A field study on estrus detection in lactating beef cattle

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A field study on estrus detection in lactating beef cattle

Christian Ducrot, Yrjö T. Gröhn, Françoise Bugnard, Yves Senlis, Philippe

Sulpice, Robert O. Gilbert

To cite this version:

Original

article

A

field

_study

on

estrus

detection in

lactating

beef cattle

Christian

Ducrot

_Yrjö

T. Gröhn

_{Françoise Bugnard’}

Yves Senlis

c

_{Philippe Sulpice}

_e

Robert

O.

Gilbert

f

&dquo;1 Centre

_{d’écopathologie}

animale, 26, rue de la Baïsse, 69100 Villeurbanne, France

n Section of

_{Epidemiology,}

_Department of Clinical Sciences, Cornell

_University,

Ithaca, NY 14853-6401, USA

’ Service de

_{pharmacologie clinique,}

162, avenue

_Lacassagne,

BP 3041, 1,

69394 Lyon

cedex 03, France

d

U.C.E.A.R., centre d’insémination Bel Air, BP 28, 69340 Francheville, France &dquo; Chambre

_{régionale d’agriculture Rhône-Alpes,}

5, rue _{Hennann-Frenkel,}

69364 Lyon

cedex 07,France

f _{Section of}

Theriogenology, Department

of Clinical Sciences, Cornell

_University,

Ithaca, NY 14853-6401, USA

(Received 25

_February

_1998;

_accepted

8 October ₁₉₉₈₎

Abstract - Estrus detection

_efficacy

and heat detection

_protocol

were studied

_by

means of a field

_study

carried out on 878

_lactating

beef cows in 60 French herds.

_Average

herd size was 48, and 75 °h! of the farmers

_partly

or

_exclusively

used artificial insemination. The cows were calved between October 1992 and March 1993. Estrus was recorded

_{daily by}

the farmers.

_Cycling

status was determined

_by

pro-gesterone

radioimmuno-assay

2 months after

_calving.

The

_relationship

between the estrus detection

protocol

and the

_{delay period}

from

_calving

to first observed estrus was

_{analysed using}

survival curves and the Cox

_proportional

hazard model,

adjusting

for confounders.

_Seventy-one

_percent of the cows were seen in estrus by the farmers; the interval between

_calving

and the first observed estrus

ranged

from 9

_days

to more than 5 months and the median was 56

_days.

Two months after

_calving,

44 % of the

_cycling

cows had not been seen in heat

_by

the farmers and 11 % of the

_non-cycling

cows had been

_reported

to have been in estrus. The heat detection

_protocol

varied

_widely

between

farm-ers,

depending

on the considered estrus

_signs,

schedule and time _spent

_looking

for

_signs.

Two factors were

_{significantly}

related to a shorter interval from

_calving

to first observed heat: the use of artificial insemination (which relates to the farmer’s interest in heat detection) and an overall

_daily

time _spent for heat detection _greater than I h. Cows in tie stalls had a _delayed interval to the first observed

estrus. These results show _{that many farmers did} not

_adapt

their

_{reproduction practice sufficiently}

to

an earlier

_{calving period.}

There is room for

_{improvement since in many}

cases the heat detection

protocol

does not match the

_required

standards for

_optimal

heat detection. © Inra/Eisevier, Paris. beef cattle / estrus /

_reproduction

/

_epidemiology

/ risk factor

*

Correspondence

and

_reprints:

Laborutoire

_{d’6cop!ithologie,}

Centre de recherche de

Clermont-Ferrand-Theix, 63122

_{Saint-Gencs-Champancllc,}

France

Résumé -

Étude

de terrain sur la détection de l’oestrus en

_élevage

bovin allaitant. Les méthodes de détection de l’oestrus et leur efficacité ont été étudiées en conditions

_{d’élevage,}

à

_partir

d’une étude de terrain conduite dans 60 _troupeaux

_français,

sur 878 vaches allaitantes _ayant vêlé entre

octobre 1992 et mars 1993. Les troupeaux

comptaient

48 vaches en _{moyenne ; 75 %} des éleveurs uti-lisaient l’insémination artificielle

_{partiellement}

ou en totalité. Les oestrus étaient notés

quotidienne-ment _{par les éleveurs} et l’existence de

_cycles

sexuels a été

_analysée

_par

_dosage

de

_{progestérone}

deux mois

_{après vêlage.}

L’effet du

_protocole

d’observation de l’oestrus sur le délai

_{vêlage - premier}

oestrus a été étudié à l’aide de courbes de survie et du modèle de Cox à

_{risques proportionnels,}

avec

ajustement

sur les facteurs confondants. Soixante et onze _pour cent des vaches ont été vues en oestrus

par l’éleveur ; l’intervalle

_{vêlage - premier}

oestrus a varié de

_{9 j}

à 5 mois, avec une médiane de 56

_j.

