Genetic parameters of feeding behaviour and performance traits in group-housed Large White and French Landrace growing pigs

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Genetic parameters of feeding behaviour and

performance traits in group-housed Large White and

French Landrace growing pigs

F Labroue, R Guéblez, P Sellier

To cite this version:

Original

article

Genetic

_parameters

of

_feeding

behaviour

and

_performance

traits in

_group-housed

Large

White

and French Landrace

growing pigs

F Labroue

R

Guéblez

P

Sellier

1

Département

de

génétique animale,

Station de

_génétique

quantitative et

appliquée,

Institut national de la recherche

_agronomique,

78352

_{Jouy-en-Josas cedex;}

2

Institut

technique

du porc, La Motte au

_Vicomte,

BP _33, 35651 Le Rheu

cedex,

France

(Received

2 November 1996;

accepted

14

_August

₁₉₉₇₎

Summary - Data on

_feeding

behaviour of 3 710

group-housed

and ad libitum fed

growing

pigs

were recorded

_using

’Acema 48’ electronic feed

dispensers.

Genetic parameters for

six

_feeding

behaviour criteria and the main

_production

traits

_routinely

recorded in French central test stations

_(three

’boar’ traits and three ’sib’

_traits)

were estimated in two

breeds

_(Large

White and French

Landrace)

using

a

_multiple

trait animal model DF-REML

procedure. Heritability

estimates for

_feeding

behaviour criteria

_ranged

from 0.36 to 0.54

and were

_{markedly higher}

than that for the food conversion ratio

(0.20).

Heritability

of

daily

feed intake was 0.42 in both

_breeds,

whereas heritabilities of rate of feed

intake,

feed intake _per meal and time _per meal were

slightly higher

(0.45-0.54).

Daily

feed intake

showed a _very close

_genetic

correlation

(around 0.85)

with _average

daily gain

but also

unfavourable

_genetic

correlations with ultrasonic backfat thickness

_(around

_0.5)

and lean

percentage

(around -0.4).

Daily

feed intake was

_{genetically independent}

of food conversion

ratio,

whereas average

daily gain

showed a favourable

genetic

correlation

_{(around -0.35)}

with that trait.

Among

the

feeding

behaviour criteria, feed intake per meal and rate of feed intake showed the

highest genetic

correlations with

_daily

feed intake

_{(around 0.5)}

and average

daily gain

(around 0.4).

They

also showed

moderately

unfavourable

genetic

correlations with ultrasonic backfat thickness

_{(around 0.25)}

and carcass lean _percentage

(around -0.25)

and seemed to be

_{genetically independent}

of food conversion ratio. The value of

_including

a trait

_relating

to feed intake pattern among traits selected for is

discussed on the basis of this set of

_genetic

parameters.

pig

/

genetic parameter

/

feeding behaviour

_/

electronic feed dispenser

/

production

trait

*

Résumé - Paramètres génétiques des critères de comportement alimentaire et des

performances de production chez des porcs Large White et Landrace _français élevés

en groupe. Les données de comportement alimentaire de 3 710 porcs en croissance élevés

en _groupes et alimentés à volonté ont été récoltées à l’aide de distributeurs automatiques

d’aliment «Acema

_48».

Les

paramètres génétiques

de six critères de comportement

ali-mentaire et des _principaux caractères de

_production

mesurés en routine dans les stations

publiques

de contrôle des

performances

(trois

caractères « candidats » et trois caractères

«

_{collatéraux»)}

ont été estimés dans deux races

(Large

White et Landrace

_français)

à l’aide

de la méthode du maximum de vraisemblance restreinte

_(REML)

appliquée

à un modèle animal multicaractère. Les héritabilités des critères de comportement alimentaire sont

com-prises

entre

_0,36

et

_0,54,

et sont nettement

supérieures

à celle de l’indice de consommation

(0,20).

L’héritabilité de la consommation moyenne

journalière

est de

0,42

dans chacune des deux races tandis _que celles de la vitesse

d’ingestion,

de la consommation moyenne par repas ou de la durée des _repas sont

légèrement plus

élevées

(0,45-0,54).

La consommation

moyenne

journalière présente

une corrélation

génétique

très élevée

_(de

l’ordre de

_0,85)

avec le

_gain

_moyen

quotidien

mais aussi des corrélations

génétiques défavorables

avec

l’épaisseur

de lard dorsal

_(de

l’ordre de

_0,5)

et le _pourcentage de muscle

_(de

l’ordre de

- 0,4).

