Variation factors of pregnancy rates after oestrus synchronization treatment in French Charolais beef cows

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Variation factors of pregnancy rates after oestrus

synchronization treatment in French Charolais beef cows

C Ponsart, M Sanaa, P Humblot, B Grimard, N Jeanguyot, Aa Ponter, Jf

Viel, Jp Mialot

To cite this version:

Original

article

Variation

factors

of

_pregnancy

rates

after

oestrus

synchronization

treatment

in French Charolais

beef

cows

C Ponsart

M

Sanaa

P Humblot

B Grimard

1

N

_Jeanguyot

₂

AA Ponter

JF Viel

JP Mialot

1 _Laboratoire

_{d’épidémiologie}

_et

gestion

de la santé animale,

École

nationale vétérinaire Alfort,

7, avenue du Général-de-Gaulte, 94704 Maisons-Alfoa,

-2_{Laboratoire de}

_dosages

_{hormonaux, Union nationale des}

_{coopératives d’élevage}

et d’insémination artificielle, 13, rue Jouet, BP 65, 94703 Maisons-Alfort;

3

_Département

_{de santé}

publique,

Faculté de médecine, 2, place

Saint-Jacques,

25000 Besançon, France

(Received

15

_September

1995;

accepted

23

_{January 1996)}

Summary ―

An

_{epidemiological study}

was conducted in

_{Bourgogne (France)}

in Charolais herds

sampled

in 1990

_(sample

1: 54

herds)

and from 1991 to 1993

(sample

2: 33

herds).

The aim was

pri-marily

to

_quantify,

at both herd and individual levels, the factors that result in variations in the response to

_{synchronization}

treatment as estimated

by

pregnancy rate after first insemination. The second

goal

was to test a similar model in

sample

2, which included

only

primiparous cows. In _{sample 1,} 329 cows

of the 627 studied

(52%)

became

_{pregnant compared}

with 122

_primiparous

cows of the 249 studied

(49%)

in

_sample

2.

_Pregnancy

rates _{after treatment per} herd

_ranged

from 0-100% across the herds. Mixed

_{logistic regression}

was used to calculate the odds ratio

(OR)

and to take into account both indi-vidual and herd level variables for each

sample.

In the first

sample, multiparous

cows were more

likely

to become _pregnant after

synchronization

treatment than

primiparous

cows

(OR

= 2.4, P <

_0.001

_The

following

individual variables also influenced the response to

_{synchronization: body weight}

at

_implant

insertion

(OR

= _{1.006, P= 0.017) and}

_{calving difficulty (OR}

= _{0.33, P= 0.008). In the second}

_sample,

results varied with the year of data collection

(OR

= _{3.46, P} =

_0.016)

_{and with two herd-level}

vari-ables: presence of a bull

(OR

= 0.48, P=

0.047)

and _type of

housing (OR

= 2.5, P=

_0.004).

In conclusion,

the results after

_{synchronization depended}

on both herd-level and individual variables and it was

nec-essary to take into account two levels of factors to reduce _variability after treatment. beef cattle / oestrus _{synchronization} /

fertility

/

epidemiology

*

Résumé &horbar; Facteurs de variation du taux de

gestation après

traitement de

_{synchronisation}

des chaleurs chez des vaches de race Charolaise. Une étude

épidémiologique

a été réalisée dans 87

élevages

de bovins Charolais en

_Bourgogne,

54 suivis en 1990

(échantillon 1)

et 33 entre 1991 et 1993

(échantillon 2).

Le

premier

objectif était de

quantifier

les facteurs à la fois individuels et liés à l’éle-vage faisant varier la

réponse

à un traitement de

synchronisation,

mesurée par la fertilité à l’c!strus induit

après première

insémination. Le second

objectif

était de voir _quels étaient les facteurs de variation sur

l’échantillon 2, constitué _uniquement de

_primipares.

Pour l’échantillon 1, 329 vaches sur 627

_{(52 %)}

ont été

_{diagnostiquées}

gestantes suite au traitement ainsi _que 122

_primipares

sur 249

₍₄₉

_{%) pour} l’échan-tillon 2. Les taux de

gestation

obtenus

après synchronisation

par

élevage

varient de 0 à 100 %. Une

régression

logistique

a _{été utilisée pour calculer les odds ratios}

_(OR).

Pour chaque échantillon a été retenu un modèle

logistique

mixte contenant à la fois des variables individuelles et des variables liées à

_l’élevage

et modélisant l’effet

_élevage

de

_façon

aléatoire. Pour l’échantillon 1, les

_multipares

ont un

odds ratio

_plus

élevé _que les

_{primipares (OR}

= _2,4,

p < _0,001). Les variables individuelles suivantes

entraînent

_également

une variation de la

_réponse

au traitement de

_{synchronisation :}

le

_poids

à la pose de

l’implant

(OR

= _1,006,

p =

_0,017)

_et les conditions de

_{vêlage (OR}

= _{0,33, p} =

_0,008),

montrant de moins bons résultats suite à un

_vêlage

difficile ou sur des vaches _trop

maigres

à la pose. En ce _qui concerne l’échantillon 2, la

réponse

au traitement

dépend

à la fois de l’année avec des taux _de ges-tation

plus

élevés en 1993 _qu’en 1991

(OR

= _{3,46, p=}

0,016)

et de deux facteurs

d’élevage qui

_sont le type de

_{logement (OR}

= _2,5,

p =

0,004)

et la

_présence

du taureau

_(OR

= _{0,48, p} =

0,047).