Deux mois

_{après vêlage,}

44 % des vaches

_cyclées

n’avaient pas été vues en oestrus et 11 % des vaches non

_cyclées

avaient été déclarées vues en oestrus. Le

_protocole

d’observation des oestrus variait considérablement entre éleveurs

_(signes

_{considérés,} horaires et _temps

_passé).

Deux facteurs

étaient

_{statistiquement}

liés à une diminution de l’intervalle

_{vêlage - premier}

oestrus détecté :

_l’usage

de l’insémination artificielle

_(qui

traduit l’intérêt de l’éleveur

_{pour la}

détection de l’oestrus) et une durée

quotidienne

d’observation de l’oestrus au moins

_égale

à une heure. Les vaches en stabulation

entra-vées ont été vues en chaleurs en _moyenne

_plus

tard que les autres. Du

_point

de vue du

_{développement}

agricole,

les résultats montrent que de nombreux éleveurs n’ont pas

adapté

suffisamment leurs

pra-tiques

de

_reproduction

à une

_période

de

_{vêlage plus précoce. Il y}

a matière à amélioration, dans la mesure où le

_protocole

d’observation des aestrus ne

_remplit

_{pas les conditions optimales} dans de nombreux cas. © Inra/Elsevier, Paris.

bovin allaitant / oestrus /

_reproduction

/

_{épidémiologie/}

facteur de

_risque

1. INTRODUCTION

Date of birth is one of the

_major

compo-nents of the market value of beef calves in

France

[10]. Therefore,

calvings

are

_planned

earlier and earlier in the winter and even in the

_fall,

_compared

to the traditional

_calving

period

which is in the

_spring.

The

conse-quence is an

_{increasing problem}

of

post-partum anestrus

_during

the

_{housing period}

[21],

which affects

_fertility

_[27]

and results

in an extended

_calving

interval

_[3].

True anestrus,

expression

of estrus

_by

cows and heat detection

_efficacy

_by

farmers

all influence _post-partum observation of

estrus

[ I

Among

them,

a _poor estrus

detec-tion is said to be one of the main causes of

long calving

intervals

_!11],

and its effect on heat detection is even

_higher

when

consid-ering

post-partum

depression

of estrous

behavior. Various environmental and social

factors

_[1]

can

_partially

inhibit estrous

behavior and these

_negative

effects have to

be counterbalanced

_by

a _very accurate and successful heat detection

_protocol.

Estrus detection in beef cows was not a

_major

con-cern when

_calvings

occurred

_during

the

spring

because

_breeding

was

_mostly

with the bull

_during

the

_pasture

_period.

However,

the

_quality

of heat detection becomes a

_key

factor in a winter

_calving

_option;

in that

case,

breeding

must take

_{place during}

the

housing period

and the farmer

_plays

an

important

role. Cattle are known to have

variable

_post-partum

anestrous

_periods

[3],

and suckled beef cows are said to have a

longer

anestrous

_period

than

_milking

cows

[8]; however,

in beef

cattle,

few studies

_[7]

have described the distribution of the inter-val from _post-partum to heat detection. Given the

_reproductive

_physiology

and the

estrus behavior of the cow, different authors

have

_{proposed guidelines}

for heat detection

protocol

[1, 11].

Some studies have been carried out to assess the

_sensitivity

and

specificity

of estrous detection

_by

farmers

[18],

mainly

in

_dairy

cattle. In beef

_cattle,

neither the current

_practices

of farmers nor

their effect on heat detection have been

reported.

anestrus under field conditions. The

_study

focused on the winter

_period

when the cows are

_housed,

because

_post-partum

anestrus

is an acute

_{problem during}

that

_period,

before cows are turned out to _pasture

_{[23 1.}

The first _part of the

_study

which concerned the risk factors for anestrus

_(determined

_by

the

_progesterone

_assay)

was

_published

pre-viously

[6].

