.28 and

repeatability

of 0.22.

Model 2

The number of

_embryos

collected was obtained in the same _way as described in

Model 1 but now the

_{logarithm of}

_{

₃

_i

was

_sampled

from a normal distribution with

mean > and variance zero. Since

parameter

{3i

is a

_constant,

there is no

variability

among donors and the

repeatability

of

_{embryo production}

is zero. The values for

the

_parameters

of the distributions _{were >} =

1.61,

0

-

2

= 0.0 and v = 1.0. These

values lead to the same mean number of

embryos

collected as in Model 1 but to a

lower coefficient of variation

_(CV

=

1.09,

R =

0.00).

Model 3

The number of

_embryos

collected _per flush and _per donor was

_{generated by sampling}

Comparison

among

breeding

schemes

The scheme with current

_values

for

_reproductive

_parameters

was used as a

_point

of reference for

_comparisons.

_Average

true

_breeding

values

_(G

_i

₎

and

_inbreeding

coefficients

_(F

_i

₎

of individuals born at the ith _year were obtained. Annual rate of

response between

years j

and

i was calculated as

AG

I

-

j

=

(Gj — G!)/(j - z),

where

j

> i. Rates

_{of inbreeding}

were obtained _{every year} as

t1F

i

=

(!—7!_i)/(l—!_i).

The rate of

_inbreeding

between

years j

and

i

_(t1Fi!j)

was obtained

_by

_taking

the _average of annual rates.

Also,

the

_following

_parameters

were calculated in the

simulations:

₁₎

genetic

variance of animals born _{every year;}

₂₎

_accuracy of selection

(correlation

between the true

_breeding

values and selection criteria of the candidates for

_{selection); 3)}

_genetic

selection differentials

_(difference

between the mean values

of selection criteria of selected individuals and candidates for

_selection)

and selection intensities for males and

females;

4)

generation

intervals

_(average

_age of

_parents

when

_offspring

are

_born)

for males and

_females;

and

₅₎

variance of

_family

sizes for male and female

parents.

The latter was calculated as described in Villanueva

et al

_(1994).

Each scheme was

replicated

200 times and the values

presented

are

the average over all

replicates.

The criteria for

_comparing

different schemes were

the rates of _response

_(AG

₁₅

_-

₂₅

₎

and

_inbreeding

_(AF

₁₅

_-

₂₅

₎

at the later years

(from

year 15 to _year

_25).

The cumulative _response

₎

₂₅

_(G

and

_inbreeding

at _year 25

_(F

_ZS

₎

RESULTS

Models for

_embryo

_production

Table I shows the

_genetic

_progress and the

_inbreeding

obtained under different models used to

_generate

the number of

_embryos

per collection. The results show

that the

_inbreeding

obtained

_depended

on the values of the coefficient of variation

and the

_{repeatability}

of

_embryo

_{yield. By}

_making

the correlation between

embryo

production

at different recoveries

_equal

to zero

(R

=

0.00),

the rate of

inbreeding

decreased

_by

4%

_(Models

1 and

_2).

The effect of the coefficient of variation of

embryo yield

on the rate of

inbreeding

was notable.

By increasing

the coefficient

of variation

_by

a factor of 2.4 the rate of

inbreeding

increased

by

14%

(Models 2

and

_3).

The rate of

_inbreeding

obtained when the number of

_embryos

collected

was fixed

(Model

4)

was between 2 and

14%

lower than that obtained when there was

_variability

in

_{embryo yield}

but the

_{repeatability}

was zero

(Models 3

and

2).

The

_genetic

progress decreased as

_variability

of

embryo production

increased. The

decrease in _response was however small. The

genetic gain

obtained with Model 1

was around

2%

lower than that obtained with

Model

4 (fixed

number of

embryos).

Improved

embryo

recovery and

embryo

transfer

Values for

reproductive

parameters

utilized in different schemes are shown in

table II. Two different situations of

improved

technology

for

_embryo

_production

were considered.

Firstly,

the coefficient of variation of

embryo yield

was decreased

and the mean was maintained.