En conclu-sion, les résultats du traitement de

_{synchronisation dépendent}

bien des facteurs à la fois individuels et

élevage,

dont il faut tenir compte si on veut en réduire la variabilité. vache allaitante

/ synchronisation

de l’cestrus / fertilité /

_{épidémiologie}

INTRODUCTION

Reproduction

is one of the most

_important

factors

_{limiting production efficiency}

in beef

cattle

_(Short

et

_{al, 1990).}

In traditional

French

_{systems, profitability}

is

_improved

when the number and

_weight

of the calves

is increased and when a uniform batch of

calves is sold in autumn

_(Grimard

and

Mialot, 1990).

Farmers are therefore

encour-aged

to shorten and advance the

_calving

season in the winter

_period.

At this

time,

however,

only

12-18% of Charolais cows

cycle

60

_{days postpartum (Chupin}

et

al,

1977;

Pelot et

al, 1977;

Grimard et

al,

1992b).

Synchronization

treatment based

on

_progestagen

associated with

_Pregnant

Mare Serum

_{Gonadotrophin (PMSG)}

can

be used to induce and

_synchronize

oestrus

in such cattle

_(Grimard

and

Mialot, 1990;

Odde, 1990).

The

results, however,

are

_highly

variable. The

_percentage

of cows

_showing

oestrus

48 h after

_implantation

or

_{intra-vaginal}

device

removal ranges from 56-100% in different

trials

_(Chupin

et

ai, 1977;

Miksch et

al,

1978)

and pregnancy rates at induced ovulation

vary from 34 to 74%

_(Miksch

et

al, 1978;

Drew et

al, 1979;

Brink and

Kiracofe,

1988).

In

_addition,

some trials have

_reported

_very

variable results between farms

_(Aguer,

1981;

Grimard et

al,

_1992a).

Such

variabil-ity

limits the usefulness of this method.

Cyclicity

before treatment

_(Chupin

et

al,

1977; Miksch

et al, 1978;

Aguer,

1981;

Beal

et

al, 1984;

Brown et

al,

1988), parity

(Aguer

et al, 1981;

_{Fogwell et al,}

1986;

Grimard et

al, 1992b),

calving

to artificial insemination

(Al)

interval

_(Pelot

et

ai, 1977;

Kiser et

_al,

1980;

Aguer,

1981;

Fogwell

et

al,

1986),

energy status of the animals and

_feeding

management

(Pelot

et

al, 1977;

Walters et

al,

1984; Grimard et

al,

1992a)

have been

reported

to _{influence the response} to the

progestagen

plus

PMSG treatment.

The results of the

_{synchronization}

treat-ment _{may also}

_depend

on other factors that are

_already

known to influence

_fertility

after

anoestrus, such as

_{suckling (Carruthers}

and

Hafs,

1980;

Terqui

et

al, 1982;

Short et

_al,

1990; Williams,

1990),

calving

difficulty

(Agabriel

et

al, 1992;

Ducrot et

al, 1994b;

Pouilly

et

al,

_1994),

date of

_{calving (Peters}

and

_Riley,

1982;

Agabriel

et

al,

1992), type

of

_housing

_(Terqui

et

al, 1982;

_Pouilly

et

al,

1994)

and the presence of a bull

_(Zalesky

et

al, 1984;

Cupp

et

al,

1993).

Among

these

factors,

many studies have

highlighted

the

_relationship

between

_parity

and

_fertility

_(Chupin

et

_{al, 1977; Odde, 1990;}

Ducrot et

al,

1994a).

First-service

_conception

rate after

_{synchronization ranged}

from

43.4-53.4% for

multiparous

cows and from

only

31.3-36.5% for

_primiparous

cows

(Aguer

et

al, 1981;

Fogwell

et

ai, 1986;

Gri-mard et

af,

1992a). Primiparous breeding

management

is more

complicated

because

primiparous

cows have

maintenance,

growth

and

_{suckling requirements}

_{which may}

com-pete

with

_{breeding requirements}

_(Short

et

al, 1990;

Agabriel

et

al,

₁₉₉₂₎

and this

results in a

_{longer period}

of

_postpartum

anoestrus

_(Ducrot

et

_{al, 1994b; Pouilly}

et

al,

1994).

In

addition,

the

_particular

needs of

primiparous

cows

_during

the

_reproduction

period

have not been

_fully

studied. The effect of herd and individual

param-eters on _{the response} to oestrus treatment

has

_rarely

been studied

_{simultaneously}

in

statistical

models,

because classical

meth-ods do not take into account the

_clustering

of animals within herds. This results in

biases

_being

introduced

(McDermott

et

al,

1994b;

Sanaa

et al,

1994).