La consommation moyenne

journalière

est

_{génétiquement indépendante}

de l’indice de consommation tandis _que le

_gain

moyen

quotidien présente

une corrélation

génétique

favorable

(de

l’ordre de

_-0,35)

avec ce caractère. Parmi les critères de _comportement

ali-mentaire, la consommation _{moyenne par repas et} la vitesse

_{d’ingestion}

sont les

_plus

liées

génétiquement

à la consommation moyenne

journalière

(environ 0,5)

et au

_gain

_moyen

quotidien

(environ 0,4).

Ces critères

présentent également

des corrélations

_génétiques

modérément

_{défavorables}

avec

l’épaisseur

de lard dorsal

(environ 0,25)

et le

pourcent-age de muscle

(environ -0,25)

et semblent être

génétiquement indépendants

de l’indice de

consommation. L’inclusion

possible

d’un critère de _comportement alimentaire parmi les

caractères sélectionnés est discutée sur la base de cet ensemble de

_{paramètres génétiques.}

porc

/

paramètre génétique

/

comportement alimentaire

_/

distributeur _automatique d’aliment

_/

caractère de _production

INTRODUCTION

The interest in

_studying

_appetite

in

_{growing pigs}

raised under ad libitum

_feeding

conditions has grown since the

early

1980s

owing

to the

genetic

trends that have

occurred as a result of selection.

Pig

populations,

which have become leaner and more efficient in terms of

_converting

food to

_liveweight

_gain,

_generally

exhibit lower

daily

feed intake

_(McPhee,

1981;

Mitchell et

_{al, 1982;}

Ellis et

_{al, 1983; Brandt,}

1987;

Smith et

_al,

_1991;

Cameron and

Curran,

1994).

Such a decrease in

_daily

feed intake under ad libitum

feeding

conditions could limit the

_{long-term genetic}

improvement possible

for

_daily

lean tissue

deposition.

The inclusion of

_daily

feed

intake,

or _any other

_feeding

behaviour

_criterion,

_among

breeding

goals

requires

the

_knowledge

of

genetic

parameters

for

_feeding

behaviour

_criteria,

_including

their

genetic relationships

with

_growth

rate,

feed

_efficiency

and carcass lean to fat ratio. The literature review made

_by

Labroue

₍₁₉₉₅₎

concerning

the

appetite

of

_growing

pigs having

ad libitum access to

_feed,

showed a rather

_large

variation in the

_genetic

parameter estimates,

especially

for the

_genetic

correlation between food conversion

ratio and

_daily

feed intake

_(range

of available estimates:

_0.01-1).

In

_France,

three central test stations have been

_equipped

with ’Acema 48’ electronic feed

_dispensers

data to

_study

the

_genetic

_variability

of

_feeding

behaviour criteria. The aim of the

_present

_study

was to estimate

_genetic

_parameters

of the

_Large

White and French Landrace breeds for

_feeding

behaviour criteria and

_production

traits

_using

a restricted maximum likelihood

_(REML)

procedure applied

to a multi-trait animal model. The data used in the last

complete

estimation of

_genetic

_parameters

for

production

traits measured in French central test stations

_(Ducos

et

_al,

₁₉₉₃₎

were collected

prior

to the establishment of electronic feed

_dispensers.

This estimation of

_genetic

_parameters

therefore is the first one

_referring

to the new central

_testing

conditions

_prevailing

in France.

MATERIAL AND METHODS

Origin

of data

Data were collected on

_Large

White

(LW)

and French Landrace

(LR)

_pigs

at three French central test stations

_(Argentr6,

Le

_Rheu,

_Mauron)

between 1988

_(beginning

of ad libitum

_feeding

in _pens of around 12

_pigs)

and 1994. Since

_1990,

most pens in these stations have been

_equipped

with an ’Acema 48’ feed

dispenser.

Feed

was distributed in

pellets

and contained 9.0

MJ/kg

net energy and 170

g/kg

crude

protein.

During

this

_{period, testing}

was

performed

both on candidates for selection

(entire males)

and

_slaughtered

sibs

_{(castrated males).}

Breeders

_usually

sent one

triplet

of

_pigs

_(two

candidates and one

_full-sib)

_per litter. Animals were tested in

successive batches

_(’all

in-all out’

_system),

each batch

being

defined as a _group of

contemporary

animals

_entering

the station within a

_10-day

_{period, having}

similar

ages and

liveweights

(around

30

_days

and 7

kg,

respectively).