Secondly,

the coefficient of variation was decreased

and the mean was increased. Under Model

1,

the coefficient of variation can be

was increased whereas _J-L was

_kept

constant. Values used for

_embryo

distribution

parameters

as well as the

_{resulting MEAN,}

CV and R are shown in table III.

The rates of response and

inbreeding

obtained with

improved

values for

parame-ters of

_embryo

recovery and

embryo

transfer are shown in table IV. The first row of

the table

represents

the current situation and is used as a reference. The

expected

number of

_embryos

transferred for each scheme is shown in the last column.

De-creasing

the coefficient of variation of

_embryo

_production

while

_keeping

the mean

approximately

constant did not have an effect on rates of response and

inbreeding.

This _may be due to the increased

_{repeatability}

that

_accompanied

the decrease in

CV in the model. The influence of the

_{repeatability}

of

_{embryo yield}

has been shown

in the

_previous

section.

_Increasing

the mean number of

embryos

transferred led to a notable increase in the rates of _response, due to increased selection intensities and accuracy and decreased

generation

intervals. In this case, the number of calves

born _{per year}

_(N

_CB

₎

was unrestricted and the number of donors was

_constant,

so

increasing

embryo yield

led to more candidates for selection. Male and female

se-lection intensities

_(i!

and

_iy)

and

_generation

intervals

_(L

_«

and

_L

_Q

₎

are shown in

table V. The rate of

_inbreeding

_(per

_year and _per

_generation)

was also increased

(particularly

when the mean number of

embryos

produced

was

9.6)

due to increased full-sib

_family

sizes and intensities of selection and decreased

_generation

intervals.

The assumed current

_frequency

of collection of

_embryos

_(FC)

was 60 d

₍₃

flushes

flushing

to 45 d

₍₄

flushes in a 6 month

period)

on rates of _response and

_inbreeding

are also shown in table IV. The increase in

flushing frequency

to this

optimistic

future value

_produced

a clear increase in

genetic

_{progress. This increase} in

genetic

progress was due to increased selection intensities and accuracy of selection and

decreased

_generation

intervals

_(table

_V).

_Inbreeding

was

_{slightly higher}

when donors were flushed 4 times _per

period. Finally, by increasing

the

probability

that an

embryo

survives until

_calving

_(ESR)

from 0.55 to 0.65 and

_0.75,

cumulative

_genetic

response was increased

_by

4 and

8%, respectively.

The rate of

inbreeding

was

also increased. Table V indicates increases in selection intensities and decreases

in

_generation

intervals with

_improved

_viability

of the

_embryos.

Tables IV and V show results obtained without a constraint on the number of

calves born in the scheme.

_{By increasing}

the mean number of

embryos

_per flush

and _per

_donor,

the

frequency

of

_flushing

or the

_embryo

survival

_probability,

the

expected

number of

_offspring

is increased. Genetic _progress obtained at _year 25 was

directly proportional

to the number of calves born each _year

_{(table IV).}

With more

offspring

born,

the selection intensities and the accuracy of selection were increased

and the

_generation

intervals were decreased

_(table

V).

Also,

the rate of

_inbreeding

(per

year and per

generation)

was increased

by improving

embryo

transfer and

embryo

recovery

techniques.

For a fixed number of sires and

donors,

the increase in

the number of

_offspring

born per year led to an increase in the variances of

family

sizes.

Comparing

schemes which differ

_widely

in the number of

_{offspring produced}

is

unfair. This is because

_{genetic gains}

are

_expected

to be

_higher

_(and

_inbreeding

is

_expected

to be

_lower)

in

_larger

_{schemes, irrespective}

of the use of

_breeding

technologies. Also,

in

_practice,

there will

_usually

be a limitation on the number

of

_embryo

collections or transfers which can be

made,

the number of

recipients

available,

or the number of

_testing

places

available for calves. These constraints are

Simulations were therefore also run with a restriction on the number of

offspring

born _{every year} and the results are

presented

in table VI.

When the mean number of

embryos

collected _per flush and the

frequency

of

collection were

increased,

some

embryos

were discarded in order to transfer a

fixed number. In these _cases, the

_expected

average number of

embryo

transfers per year was 540

(this

is the

expected

number of transfers with current values for

reproductive

parameters

and 18

_donors).