The aim of the

_present

_study

was

pri-marily

to

_quantify,

in an

_{epidemiological}

trial,

both herd-level and individual-level factors

which

_provoke

variations in _pregnancy rate

after insemination at a fixed time after

syn-chronization treatment in Charolais beef

cows. The second

_objective

was to

_study

the factors of variation in Charolais

primi-parous cows, as _{these animals may have}

special requirements

for successful

breed-ing.

MATERIALS AND METHODS

Study population

The

_{study population}

consisted of 87

_randomly

selected Charolais beef cow farms located in Sa6ne-et-Loire

(Bourgogne, France).

These herds

were

_registered

at the insemination

cooperative

of

Verdun-sur-le-Doubs, where their

_reproduction

management systems are based on the

coupled

use of insemination in winter and natural

breed-ing

in

spring.

In order for these farms to be

selected, at least three Charolais cows had to

have received

_{synchronization}

treatment at the

same time and the

calving period

had to have been between 1 October and 31

_January.

For each herd, all cows

_synchronized

were _included,

which

_represented

a total of 876 cows.

In autumn 1990, a first

_sample

was collected;

this included 54 herds

(627 cows) satisfying

the above selection criteria and

containing

both

mul-tiparous

and

primiparous

cows. A second

_sample

of another 33 different herds

_{(249 cows)}

was also selected

during

the autumn

_periods

of 1991, 1992

and 1993.

Only primiparous

cows were retained for this second

analysis.

Treatment

All cows retained for the

study

received a 3 _mg

Norgestomet

ear

implant

for 9-10

days

and an

intramuscular

injection

of 3 mg

Norgestomet

and 5 _{mg Estradiol} valerate at the

beginning

of

implant

treatment

_(Crestar

ND, Intervet,

Angers, France).

In addition, 600 IU of PMSG was _injected at the

time of

_implant

removal

_(Chronogest

PMSG ND,

Intervet,

Angers, France).

Artificial insemination

was

_{performed systematically}

48 and 72 h after the

implant

removal.

Outcome measures

The response to

_{synchronization}

treatment was

estimated

_by

the first service

_conception

rate, as

defined

_by

the pregnancy rate after the two fixed-time inseminations

following

induced ovulation, pregnancy

being diagnosed by ultrasonography

For the

_analysis,

we modelled the occurrence

of pregnancy.

Collection of data associated

with

_{reproductive performance}

Herd information was obtained

by

an Al technician

during

an interview with the farmer

_early

in the winter of each year. The responses were recorded

on a

_{questionnaire.}

The information collected included the size of the farm, eg, number of cows

per herd, number of farmhands and usable

agri-culture area

_(De

_Bonneval,

_{1993), housing}

_type

and feed _management. Other variables were

recorded from 1991, such as nutritional

flushing

defined as an _energy

_{supplement (2-3}

Unites

Fourrag6res Lait) during

a short

_{period preceding}

mating

(Petit,

1988)

and the

physical

presence of a bull.

Production records, such as

_calving

date,

calv-ing difficulty

and date of AI, were collected from the farm records and from the records

kept by

the Al technician.

_{Calving difficulty}

was indexed as

calving

without assistance, with minor assistance,

forced extraction or caesarean section

(Grimard

et at, 1992a,

b).

For each cow, the

following

infor-mation was also collected

by

an Al technician at

calving

time in autumn 1990 and at

_implant

inser-tion each year:

body weight

was estimated

by

thorax-girth

measurement

_(Rossignol

and

Dechambre,

1984)

and a

_{body-condition}

score was estimated based on the INRA

(Institut

National de Recherche

Agronomique)

scale of 0 to 5

(Agabriel

et al,

1986).

Data

_analysis

The data were verified and

_{analysed using}

the statistical

packages

SAS

_(SAS

User’s

guide,

1988)

and

Egret (Egret, 1990).

Pooled variance Student’s t-tests for continuous variables

_and

_X

₂

analysis

for _{category-type} variables were

per-formed to test the

simple

association between the _potential factors of variation and pregnancy. A

descriptive comparison

between the two

samples using pooled

variance Student’s t-tests for continuous variables

_{and x}

₂

analysis

for

cat-egory-type variables is _presented in table I: the

sample

1 is similar to the

sample

2 for herd size, number of farmhands and _type of

_housing.

Calv-ing difficulty (P

< 10-3)

and

calving

to Al interval

(P

<

_10-

₃

₎

were

_{significantly}

different for each

sample,

is in _agreement with a

_parity

effect. The variables were therefore examined for their

rela-tionship to _pregnancy in both the data collected in 1990 and in the second,

primiparous sample

with data collected between 1991 and 1993.

Only

variables with p <- 0.10 were considered for further

analysis,

except

calving

to Al interval,

which was also tested because this factor has been _reported in _many studies

_(Pelot

et al, 1977;

Kiser et al, 1980;

Aguer,

1981;

Fogwell

et al,

1986).