Young

boars

_(candidates

for

_selection)

were tested between 35 and 95

_kg

liveweight.

Beginning

in

_1990,

feed intake was recorded

_{individually throughout}

the test

_period

_(ie,

the establishment of electronic feed

_dispensers).

Backfat thick-ness was measured twice at the end of the test at

_liveweights

around 95

_kg.

The ultrasonic measurements were taken on each side of the

_spine,

4 cm from the

mid-dorsal line at the level of the

shoulder,

last rib and

_{hip joint, respectively.}

Castrated males

_(sibs)

were tested between 35 and 100

_kg

liveweight. They

were fed ad libitum

during

the whole test

_period,

but individual feed intake was not recorded on all sibs in two stations. At these

stations,

the boars were

preferentially

raised in the pens

equipped

with an electronic feed

dispenser.

Sibs were

slaughtered

in a commercial abattoir at an _average

liveweight

of 100

kg.

On the

day

after

slaughter,

a standardized

_cutting

of one half-carcass was

performed

(Anonymous,

1990)

and three meat

_quality

measurements

_(ultimate

_pH,

reflectance and

water-holding

capacity)

were taken on ham muscles as described

by

Tribout et al

(1996).

Two data sets

_(one

_per

_breed)

were created

_{by considering}

all LW and LR boars

and sibs tested from 1988 to 1994 in the three French central test stations

_{(table I).}

Detailed information on individual

_feeding

behaviour was available for all boars and

a

_portion

of the sibs tested between 1992 and 1994. For

computational

_reasons,

only

two

_generations

of

_ancestors,

_ie,

the

parents

and

_{grand-parents}

of tested

animals,

Traits

_analyzed

Production traits

Six

_performance

traits were

_{studied, namely:}

three ’boar’ traits: average

daily gain,

food conversion ratio and ultrasonic

backfat

_thickness;

three ’sib’ traits:

_dressing

_percentage

_computed

as the ratio of carcass

_weight

over

_{slaughter liveweight,}

carcass lean content

_predicted

from the

proportions

of six

joints

in the half-carcass

_(Anonymous,

1990;

Bidanel and

_Ducos,

_1996),

and meat

quality

index established as a

_predictor

of the

technological yield

of cured-cooked ham

_processing

and

consisting

of a linear function of the three above-mentioned meat

_quality

measurements

_(Gu6blez

et

_{al, 1990;}

Tribout et

_al,

_1996).

Feeding

behaviour criteria

After each visit to the feed

dispenser,

animal

number,

time at the

_beginning

and at the end of the visit and amount of feed consumed were recorded. Successive visits

performed by

the same animal within 2 min were

_grouped

into the same meal as described

_by

Labroue et al

_(1994b).

The

_following

six traits were defined for each

pig

:

three criteria

_relating

to meal characteristics: average feed intake per meal

(g),

average total time per meal

(min)

including

eating

time and time intervals between the

visits,

average number of meals per

day;

three criteria

_relating

to

_daily

characteristics: _average feed intake per

day

(kg),

average total

eating

time per

day

(min)

defined as the total duration of all visits made on the same

_day,

_average rate of feed intake

_(g/min)

defined as the ratio

of

_daily

feed intake over

_daily

_eating

time.

Feeding

behaviour data were collected over a fixed

_period

of 12 weeks for boars and 13 weeks for

_{sibs, ie,}

the

_respective

average times on test for entire and castrated males. The calculation of _average

_feeding

behaviour traits was

_{performed using}

only

’full-record’

_days

_(Labroue,

_1996).

Some

_pigs

did not

_complete

the test. The minimum duration of the test

_period

was set to 10 weeks.

_Any

_pig

dead or discarded before the 11th week of test was removed from the

_analysis.

The number of

_pigs

per pen

(’group size’)

was based on the number of

_pigs

that started the test. An animal

_present

for less than 10 weeks was

_given

a

_weight

of 0.1 _per week of presence.

However,

there were

only

_very few accidental losses and

group size

usually

remained

unchanged throughout

the test

period.

The average group size was

11,

with

85%

of the

pigs

housed in pens of 9 to 13 animals. Boars and sibs were not raised

_together

in the same _pen, whereas LW and LR

_pigs

were

occasionally

mixed

_together.

An earlier

study

(Labroue

et

_al,

_1994b)

had

_suggested

that

mixing pigs

from these two breeds in the same _pen could influence the

feeding

pattern

of LR

_pigs.