Estrus detection

_by

the

farmer,

based on his current and usual

_practices,

was the

subject

of interest for the second

part of the

_study.

The

_goals

were two-fold:

first,

to describe the initial time of estrus

detection after

_calving,

and the

_relationship

between heat detection and true

_cycling

sta-tus 2 months after

_calving

and

second,

to

examine

_specific

estrus detection

_strategies

practised

by

farmers,

and their

_relationship

to the initial estrus detection after

_calving.

2. MATERIALS AND METHODS

2.1.

Study

design

A

_longitudinal

and

_{prospective study}

[ 17] was carried out under field conditions to

_analyze

both

anestrus and heat detection. The

_study

involved a

_{pluriprofessional workgroup composed}

of vet-erinarians, inseminators, technicians and scien-tists.

_They

defined the

_hypotheses,

assembled the

_protocol

and the

_{questionnaires}

and tested them for

_{comprehension}

_{and accuracy, and} dis-cussed the results. The data collection on the farms was

_{performed by}

44

_participants

_(same

professions

as the

_workgroup)

that were

_given

a

_{1-day training}

session about the

_{study goals,}

the

_protocol

and

_{questionnaires,}

and the

_body

condition

_scoring.

The

_study

was carried out from the fall of 1992 to the end of the

_spring

of 1993. The

sub-jects

of interest were the cows that calved in fall and winter. Cows that were housed at

_calving

and were turned out to _pasture within 2 months of

calving

were excluded from the

_study.

Thus, two

kinds of cows were

_represented;

those that were housed for a 2-month

_period

after

_calving

and those that had access to _pasture

_throughout

the

study.

2.2.

_{Study population}

A

_sample

of 60 farms was selected _[25] from the farms

_{proposed by}

_{44 surveyors}

(veterinari-ans, inseminators, advisors and scientists) who

participated

in the

_project.

All farmers were

vol-unteers and, in order to be included, each farm had to have at least ten

_calvings

between October and

_February

and facilities to confine the cows for

_sampling.

Farms

_using

artificial

insemina-tion (AI) (75 %) as well as farms

_using

bulls for

breeding

were included in the

_study.

Even farm-ers

_using

a bull had to observe the heat when the cows were _housed,

_especially

in tie-housed farms. This was the case for the winter, the

_period

of interest for this

_study.

These farms were located in the

_{Rhone-Alps region}

in France, in a

tradi-tional area for beef-cattle

_production.

On each _farm, all cows

_calving

between Octo-ber and March (up to a _{maximum of 30 per farm)} were _{followed up if they} were still housed 2 months after

_calving.

2.3. Data collection

For each cow, the farmer noted breed,

parity,

date

_{of calving, calving}

circumstances

(diffi-culty, complications), suckling

type,

housing

characteristics and

_feeding

method. In addition,

he

_reported

the

_daily

observed heats on a calen-dar

_{posted in the barn. The farmer also gave}

descriptive

information about his farm, his beef-cattle

_enterprise

and the

_feeding

and

_breeding

methods.

_Among

others, he

_completed

a ques-tionnaire about his

_practices

in heat detection.

This was

_performed

at the

_beginning

of the

calv-ing period in the presence of the surveyor. The

usual habits were

_{explored. They}

concerned the persons involved, the

_daily

observation

sched-ule, and those

_{important signs}

considered for heat

_{diagnosis (excitability,}

vulva

characteris-tics, immobilization response [I], mounts

oth-ers, etc.). Each farm was followed up

by

a sur-veyor who visited the farm once a month to check the

_quality

of the farmer’s records and,

during

the

_{requisite period,}

also took blood

_samples

for

progesterone measurement.

2.4.

_Samples

and assays

The anestrus determination was based on

The method used for these assays from milk [28]

or _{serum [ 141}

_samples

has been detailed in a pre-vious paper [6!. The first _progesterone

measure-ments were

_performed

60 ± 5

_days

after

calv-ing.

If thc result was not

_positive

_{(progesterone}

concentration < 2

_ng/mL),

a second

measure-ment was

_performed

9 ±

_{2 days}

later. The cow was considered to have

_cycled

if at least one of the two measurements was

_positive,

and was oth-erwise considered to be anestrous.

2.5.

_Analysis

Estrus detection after

_calving

was described at

the animal level

_by

the time interval between

calving

and first heat observed

_by

the farmer. Cows that were not seen in heat

_by

the farmer

(due either to anestrus, silent hcat or _poor

obser-vation) were classified as ’no heat observed’.