A multivariate

_{logistic regression}

with a random effect model

(Egret, 1990)

was used to evaluate the effect of each variation factor

_controlling

the effect of other variation factors. Two models were

developed:

one for the first

_sample

in 1990 and another for the

_sample

1991-1993.

The choice of a

_{logistic regression}

model with random effects was _{justified by}

_clustering:

the

grouping

of cattle into herds means that cows in the same herd live under the same _management

and environmental conditions. Therefore, classi-cal

_regression

models, which are based on the

assumption

that observations are

_independent

and where cluster

_adjustment

methods are

ignored,

introduces biases because variance esti-mates are too small

_(McDermott

et al,

1994b).

Appropriate

statistical methods for

_binary

corre-lated data can be _grouped into two classes:

clus-ter-specific (individual level)

and

population-aver-aged (herd level).

Therefore a mixed model was

used, which

_gives

a

_good

estimate of both indi-vidual and herd

_regression

parameters

(Sanaa

et al,

1994).

All variables were introduced in the multivariate model without a _stepwise

_procedure

to calculate

adjusted

OR. A classical

logistic regression

model and a mixed model were

_{compared using}

a

single-rated likelihood ratio test

(Egret, 1990).

RESULTS

Average

herd size was similar in both

sam-ples (75

cows in

_sample

1 and 81 cows in

sample

2).

The average usable

agriculture

area _{and the average number of}

_people

working

on the farm were 119 ha and 1.8

respectively,

and no difference between

In the first

_sample,

329 cows of the 627

studied

_(52%)

became

_{pregnant (156}

prim-iparous

cows and 173

multiparous,

repre-senting

pregnancy rates of

_respectively

42.7 and

_66%).

In the second

_sample,

122

primiparous

cows of the 249 studied

_(49%)

were found to be

pregnant

after

synchro-nization treatment

_{(78 (45.6%), 17 (44.7%)}

and 27

_(67.5%)

_primiparous

cows in

1991,

1992 and 1993

_{respectively);}

these

per-centages

need to be

_{interpreted carefully}

because of the small

_sample

size collected

each year.

Pregnancy

rates after treatment across

herds were _very

_{heterogeneous}

as shown in

figure

1. In the two

_samples,

_pregnancy rates _{per herd varied from 0-100 %.}

Sample

1

The univariate association of each variable

with pregnancy after

synchronization

gave

the

_following

variables to be considered in

the final model: number of

_{farmhands, body}

weight

at

_{calving, body weight}

at

_implant

insertion,

calving difficulty, parity

and

_calving

to Al interval.

In the multivariate mixed model

pre-sented in table

II,

three variables were

sig-nificantly

associated with pregnancy rate

after treatment:

_{parity, calving difficulty}

and

body weight

at

_implant

insertion.

_Multiparous

cows had a

_greater

chance of

_becoming

pregnant

after treatment than

primiparous

cows. The odds ratio of _pregnancy

decreased

_{significantly}

with

_calving

diffi-culty, especially

for cows

_following

forced

extraction at

_calving,

where their chances

of

_{becoming pregnant}

decreased threefold

compared

with cows not

_requiring

assis-tance.

Moreover,

body weight

at

_implant

insertion influenced the result of

synchro-nization treatment: an increase of 50

_kg

in

body weight

resulted in an _{odds of}

preg-nancy of

1.37. The number of

farmhands,

body weight

at

_calving

and

_calving

to Al

interval were not found

_significant.

To test the interaction between

_body

weight

and

_parity

and the interaction

between

_{calving difficulty}

and

parity,

we

added these terms to the final model. The

likelihood was not

_{significantly improved}

(likelihood

ratio statistic

_(LRS)

= 2.94 with 8

A likelihood ratio test

_comparing

a classic

logistic regression

model with the mixed

model was also

_{highly significant}

_(LRS

=

9.433 for 1

df,

p =

0.001

which

is in

agree-ment with a random herd effect.

Sample

2

The univariate

_analysis

led to the inclusion

of the

_following

variables in the final model:

calving

difficulty,

number of

farmhands,

the

presence of a

bull,

_{housing type}

and

_calving

to Alinterval.

As shown in table

III,

the

_physical

pres-ence of a

_bull,

_type

of

_housing

_{and year of}

data collection were

_{significantly}

related to

the success of the treatment after

adjust-ment for a herd effect.

_Primiparous

cows in

a herd with a bull

_appeared

to have an odds

of pregnancy after treatment twice as

_high

as cows in herds that did not have a bull.

Tethered

_housing

was associated with an

increased rate _{of pregnancy}

_compared

with

loose

_housing.

The year of data collection

influenced the results after treatment, with

1993 data

_showing

a better _pregnancy rate

than

_previous

years. No

significant

effect

was found for

_{calving difficulty, body}

score

at

_implant

insertion,

_calving

to Al interval or

number of farmhards. The likelihood ratio

test

_comparing

a classic

_logistic

model with

the mixed model was not

_significant

_(LRS

=

0.00 for 1

df,

p =

_0.500).