This was not confirmed in the

_present

_sample

of

_pigs,

and the effect of breed

_mixing

or not was not included in the statistical model.

Statistical model

The model varied

_depending

on the

_trait,

but had the

_following

basic form in matrix

notation:

where y is the vector of

_{observations,}

b is the vector of fixed

effects,

p is the

vector of random litter

_effects,

a is the vector of random additive

_genetic

values of

animals,

e is the vector of

_residuals,

and

_{X, W,}

Z are incidence matrices

_relating

observations to the effects included in the model.

The statistical model used for each trait or _group of traits is shown in table III.

For

_feeding

behaviour

_traits,

the model used was chosen

_following

the results of two earlier studies on factors

_{influencing feeding}

behaviour in

_group-housed

_growing

pigs

(Labroue

et

al, 1994a,

b).

The three fixed effects taken into account were:

sex

(entire

or castrated

males),

batch

(35

or 36

_{levels, depending}

on the

breed)

and _group size

_(<

_7,

_{8, 9, 10, 11,}

_12,

_{13, !}

_14).

_{Preliminary analyses}

showed that the first-order interactions among fixed effects were not

_significant

for any

trait,

and no interaction term was included in the model. The random litter effect was not taken into account in the model

_applied

to the three ’sib’ traits.

Indeed,

there was

_only

one castrated male in

98.7%

of LW and

99.1%

of LR

_{litters, precluding}

the

_possibility

of

_obtaining

a reliable estimation of litter effects for ’sib’ traits. A random

_sampling

of one castrated male was therefore

performed

in the _very few

litters

_containing

two castrated males.

Computing

strategies

Variance and covariance

_components

were estimated

by

the multivariate REML

using

the derivative-free

_algorithm

described

_by

Groeneveld

_(1991).

It was not

computationally possible

to

_analyze

all the traits at once. For the traits

_sharing

the same model of

analysis, only

4- or 5-trait

analyses

reached _convergence within an

_{acceptable computing}

time. As a

_result,

several

_analyses

_using

different com-binations of traits were

performed

for each breed.

Moreover,

for

_estimating

the

(co)variance

components

for traits submitted to different models of

_{analysis, only}

2-trait

_analyses

_(including

one

_production

trait and one

_feeding

behaviour

trait)

could be

performed

for each breed. In all _cases, a

Quasi-Newton

(DF-QN)

algo-rithm

_(UNCMIN

_option

of VCE 3.2 software

_package)

was used to maximize the likelihood function because of its

_good

_{convergence rate}

_{(Groeneveld, 1993).}

The

convergence criterion

(CC)

was defined as CC = max

[ø(

t

) - !(t-1)!,

where

0(’)

and

!!t-1)

are the vectors of

_parameters

estimated at iteration t and t -

1,

respectively.

The

_stopping

criterion was set at

5.10-

3

.

The total number of iterations

_ranged

from 35 to 42 for the 2-trait

_analyses

and from 58 to 120 for the 4- or 5-trait

analyses.

Lower bounds of standard errors of

_genetic

_parameters

were obtained from the

RESULTS

Production traits

As shown in table

IV,

traits

_pertaining

to carcass lean to fat ratio showed the

_highest

heritabilities

_(h

₂

_ranging

from 0.60 to

_0.76).

_Heritability

estimates for _average

_daily

gain

were about 0.35.

Heritability

values were similar in both breeds

(around 0.20)

for food conversion ratio and meat

_quality

_index,

but were

_larger

in the LW than

in the LR breed for

_dressing

_percentage.

Common environmental effects

_(c

₂

₎

were small for live backfat thickness but were

_larger

for _average

_daily

_gain

and food conversion ratio.

The two traits

_predicting

carcass lean to fat

_{ratio, ie,}

live backfat thickness in boars and carcass lean content in

sibs,

showed

_{high genetic}

correlations

_(-0.84

and -0.79 in LW and LR

breeds,

respectively).

Average daily

gain

and food

conversion ratio were

_negatively

_{(ie, favourably)}

_correlated,

with a more

_pronounced

genetic

association in the LR than in the LW breed

_(-0.47

versus

_-0.24).

Genetic

relationships

between _average

_daily

_gain

and carcass lean to fat ratio were moderate

in both breeds

_(r

_A

of about

_-0.20).