_Embryos

were not discarded at

random;

most were discarded from donors

producing

more

_embryos,

in order to

_equalize

family

sizes. The decision on which

embryos

were transferred was made within

individual flushes.

First,

the number of

_embryos

recovered from each donor was

obtained

using

Model 1. After

_that,

1

_embryo

from each donor

_(if

_available)

was

allocated

_(for

_transfer)

in succession and this _process was

repeated

until the desired

total number of

_embryos

was reached. In these _cases, the maximum number of

embryos

transferred after a

_single

flush was 18 x 5 = 90. When the survival rate

of

_embryos

from transfer until birth was

_increased,

the number of transfers was

decreased in order to obtain a fixed number of calves born per year.

Again,

more

embryos

were discarded from donors with

higher embryo production.

Table VI

shows that with these

_strategies,

the number of calves born per year

(N

CB

)

was

approximately

constant in all schemes.

Increasing

the mean number of

embryos produced

per flush and per donor from

5.0 to 7.4 and 9.6 decreased the rate of

_{inbreeding by}

_up to

10%

even with the

increased

_{repeatability}

_(table

_VI).

_{Thus, restricting}

_family

sizes nullified the effect of

_{repeatability.}

The decrease in

_inbreeding

rates was due to decreased variances of

family

sizes. The increase in

_frequency

of

_flushing

also led to a decrease in the rate

of

_inbreeding

_{(by 11%)}

whereas the

_genetic

_progress was not affected.

_{Finally, by}

increasing

the

_probability

of

_{embryo survival,}

the rate of

_inbreeding

was reduced

_by

up to

5%

with no effect on _response. These latter schemes

(ESR

>

_0.55)

were not

compared

on an

equal

basis with the

others,

since fewer

embryos

were transferred

and less

recipients

were used.

Sexing

of semen

Table VII shows the results obtained when sexed semen was used to

_change

the sex

ratio from 1:1 to 1:2 and 1:3 in favour of

females,

in order to increase the selection

intensity

in this sex. The number of

embryos

obtained _per flush and _per donor was simulated

_using

Model 1. The number of transfers per year was

expected

to be the same for all schemes

(540

transfers per

year).

There was no benefit from

_using

sexed semen when the number of _progeny tested per year was fixed. Table VIII

shows

_generation

intervals and selection intensities for schemes with different sex

ratios. As

_expected,

the selection

_intensity

of females increases as the

_proportion

of

female

_offspring

increases.

_However,

at the same

_time,

there is a reduction in the

selection

intensity

of males and the average selection

intensity

is not increased. This led to similar rates of response when different sex ratios were simulated. Generation

DISCUSSION

Two novelties of the

_present

_study

are that the coefficient of variation of

embryo

yield

used

corresponds

to that observed in real data

_(and

is

_higher

than that used in

_previous

_studies)

and that the

_{repeatability}

of

_embryo

_yield

has been included. Studies

_evaluating

the use of MOET for

_{genetic improvement}

of ruminants have

frequently

assumed a fixed number of

embryos

_per collection

(Land

and

Hill, 1975;

Nicholas and

Smith, 1983;

Juga

and

Maki-Tanila, 1987;

Gearheart et

_{al, 1989;}

Keller

et

_{al, 1990; Meuwissen, 1991;}

Ruane and

Thompson, 1991;

Toro et

_al,

_1991;

Bondoc and

_{Smith, 1993;}

Leitch et

al,

1994).

Several studies have considered variable

_family

sizes but

_using

_hypothetical

distributions.

Ruane

₍₁₉₉₁₎

simulated variable

_family

sizes

_by

_obtaining

the number of

_embryos

recovered per donor from a normal distribution with mean 16 and variance _up to

64

_(he

assumed 4 flushes per

generation

and an _average of 4

_embryos

_per

flush).

Thus the

_highest

coefficient of variation for

_embryo

_production

simulated was 0.50.

His results showed a small effect of variation in

family

sizes on rates of _response

and

_inbreeding.

The reduction in response

by including

variation in the number of

embryos

collected per donor was around

4%

whereas the increase in

inbreeding

was

up to

3%.

Colleau

₍₁₉₉₁₎

used a ’scaled’ binomial distribution to model

embryo

production.