DISCUSSION

The aim of this

_study

was to describe the

factors

_affecting

pregnancy rates _after

syn-chronization,

using

a statistical method that

accounts for within-cluster correlations. The

principal problem

if herdmates have

corre-lated responses is that error _and

parame-ter variance estimates are so small that null

hypotheses

are too

_{easily rejected.}

The choice between the different models

depends

on both

_logistical

and theoretical

considerations,

such as

_type

of

data,

sta-tistical software

_availability

and the

We therefore used a mixed

model,

that

_gives

a correct estimate of both individual and

herd

_{regression parameters}

and allows the

quantification

of the herd effect

_(Sanaa

et

al,

1994).

The use of this model for our data

is also

_{supported by}

the

_great

_variability

in

pregnancy rates _{per herd and} a

_significant

herd effect in the first

_sample.

In

addition,

this statistical method of

_modelling

the herd

effect as a random effect has

_rarely

been

used in studies on beef cow

_fertility

(McDer-mott et

al, 1994a;

Sanaa et

al,

1994).

The

_design

of this

_study

therefore allowed

the simultaneous

_analysis

of

_parameters

at

an individual and herd level under farm

con-ditions. Previous studies have

_already

tested

individual factors

_(Aguer,

1981;

Fogwell

et

al,

1986;

Grimard et

_al,

1992a,

b)

or herd

fac-tors

_(Terqui

et

al, 1982;

Cupp

et

al,

1993),

but the

adjustment

for factors at two levels

may

improve

the

_{understanding}

of variation

factors

_concerning

_fertility.

However,

an

observation

_study

under farm conditions

parameters

that cannot be

_readily

controlled,

for

_example,

_feeding

level. In

addition,

data

were collected over the

_period

from 1990

to 1993 and an _{evolution in farm}

manage-ment _{may have occurred}

_during

this

_period.

Another

_problem

_{may be related} to the

choice of animals within a herd

_being

treated. This

selection bias, however,

has

been

limited

_by

the

_fairly

restricted

period

of inclusion in the

_sample.

Farmers had to

synchronize

the cows that had calved

between 1 October and 31

_January.

Pregnancy

rates after the first

insemina-tion for the

_multiparous

cows and the

prim-iparous

cows in the two

_samples

_{agree with}

previous

results

_varying

from 34-74%

(Miksch

et

al, 1978;

Drew et

al, 1979;

Brink and

_{Kiracofe, 1988).}

In the first

_sample,

the most

_important

factor that influenced pregnancy rates after

synchronization

treatment was

_parity.

Some

other studies have

_highlighted

this

relation-ship

between

_parity

and

_fertility

after oestrus

synchronization

and obtained first-service

conception

rates

_ranging

from 43.4-53.4% for

_multiparous

cows and from as low as

31.3-36.5% for

_primiparous

cows

_(Aguer

et al, 1981;

Fogwell et al,

1986;

Grimard et

al,

_1992b).

In the

_present

_survey,

_fertility

was about 10%

_higher

_{for each group than}

previous

values in the

literature,

but the

observed difference between

_primiparous

and

_multiparous

cows demonstrated a

sim-ilar

_pattern.

Calving difficulty

was also

_strongly

asso-ciated with

_fertility

after

_{synchronization.}

Similar

_calving

effects have

_already

been

shown to influence the

_length

of

postpar-tum anoestrus and cows after a difficult

calv-ing

have a

_{longer period}

of anoestrus

(Ducrot

et

_{al, 1994b;}

_Pouilly

et

al,

1994).

Ducrot et al

_{(1994a) recently reported}

that

the risk of

_infertility

is

_especially

_high

with a

caesarean section or

_calving

_requiring

a

high

level of assistance.

_Agabriel

et al

₍₁₉₉₂₎

also described a

_longer

_calving

to

_calving

interval,

which could increase to 30

_days

after a difficult

_calving.

These

results,

how-ever, were obtained with

_{unsynchronized}

breeding

methods,

which could

_explain

the differences between these observations.

Another individual factor that

_{significantly}

influenced pregnancy rates after induced

ovulation was

_{body weight}

at

_implant

inser-tion.

_Pregnancy

rates and first-service

con-ception

rates of beef suckler cows are

affected

_{by precalving}

and

_postcalving

energy intake

(Randel, 1990).

To obtain

cows in a favourable energy status at the

time of

treatment,

previous

studies

reported

the beneficial effect of

_flushing,

which

improved

pregnancy rates

_by

10-20%

_(Petit

et

al, 1977;

_Aguer

et

al,

₁₉₈₁

_Thin

cows

showed the

_greatest

_improvement

(Kaban-dana et

al,

1993)

and we can _{suppose that}

this

_{body weight}

effect is to a

_large

extent

related to

_primiparous

cows, as

_previously

described

_by

Grimard et al

_(1995),

even if

the interaction was not

_significant

in the

model. A non

_significant

interaction term in

our model

_might

also have been due to the

relatively

small number of observations.