Genetic correlations between meat

_quality

index and average

daily

gain

were low in both breeds and the

_genetic

correlation between meat

_quality

index and food conversion ratio were unfavourable. A noticeable

genetic antagonism

was also found between meat

_quality

index and carcass lean to fat ratio

_(r

_A

of about

_-0.35)

whatever the breed.

Feeding

behaviour criteria

Most

_heritability

estimates of

_feeding

behaviour criteria were in the _range 0.42-0.50

_{(table V).}

Whatever the

breed,

the

_highest

heritabilities were found for rate of feed intake

_{(about 0.50)}

and the three criteria

_relating

to meal characteristics

(0.42-0.54).

The

_heritability

value of feed intake _per

_day

was 0.42 in both breeds. Common environmental effects were

higher

in the LR

(7-11%

of the

phenotypic

variance)

than in the LW breed

_(2-6%).

Genetic correlations among

feeding

behaviour criteria were similar in both breeds.

_Phenotypic

correlations in absolute value were most often lower than

_genetic

correlations. In both

breeds,

high

genetic correlations, larger

than 0.79 in absolute

value,

were found between

_daily

_number,

size and duration of meals.

_{Thus, pigs}

eating larger

meals consumed a few

_long

meals per

day,

and there seemed to be

a _range of

feeding

_patterns

_varying

from

_’large

meal eaters’

(a

few

_long

meals _per

day)

to ’nibblers’

_(many

short meals per

day).

Feed intake per

day

showed

_positive

genetic

correlations

_(0.40-0.60)

with feed intake per meal and rate of feed intake.

These

_fairly

_{high genetic}

correlations as well as the

_{negative genetic}

correlation

(around -0.33)

found between feed intake _per

_day

and number of meals _per

_day

indicate that

_breeding

for increased

_appetite

would lead to

₁₎

_’large

meal eaters’ rather than ’nibblers’ and

₂₎

_pigs

_having

a

_higher

rate of feed intake. In

_contrast,

daily eating

time would not be

_greatly

affected.

Genetic correlations between

_production

traits and

_feeding

behaviour criteria

Among

the studied

_feeding

behaviour

_criteria,

feed intake _per

_day

was the most

(around

0.85)

were found between feed intake _per

day

and _average

daily gain.

Whatever the

_breed,

the

_genetic

correlation between

_daily

feed intake and food conversion ratio was close to zero.

_However,

the

_{genetic antagonism}

between

_daily

feed intake and carcass lean content was

noticeably

_stronger

in the LR than in the LW breed. Genetic correlations of feed intake _per

_day

with

_dressing

_percentage

or meat

_quality

index were rather low in both breeds.

Among

behavioural criteria other than feed intake _per

_day,

feed intake _per meal and rate of feed intake showed the closest

_genetic

associations with

_production

traits.

_They

were

_positively

correlated with _average

_daily

_gain

(about

0.50 and 0.30 in LW and

LR,

respectively)

but

_negatively

correlated with carcass lean content

(about

-0.30 and -0.20 in LW and

_LR,

_{respectively).}

Genetic correlations of rate of feed intake or feed intake _per meal with

_production

traits were of the same

_sign

as those found between feed intake per

day

and

production

traits,

while

being

lower in

absolute value. Other

_feeding

behaviour criteria

_(eating

time per

day,

number and duration of

_meals)

showed

_fairly

low

_genetic

correlations with

_production

traits.

However,

in

_LR,

food conversion ratio was

_genetically

correlated with duration of meals and rate of feed

_intake,

whereas carcass lean content was

_genetically

correlated with number of meals per

day.

DISCUSSION

Methodological

aspects

There is a

_general

_agreement

that REML

_methodology

_applied

to an individual animal model

_(IAM)

is the method of choice for

_estimating

location and

dispersion

parameters

for traits described

_by

linear

_models,

because of its desirable statistical and

_{genetic properties.}

In

_particular,

this method accounts for the effects of selection if all the information related to selection is included in the

_analysis

_(Sorensen

and

Kennedy, 1984;

Gianola et

al,

1989).

Nevertheless,

the use of multivariate REML-IAM for a

_{single analysis}

of

_large

data sets

_requires

substantial

_{computational}

facilities,

and

_generally

researchers use limited

_{applications,}

which can be

_performed

with some deviations from the

optimal

situation.

The

_present

data set had several

_drawbacks,

such as different traits

_being

mea-sured on different

individuals,

low numbers of

offspring

recorded _{per sire} and _per

litter,

and a _very low

_proportion

of

_{performance-tested}

_parents.