The

proportion

of treated cows

_responding

to

_{superovulation}

_(p)

was

0.7 and the number of

_embryos

collected per flush and per treated cow

(m)

was

4 or 5. Thus the number of

embryos

collected per flush and per donor was

_m8,

where 0 is a random variable

having

a binomial distribution with the

_parameters

n = 1

(flushes)

and

distribution is _mnp and the variance is

_m

₂

_{np(1 -}

_p)

which leads to a coefficient of

variation of

_{[(1- p)/pj}

_I/2

= 0.65. Poisson distributions

(with

_constant

parameters)

have been used also for

_modelling

the number of

_embryos

recovered

_following

superovulation.

Schrooten and van Arendonk

₍₁₉₉₂₎

used a Poisson distribution

with

parameter

5 to obtain the number of live calves per flush. This

implies

a

coefficient of variation of 0.45.

More realistic models have been

_proposed

to

_generate

_{embryo yield}

distributions

(Foulley

and

_{Im, 1993; Tempelman}

and

_Gianola,

_1993;

_1994).

In a simulation

study,

Tempelman

and Gianola

₍₁₉₉₄₎

_{generated embryo yields}

in MOET nucleus

herds

_using

a model which includes

_repeatable

_{variation among} donors.

Within-dam variation was modelled

_using

a Poisson distribution.

However,

Woolliams et al

(1995)

have shown that extra-Poisson variation is observed in

_practice.

Actual data

on ovulation rates and

_embryo

recoveries are better described

_{by negative}

binomial

distributions than

_by

Poisson models. In the model of

_Tempelman

and Gianola

(1994),

more within-donor variation could be

_generated

by

including

an additional random term.

Wray

and Simm

₍₁₉₉₀₎

used a distribution based on commercial data to

_generate

the number of calves per flush in beef cattle. The coefficient of variation used was

1.15. The coefficient of variation of the number of calves born _per flush can be

obtained from the mean and the coefficient of variation of

_{embryo yield}

as

!ESR(1 -

ESR)MEAN

+

ESR

2

CV

2

MEAN

2

]

1

/

2

/[ESR

x

_MEAN]

The value used in the

_present

_study

_(1.34)

is

_higher

than that used in

_previous

studies.

_Changes

in

_inbreeding

also

_depend

on the value of

repeatability

of

embryo

yield

(table

I).

With increased

_{repeatability}

of

_{embryo yield,}

an increase in the variance of

_family

size is

_expected

_(since

the variance between donors

_increases),

with the

_potential

for fewer families to make a

_greater

contributions to successive

generations. Thus,

models used

_previously

_(which

have assumed a constant number of

_embryos

_per collection or a variable

number,

but with lower coefficient of variation

and no correlation between different

recoveries)

have underestimated the rate of

inbreeding

and overestimated

genetic gain.

Improved

embryo

recovery and transfer success rates lead to

_higher

rates of

response and

inbreeding

than current success

_rates,

providing

the number of calves

tested per year is unconstrained. This is due to

_higher

selection intensities and

accuracies,

lower

_generation

intervals and

_higher

and more variable

_family

sizes.

However,

it is unrealistic to assume unconstrained number of calves born

_since,

in

centralized nucleus

_herds,

costs will

depend

of the total number of animals in the scheme. When different schemes are

compared

at a constant number of

_offspring

born per year,

improved

success rates do not increase _progress, since selection intensities and

generation

intervals are maintained.

However,

inbreeding

rates can

be

_markedly

reduced

_{by discarding}

more

_embryos

from donors with the

_highest

embryo

production,

to

_equalize

_family

sizes.

_Although

the results

presented

are for

the later years of

selection,

rates of response and

inbreeding

were also obtained for

the

_early

years

(from

year 5 to year

15).

There was no

_significant

effect of time on

the results of

comparisons

among schemes.

Schemes which assumed

_{improved embryo}

transfer

techniques

(improved ES’R)

schemes

_(see

_E(ET)

in table

_VI).