Body

condition score is another

com-monly

used indicator of energy status

(Agabriel

et

_al,

_{1986; Randel, 1990;}

Gri-mard et

_al,

_1992a).

This

factor,

however,

seemed to have no influence on the result of

synchronization

treatment and was not

retained for the multivariate model. On one

hand,

body

condition score is

_strongly

cor-related to

_{body weight,}

so that these

fac-tors could not all be

_significant

simultane-ously.

On the other hand this could be

_partly

explained by

the

_long

_calving

to Al interval of

most of the cows in the

_sample.

Grimard et

al

_{(1995) recently reported}

that mobilization of

_body

reserves decreases with time

post-partum

in underfed suckled beef cows, so

that cows,

despite

their lower

_body

reserves,

can recover an

_equilibrated

_{energy balance}

compatible

with normal

_reproductive

function

at

_day

70

postpartum.

Previous studies have indicated a

the variation of

_{synchronization}

results and

obtained a

_calving

rate which

_ranged

from

18-45% for

_{synchronization}

treatment

beginning

earlier than 40

_days

after

calv-ing,

but was

_greater

than 50% for treatment

starting

60

days

after

_{calving (Pelot}

et

al,

1977;

Holtz et

_{al, 1979;}

Kiser et

al, 1980;

Aguer, 1981

). In

the

present

study,

this effect

was not observed

_probably

because

_only

23% of the cows from the first

_sample

had a

calving

to Al interval shorter than 70

_days,

which

_greatly

_{reduces the power of the}

sta-tistical test.

In the second

_sample,

the

_large

varia-tion between years,

especially

for

1993,

has

been taken into account in the model. In

this

_sample,

an

_important

factor

_influencing

the

_fertility

after

_{synchronization}

was

hous-ing type.

Pregnancy

rates were

_higher

for

primiparous

cows in tethered

_stalling

than for cows in loose

_housing.

_{This result may}

be related to a

_suckling

effect,

because the

calf can suckle

_throughout

the

_day

in loose

housing, compared

to twice a

_day

in

teth-ered

_housing.

Reduced

_{suckling frequency}

or

_temporary

calf removal are

_reported

to

decrease the

_length

of

_postpartum

anoestrus

and to

_improve

the pregnancy rate _after

syn-chronization

_(Odde,

1990;

Williams,

1990).

However,

this increased OR in tethered

stalling

is not in

_agreement

with the

obser-vations of

_Pouilly

et al

₍₁₉₉₄₎

or

_Terqui

et

al

_(1982),

who noted that

_cyclicity

is

_higher

in loose

_housing.

In the

present

study,

the presence of a

bull seemed to

_{improve primiparous}

preg-nancy rates after

_{synchronization,}

as has

already

been described in

_previous

studies

(Pelot

et

al, 1977;

Cupp

et

al,

1993).

Individual factors were not

_{significantly}

associated with pregnancy results after

syn-chronization in this

_{sample. Firstly, calving}

difficulty

was not

_significant.

Ducrot et al

(1994a)

and

_Pouilly

et al

_{(1994) reported}

an effect of

_{calving difficulty}

on the

_length

of the anoestrus

_period

and Sanaa et al

(1994)

found an association between

calv-ing

difficulty

and

_fertility

at induced

ovula-tion.

However,

it should be noted that this

sample

is made _up

_only

of

_primiparous

cows

and that

_primiparous

cows often have diffi-

i-cult

_calving

_(Agabriel

et

al, 1992;

Pouilly

et

al,

1994). Therefore, only

18% of the

prim-iparous

cows

_required

no assistance at

calv-ing.

This results in a _{reduction in the power}

of the statistical test.

Secondly,

body

score at

_implant

inser-tion was not a

_significant

factor in the

vari-ation of the

_{synchronization}

results: in this

sample, only

23% of the

_primiparous

cows

had a

_body

score

_greater

than 2.5 and

con-sequently

77% had insufficient

_body

reserves

_according

to Grimard et al

(1992a, b)

thus

_{partly explaining why}

this

factor is not

_{discriminatory.}

It is

_surprising

to note the differences

between the two

_{samples. Why}

were

indi-vidual factors not

_significant

in the second

sample compared

with the first

_sample?

This result may be

partly

related to the small

number of animals in each

_category,

espe-cially

for the last two _{years. In}

addition,

these differences may also arise from the evolution

in time of herd

_management:

for

_example,

the

_primiparous

animals of the second

sam-ple

had on _average a

_{higher body weight}

(by

20

_kg)

than

_primiparous

cows in the first

sample.

Moreover,

cows in the second

sam-ple

had a

_{significantly longer calving}

to Al

interval than cows in the first

_sample,

indi-cating

an increase in this interval across

years

(table I),

probably

because the

farm-ers followed the UNCEIA’s

_(Union

nationale

des

_{cooperatives d’61evage}

et

d’ins6mina-tion

_{artificielle)}

advice for

_{synchronization}

treatment, which recommended a

_calving

to Al interval

_greater

than 60

days

and a

body-condition

score

_greater

than 2.5 at

calving.