As a

_result,

there

were _convergence

_problems,

which were solved

_by

1)

_limiting

the number of ge-nerations of ancestors taken into account in the

_{pedigree file,}

₂₎

_setting

the litter covariance

_components

to zero when

_analyzing

traits described

_by

different sta-tistical

_models,

and

₃₎

_{running analyses}

that included at most two to five traits.

Limiting

the number of ancestors and the number of covariance

components

re-sulted in a reduction of the number of likelihood functions to be

_computed

and of the CPU time per likelihood. The

_impact

of

₁₎

was

_investigated

in LR.

_Adding

a third

_generation

of ancestors increased

_computing

time

_{considerably,}

but did not

change

the estimates of variance

_components

at all

_{(Labroue, 1996).}

The

_impact

of

₃₎

is

_{theoretically}

more critical. All selected traits should be included in the

estimates of variance

_components

obtained for a

_given

trait from different

_analyses

(within

or between _groups of

_traits)

tends to indicate that these _consequences should be rather

_limited,

at least for variances.

However, positive

definiteness of the reconstructed variance-covariance matrices is no

_longer

_guaranteed.

The

consistency

of variance-covariance matrices was tested in both breeds for each _group of

_traits,

and

_positive

definiteness was obtained for all these matrices

(Labroue,

1996).

Another drawback of the

_present

data set was the imbalance between numbers of boars and sibs measured for

_feeding

behaviour criteria.

_Owing

to the low numbers of sibs recorded for

_feeding

_behaviour,

it was

_thought

that

_studying

’boar’ and ’sib’

_feeding

behaviour criteria

_separately

would not

_provide

reliable estimates of

_genetic

_parameters

for sibs.

Heritabilities

Heritability

estimates for

production

traits are

generally

in

_agreement

with those found in recent studies

_dealing

with the traits

_routinely

recorded in French central

test stations

_(Ducos

et

_{al, 1993;}

Bidanel and

_Ducos,

_1996).

In

_comparison

with earlier French studies

_(Ollivier

et

_{al, 1981;}

Tibau i Font and

Ollivier,

1984)

and with the literature review of Ducos

_(1994),

the most

_pronounced

differences in

_heritability

estimates concern the

_relatively

low values obtained for food conversion ratio as well as the

_{relatively high}

values obtained for ultrasonic backfat thickness. These differences

_{probably originate}

from the differences in

_feeding

conditions

_(ad

libitum versus restricted or ’to

_appetite’

feeding)

_knowing

that the variation in

_genetic

parameter

estimates due to

_feeding

_regime

is well established in

_pigs

_(Cameron

et

_al,

_1988).

For

_heritability

of food conversion

_ratio,

our estimates

(around 0.20)

are the same as that found

_by

Von Felde et al

₍₁₉₉₆₎

for similar breeds and

_testing

environment.

The

_present

_heritability

estimate for feed intake per

day

(0.42

in both

_breeds)

is

slightly

greater

than the _average literature value of 0.32

_reported

_by

Labroue

_(1995).

For the other

_feeding

behaviour

_criteria,

the

_present

results can be

_compared

with those obtained in recent studies carried out under

_{group-housing}

conditions

_using

electronic feed

_dispensers,

either ’IVOG’ stations

_(De

Haer and De

Vries,

1993)

or

’Acema 48’ feeders

_(Von

Felde et

_al,

_1996).

In the

_study

of De Haer and De Vries

(1993),

heritability

estimates for

_feeding

behaviour criteria

_ranged

from 0.24 to

0.49,

but with

_fairly

_large

standard errors

_(0.16-0.24)

due to the limited size of the data set.

_Feeding

duration was the least heritable criteria

(h

2

= 0.25 on

average),

whereas the

_daily

number of meals was more heritable

(h

2

=

0.45).

Feed intake

per meal had a rather

high heritability

(0.47),

whereas feed intake _per

day

had a

_markedly

lower

_heritability

(0.16)

than in the

_present

_study.

_According

to Von Felde et al

_(1996),

_heritability

of feed intake _per

_day

between the

_liveweights

of 48 and 117

_kg

showed some variation over time and reached its maximum value

(h

2

=

0.30)

in the middle of the test

period.

Over the whole

testing

period,

the

heritability

estimates

_{reported by}

these authors are 0.22 ! 0.06 for feed intake per

Genetic correlations

For ’boar’ and ’sib’

_{production traits,}

the set of

_genetic

correlations estimated

in the

present

study

shares several common features with that from the most

recently published study

carried out in France on the same traits and breeds

(Ducos

et

_al,

_1993).