An alternative basis for

_making

fair

comparisons

would be to transfer 540

_embryos

and,

from the 351

_(ESR

=

0.65)

and 405

(ESR

=

0.75)

calves

expected,

choose for

performance testing

the 297 animals

which would best

equalize

family

sizes. In this case, schemes with

improved

ESR

could benefit from

_selling

surplus

calves. On the other

_{hand, improved embryo}

recovery

techniques

(increased

MEAN and

FC)

give

the

opportunity

of

selling

surplus embryos

(see E(ER)

in table

_VI).

However,

this benefit is difficult to

quantify

as there is not

_currently

a

_large

or stable market for beef cattle

embryos.

Variation _{in response to} selection can be an

_important

limitation of MOET

nu-cleus schemes. Nicholas

₍₁₉₈₉₎

_suggested

that the maximum

_acceptable

coefficient

of variation of _{response after 10 years of selection}

_ranged

from 5 to

10%.

For the schemes considered in this paper, the coefficients of variation of response over a

10 year

period

varied from 10 to

14%.

Thus,

the size of the nucleus needs to be

larger

than that considered

here,

or

_strategies

for

_{controlling inbreeding}

should be

applied,

to have a reasonable level of risk.

In

_breeding

_programmes,

_sexing

of

_embryos

or semen could be used to increase the selection

intensity applied

to females.

_However,

there seems to be no benefit from

sexing

when

_performance

information is available on both sexes and

_comparisons

are made at a fixed number of individuals tested _{per year}

(table VII).

As

expected,

the selection

intensity

of females increased as the

_proportion

of female

offspring

increased.

_However,

at the same

time,

there was a reduction in the selection

_intensity

of males and the overall selection

_intensity

was

unchanged

(table

VIII).

Wray

and

Goddard

₍₁₉₉₄₎

have

_investigated

a different

_strategy,

in

_sheep

MOET

_schemes,

which used sexed semen

(female)

to inseminate the selected dams with lower EBVs whereas the

top

selected dams were inseminated with unsexed semen

_(to

avoid

decreases in male selection

_intensity).

However,

in MOET

_schemes,

the overall

selection

_intensity

did not

_{change. They}

found a benefit from

sexing

in conventional

schemes

_(without

_MOET),

_suggesting

that

_sexing

can be beneficial when the male

and female selection intensities differ

_greatly.

On the other

_hand,

Colleau

₍₁₉₉₁₎

_reported,

for

_dairy

cattle,

slightly higher

gains

for adult MOET schemes with

sexing

of

_embryos

than for

_juvenile

MOET schemes without

_sexing.

Since

_juvenile

schemes are

_expected

to be

superior

to adult

schemes without

_sexing

_(with

_respect

to

_genetic

_progress),

his results

_suggest

a clear

benefit from

_sexing.

Whereas the studies discussed in the

_previous

_paragraph

have considered fixed numbers of

transfers,

sires and

dams,

Colleau’s model allowed the

number

of dams to _vary. The nucleus considered assumed females

_dispersed

across

many recorded herds. For his adult

scheme,

the overall number of dams used for

replacements

was much

_higher

than in the

juvenile

scheme. This allowed selection

differentials in the adult scheme to be

high enough

to

_compensate

for the

_longer

generation

intervals. In centralized nucleus

schemes,

a constraint on the number of dams would be also nParlarl, In these circumstances the

_advantage

of

_sexing

would

be doubtful.

In

_conclusion,

the values of the coefficient of variation and the

repeatability

of

embryo

yield

are

important

in

determining

rates of

inbreeding.

When the number of

testing

places

is

_{constrained, improved technologies}

can

_greatly

decrease the rate of

is available on both sexes and

_comparisons

are carried out at a fixed number of

individuals tested per year, there is no

_apparent

benefit in response from

sexing.

ACKNOWLEDGMENTS

We would like to thank Drs B

_McGuirk,

R

Thompson

and a referee for their valuable

suggestions.

This work was funded

_by

the

_Ministry

of

_Agriculture,

Fisheries and

_Food,

the Milk

_Marketing

Board of

_England

and Wales and the Meat and Livestock

Commis-sion. SAC receives financial _support from the Scottish Office

_Agriculture

and Fisheries

Department

and the Roslin Institute receives financial support from the

_{Biotechnology}

and

_Biological

Sciences Research Council and the

_Ministry

of

_Agriculture,

Fisheries and Food.

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