However,

the

_analysis

has indicated

other factors

_affecting

_pregnancy rate, such

as _{the presence of} a bull or

_type

of

hous-ing.

In

conclusion,

this

_study

underlines the

clus-tered

data,

through

its

_{quantification}

and

influence on

_{synchronization}

results.

Con-sequently,

it _{is necessary} to take it into

account

_during

_analysis

with

_adapted

sta-tistical methods.

_Therefore,

we used a mixed

logistic

model,

that allowed the effect of both

individual- and herd-level factors to be

high-lighted

within the same

_study.

The

variabil-ity

in

_reproductive

success after oestrus

syn-chronization may be reduced

by taking

into

account these factors.

ACKNOWLEDGMENTS

The authors are

_grateful

to UNCEIA for data and for the hormone assays, to the Chambre

d’agri-culture de Sa6ne-et-Loire, to the

_Cooperative

d’insdmination artificielle de Verdun-sur-le-Doubs,

to Intervet for

providing

the treatments, and to the farmers and F Bendali for their

_{daily help.}

REFERENCES

Agabriel J, Giraud JM, Petit M (1986) Determination et

util-isation de la note d’6tat d’engraissement en _elevage

allaitant. Bull Tech CRVZ Theix INRA 66, 43-50

Agabriel J, Grenet N, Petit M (1992) ttat corporel et

intervalle entre _vblages chez la vache allaitante. INRA Prod Anim 5, 355-369

Aguer D (1981) Les progestagenes dans la maitrise des

cycles sexuels chez les bovins. Reci Med Vet Ec

Alfort 157, 53-60

Aguer D, Pelot J, Chupin D (1981) Reproduction du

trou-peau a viande et _{synchronisation} de I’oestrus. Bull

Group Tech Vet 33-57

Beal WE, Good GA, Peterson LA (1984) Estrus syn-chronisation and pregnancy rates in _cyclic and

non-cyclic beef cows and heifers treated with synchro-mate B or norgestomet and alfaprostol.

Theriogenology 22, 59-63

Brink JT, Kiracofe GH (1988) Effect of oestrus cycle stage at _synchro-mate B treatment on conception and time to estrus in cattle. Theriogenology 29, 513-519 9

Brown LN, Odde KG, Lefever DG, King ME, Neubauer

CJ (1988) Comparison of MGA-PGF2a to

synchro-mate B for oestrus _{synchronisation} in beef heifers.

Theriogenology 30, 1

Carruthers TD, Hafs HD (1980) Suckling and four times

daily milking, influence on ovulation estrus and

serum-luteinizing hormone, corticoids and _prolactin

in _{post partum} Holsteins. JAnim Sci50, 919-925

Chupin D, Pelot J, Petit M (1977) Induction et

synchro-nisation de [’ovulation chez les femelles de race à viande. In: Physiologie et _{pathologie de la} repro-duction, Journées ITEB-UNCEIA, ITEB, Paris, 45-49

Cupp AS, Roberson MS, Stumpf TT, Wolfe MW, Werth

LA, Kojima N, Kittok RJ, Kinder JE _{(1993) Yearling}

bulls shorten the duration of postpartum anoestrus in beef cows to the same extent as do mature bulls. J Anim Sci 71, 306-309

De Bonneval (1993) Vocabulaire des systemes agraires

systbmes de _production. Vocabulaire _français

-anglais avec index _anglais. Inra

Editions,

Paris

Drew SB, Wishart DF, Young IM _{(1979) Fertility} of norgestomet treated suckler cows. Vet Rec 104, 523-525

Ducrot C, Cimarosti I, Bugnard F, Luquet F (1994a)

Calving effects on French beef-cow fertility. Prev Vet Med 19, 129-136

Ducrot C, Gr6hn YT, Humblot P, Sulpice P, Gilbert GO

(1994b) Postpartum anoestrus in French beef cattle:

an _{epidemiological study. Theriogenology 42,}

753-764

Egret (1990) EGRET statistical software manual, 1990

version. Statistics and _Epidemiology Reseach

Cor-poration, Seattle, WA, 279-282

Fogwell RL, Bartlett BB, Reid WA (1986) Synchronized

oestrus and fertility of beef cows after _weaning calves for short intervals. J Anim Sci 63, 369-376 Grimard B, Mialot JP (1990) Avancer et regrouper les

v6lages grace a la maitrise des cycles sexuels dans

les _systemes allaitants traditionnels. Elev Insem 240,

15-30

Grimard B, Humblot P, Parez V, Mialot JP, Thibier M

(1992a) Synchronisation de I’oestrus chez la vache

charolaise : facteurs de variation de la cyclicit6

pre-traitement, du taux d’ovulation aprbs traitement et du taux de fertilite a I’oestrus induit. Elev Insem 250, 5-17 7