Both studies show very close

genetic

relationships

between

similar traits measured on animals of different sex

_types

(r

A

of -0.8 to -0.9

between live backfat thickness of boars and carcass lean

_percentage

of

sibs)

as well as

_moderately

unfavourable

_genetic

correlations of _average

_daily

_gain

with ultrasonic backfat thickness of boars

_{(around 0.3)}

or carcass lean

percentage

of sibs

(around -0.2).

However,

a noticeable difference between the two studies concerns the

_{relationships}

between average

daily gain,

food conversion ratio and live backfat

thickness. The

_study

of Ducos et al

₍₁₉₉₃₎

dealt with data collected in

_1980-1990,

and most of the boars involved had been fed ’to

_appetite’

_(two

meals per

day).

In that

_study,

food conversion ratio was much more

closely

associated at both

phenotypic

and

_genetic

levels with _average

_daily

_gain

_(rp

and rA of about -0.7 and

_-0.6,

_{respectively)}

than with ultrasonic backfat thickness

_(rp

and rA of about 0.1 and

_0.3,

_{respectively).}

_Conversely,

all boars involved in the

_present

_study

were fed ad

libitum,

and food conversion ratio

appeared

to be associated to the same extent with _average

_{daily gain}

and backfat thickness. When

_averaged

over LW and

LR

_breeds,

the

_phenotypic

as well as

_genetic

correlations turned out to be around - 0.40 for average

daily gain

and 0.35 for backfat thickness.

To our

_knowledge,

estimates of

_genetic

_{correlations among}

_feeding

behaviour criteria

_have,

so

far,

been

reported

_{only by}

Von Felde et al

_(1996).

_However,

De Haer and Merks

_(1992),

Labroue et al

_(1994b),

_Young

and Lawrence

₍₁₉₉₄₎

and

Hyun

et al

₍₁₉₉₇₎

_{reported phenotypic}

correlations between these criteria. The main differences between the

_study

of Von Felde et al

₍₁₉₉₆₎

and the

_present

one concern the

_{genetic relationships}

between feed intake _per

_{day, eating}

time per

day

and rate of feed intake. As

_previously

_{reported by}

De Haer and Merks

₍₁₉₉₂₎

and

_Young

and Lawrence

₍₁₉₉₄₎

at the

_{phenotypic level,}

Von Felde et al

₍₁₉₉₆₎

_reported

a

fairly

low

genetic

correlation

_(r

_A

=

0.31)

between

daily

feed intake and _rate of feed

intake but

_higher

_genetic

correlations of

_{daily eating}

time with

_daily

feed intake

(r

A

=

0.44)

and rate of feed intake

(rp

=

-0.62).

In the

_present

study,

rate of feed intake was

_closely

correlated with

_daily

eating

time

_(rp

= -0.73 and _rA = -0.82 when

_averaged

over the two

_breeds),

_and,

to a lesser

_extent,

with

_daily

feed intake

(rp

= 0.41 and _{rA =}

0.43),

but

daily

feed intake and

daily eating

time were

_poorly

correlated

_(rp

= 0.22 and _rA =

0.15).

In

_contrast,

there is

good

_agreement

between the two studies

_regarding

the _very close

_genetic

correlations

_(about

0.8 in absolute

value)

between

_size,

duration and

_daily

number of meals.

Concerning

the

_{genetic relationships}

between

_daily

feed intake and

_production

traits,

the

_genetic

correlation estimated in the

present

study

between feed intake

per

day

and average

daily

gain

(0.84

on _average over the two

_breeds)

is

_slightly

higher

than the _average literature value of 0.71

_reported

_by

Labroue

₍₁₉₉₅₎

and the value of 0.68 found

_by

Von Felde et al

_(1996).

The

_genetic

correlation between feed intake per

day

and ultrasonic backfat thickness

(0.48

on

_average)

is _very close to the value of 0.45

_{reported by}

Labroue

₍₁₉₉₅₎

and Von Felde et al

_(1996).

This

a

_genetic

_independence

between feed intake _per

_day

and food conversion ratio

_(r

_A

of 0.11 or -0.06

_depending

on the

_breed),

which _agrees with the

_{corresponding}

value of 0.13 ±0.28

_reported

_by

Von Felde et al

₍₁₉₉₆₎

but not with the _average literature value of 0.37

_{quoted by}

Labroue

_(1995).