Grimard B, Humblot P, Thibier M (1992b)

Synchronisa-tion de l’oestrus chez la vache charolaise: effets de

la parite et de la cyclicit6 pr6traitement sur les taux

d’induction d’ovulation et de gestation. Elev Insem 247, 9-15 5

Grimard B, Humblot P, Ponter AA, Mialot JP, Sauvant D, Thibier M (1995) Influence of postpartum energy restriction on energy status, plasma LH and oestra-diol secretion and follicular development in suckled beef cows. J Reprod Fertil 104, 173-179 Holtz W, Herrmann HH, Voss HJ (1979) Estrus

syn-chronisation and superovulation with a subcutaneous gestagen implant (Norgestomet, Intervet) in suckler

cows and heifers. _{Theriogenology} 12, 197-205 Kabandana F, Grimard B, Humblot P, Thibier M ₍₁₉₉₃₎

synchroni-sation de l’oestrus chez la vache allaitante : references particuli6res aux _primipares non _cyclées.

Elev Insem 258, 1-14 4

Kiser TE, Dunlap SE, Benyshek LL, Mares SE ₍₁₉₈₀₎ The effect of calf removal on estrous response and pregnancy rate of beef cows after _synchro-Mate B treatment. Theriogenology 13, 381

McDermott JJ, Allen OA, Martin SW, Leslie KE, Meek

AH, Etherington (1994a) Reproductive performance

of Ontario beef breeding herds. Prev Vet Med 18, 115-127

McDermott JJ, Schukken YH, Shoukri MM (1994b) Study

design and analytic methods for data collected from

clusters of animals. Prev Vet Med 18, 175-191 Mialot JP, Levy I, Grimard B (1991) L’echographie dans

la gestion de la reproduction des bovins. Recl Med Vet Ec Alfotl 167, 21-31

Miksch ED, Lefever DG, Mukembo JC, Spitzer JC, Wilt-bank JN _{(1978) Synchronisation} of oestrus in beef cattle. II. Effect of an injection of _norgestomet and an

oestrogen in conjunction with a _{norgestomet implant}

in beef heifers and cows. _{Theriogenology} 10, 201-218 8

Odde KG ₍₁₉₉₀₎ A review of synchronisation of oestrus in postpartum cattle. J Anim Sci 68, 817-830 Pelot J, Chupin D, Petit M (1977). Influence de quelques

facteurs sur la fertilit6 a l’oestrus induit. In:

Physi-ologie et _pathologie de la reproduction, Journées ITEB-UNCEIA, ITEB, Paris, 49-52

Peters AR, Riley GM (1982) Milk progesterone activity

and factors _affecting postpartum ovarian _activity in beef cows. Anim Prod 34, 145-153

Petit M (1988) Alimentation des vaches allaitantes. In : Alimentation des bovins, ovins. lnra

Editions,

Paris, 159-184

Petit M, Chupin D, Pelot J _{(1977) Analyse} de I’activite ovarienne des femelles bovines. In:

Physiopatholo-gie de la reproduction, Journées ITEB-UNCEIA, ITED ed, Paris, 21-28

Pouilly F, Viel JF, Mialot JP, Sanaa M, Humblot P, Ducrot C, Grimard B (1994) Risk factors for postpartum anoestrus in Charolais beef cows in France. Prev

Vet Med 18, 305-314 4

Randel RD (1990) Nutrition and postpartum rebreeding

in cattle. J Anim Sci 68, 853-862

Rossignol H, Dechambre P (1984) E16ments d’hygiène

et de zootechnie, Vol. 2. Rueff et Cie, Paris Sanaa M, Puyalto C, Grimard B, Humblot P ₍₁₉₉₄₎

Binary correlated data analysis: a model’s review

and applications to _{veterinary epidemiology. Kenya}

Vet18,152-155

SAS, 1988. STA T user’s _guide. SAS Inst Inc, Cary, NC, USA

Short RE, Bellows RA, Staigmiller RB, Berardinelli JG, Custer EE (1990) Physiological mechanisms

con-trolling anoestrus and infertility in postpartum beef cattle. J Anim Sci 68, 799-816 6

Terqui M, Chupin D, Gauthier D, Perez N, Pelot J,

Mauleon P (1982) Influence of management and nutrition on postpartum endocrine function and

ovar-ian activity in cows. Curr Top Vet Med Anim Sci 20,

384-408

Walters DL, Burrel WC, Wiltbank JN (1984) Influence of exogenous steroids, nutrition and calf removal on

reproductive performance of anestrus beef cows.

Theriogenology 21, 395-406

Williams GL _{(1990) Suckling} as a _regulator of

postpar-tum _rebreeding in cattle: a review. J Anim Sci 68,

831-852

Zalesky DD, Day ML, Garcia-Winder M, lmakawa K,

Kit-tok RJ, D’Occhio MJ, Kinder JE ₍₁₉₈₄₎ Influence of exposure to bull on _resumption of estrous _cycles

fol-lowing parturition in beef cows. J Anim Sci 59,