_However,

the latter author

pointed

out that this

_pair

of traits shows an

_extremely

broad range of variation

(0.01-1.00)

for the available estimates of

_genetic

correlation between the two traits under ad libitum

feeding

conditions.

Feeding

behaviour criteria other than feed intake _per

_day

are also somewhat associated with

_production

traits.

_According

to De Haer et al

₍₁₉₉₃₎

and

_Hyun

et al

_(1997),

feed intake _per meal and rate of feed intake are the most

_closely

correlated with

_production

traits at the

_{phenotypic level,}

the

_highest

correlations

(around

0.4)

occurring

for _average

_{daily gain.}

The same

_general

_pattern

was found here at the

_{genetic level,}

but

_{relationships}

were

_slightly

less close. In our

study,

the

_highest

_genetic

correlations for _average

_{daily gain}

were found with rate of feed intake and feed intake _per meal

_(around

0.4 when

_averaged

over LW and LR

breeds).

The

_{corresponding}

estimates

_{reported by}

Von Felde et al

₍₁₉₉₆₎

tended to be lower

_{(around 0.25).}

In the

_present

_study,

feed intake per meal and rate of feed intake also showed

_moderately

unfavourable

_genetic

correlations with ultrasonic backfat thickness

_{(around 0.25)}

and carcass lean

_percentage

_{(approximately -0.25).}

Considering

this moderate

_{genetic antagonism}

with carcass lean to fat

_ratio,

these

two criteria could form an

_interesting

alternative for selection. Von Felde et al

₍₁₉₉₆₎

also

reported

a

_{positive genetic}

correlation of 0.32 between average

daily

gain

and

daily eating time,

whereas the

_{corresponding}

estimate was

_only

_0.11,

when

averaged

over the two

breeds,

in the

_present

_study.

As a

_{general rule,}

food conversion ratio and carcass lean to fat ratio showed low

_genetic

correlations with

_feeding

behaviour criteria

_apart

from

_daily

feed intake in the

_present

_study

as well as in the

_study

of Von Felde et al

_(1996).

A feature common to the two studies _appears to be the moderate

_genetic

correlation

_(around

_-0.2)

of carcass lean to fat ratio with rate of feed intake.

CONCLUSION

Reliable estimates of

_genetic

_parameters

make it

_possible

to consider _ways of

enhancing

genetic improvement

of

_{growing pigs}

while

_preventing

a decrease in

_daily

feed intake. The direct inclusion of the latter trait in the overall

_breeding

_objective

would underline two

problems:

1)

the choice of an economic

_weight

and

₂₎

the

genetic antagonism

between

_daily

feed intake and carcass lean to fat

_ratio,

which

may

adversely

affect the

efficiency

of their

_joint

selection. As more unfavourable

genetic

correlations of carcass lean to fat ratio are found with

_daily

feed intake

than with feed intake _per meal or rate of feed

_intake,

the two latter traits could form an

_interesting

alternative for selection.

_However,

the

_{expected genetic}

_response for

_daily

feed intake would be lower because the

_genetic

correlations of those traits with

_daily

feed intake itself are

_only

0.4. It

_might

be worth

_{investigating}

whether

taking

into account certain

_feeding

behaviour criteria could

_improve

the overall

efficiency

of selection or not. The

_present

_study

_highlights

that heritabilities of feed intake per

day,

rate of feed intake or feed intake _per meal are twice as

_large

as that

predicted

at the

_genetic

level

by

the combination of average

daily gain

and carcass lean to fat ratio under ad libitum

feeding

conditions. It is

_suggested

that it

_might

be valuable to

_replace

food conversion ratio

_by

a trait

_relating

to feed intake

pattern

in the selection indexes used in

_{pig breeding}

_programmes.

ACKNOWLEDGMENTS

We

_{gratefully acknowledge}

M

_Bouffaud,

D

_Brault,

D

_Breton,

E

_Cherel,

C Perrocheau

and the staff of French central test stations for their valuable

_help.

Thanks are due to

D Boichard for

_{having developed}

methods of

_{computing sampling}

variances of

_genetic

parameters estimated

_by

the VCE

_package

as well as to the anonymous reviewers for their pertinent comments. This work was

_{supported by}

a doctoral thesis

_{scholarship given}

to the first author

_by

INRA and ITP and

_by

_grants from INRA

_(AIP

’D6terminisme

_g6n6tique

de

_{I’app6tit’)}

and the French

_Ministry

of

Agriculture.

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