Data about the first observed heat was then

com-pared

to rcferenced _progesterone measurements

performed

2 months after

_calving.

The percent-age of cows that were not seen

_previously

in heat was determined, based on _{progesteronc assays.} among

cycling

cows. The _percentage of those

cows that the farmer

_reported

to have seen in heat was determined, based on _progesterone assays, among anestrous cows.

The

_protocol

for estrus detection used

_by

the farmer was described at the farm level. An

anal-ysis

was then

_performed

at the cow _level, to

_study

the

_{relationships}

between factors relative to the heat detection

_protocol

and the time interval from

calving

to the first observed heat. Main factors were the number of heat observation

_periods

_per

day, time of the first

_daily

observation, time of the last

_daily

observation, first-to-last

_daily

obser-vation

_{delay, length}

of each observation

_period,

overall

_daily

time _spent for heat detection, num-ber

_{of people}

involved in heat detection, breed.

parity, body

condition at

_calving

and

_change

in

body

condition within 2 months

_{of calving.}

Those cows that were never seen in heat after

_calving

were declared censored 6 months after

_calving,

when heat detection was no

_longer

an

_important

issue.

A _two-step

_analysis

was

_{pe!fionned, using}

the

hazard ratio _[2! as the

_relationship

estimator. The

hazard ratio is

_{interpreted similarly}

to a risk ratio

being

observed for heat from 0 to _{30 min per}

_day

have a 1.3 times

_{higher probability}

not to have been seen in estrus

_compared

to cows that have been observed for more than 60 min _every

_day

(reference group). If

the confidence interval of the hazard ratio of a _category includes 1, the hazard for this category is not

_{significantly}

different from those of the reference _category. A univari-ate

_analysis

was first carried out in order to screen the

_{existing relationships.}

The variable of

inter-est was a time interval

_(days)

and was checked

using

survival curves _[25]; for each category studied, these curves described the _percentage of cows not _yet seen in estrus,

against

the time since

_{calving. Log-rank}

statistics [25] were used to test the difference in the curves and were per-formed with SAS software _(LIFEREG proce-dure [26]).

The

_significant

factors (P < _0.20) linked to

the

_{calving-to-first-observed-estrus}

interval were then

_incorporated

in a multivariable

_analysis

per-formed with the Cox

_proportional

hazards model

[2] (SAS software, PHREG

_procedure

[26]). Sev-eral

_potential

confounders were introduced in the model because

_they

can interfere with the effect of heat detection

_{procedure: cycling}

status

(based on _progesterone

_assays)

determined 2 months after

_calving,

in order to

_adjust

for a real anestrus

_problem;

the

_housing

type and the most

important signs

considered

by

the farmer to

diag-nose estrus. The

_assumption

of

_equality

of the hazard ratio over time was checked

_graphically

[12] with a

_Log[-Log

_{(survival function)]} versus

Log

(time)

plot.

The curves for the different cow

categories

were

_roughly

_parallel

and the hazard ratio was assumed to be

_{equal. Only}

confounders and factors linked to the

calving-to-first-observed-estrus interval that were

_significant

_(P < 0.05) were considered in the final result.

3. RESULTS

3.1.

_Description

of the farms and cows The _average herd size was

_48,

and 12 %

of the herds had more than 70 cows.

_Only

33 % of the farms were

_specialized

in beef

production;

the others had

_{complementary}

operations

such as

_dairy

or

_poultry

enter-prises. Among

the 878 cows

_followed,

304

(34.6 %)

were

_primiparous;

the others

ranged

from

_parity

2 to 12.

_Ninety-one

per-cent were of the Charolais

breed,

5 % of the Limousine

breed,

and the others of mixed

breeds.

_Eighty-nine

_percent

of the cows calved before

_February.

Different

_housing

types

were

_represented:

26 % of the cows were

_kept

in tie

_housing,

40 % in loose

hous-ing

and 34 % had free access to

_pasture.

3.2. Estrus detection after

_calving

The distribution of the

_calving

interval

to the first observed estrus is shown

_in

relates to the observation made

_by

the

farmer;

it does not represent the real anestrus rate. The interval from

_calving

to the first observed heat

_ranged

from 9

_days

to more

than 5 months.

_Among

the cows that were

seen in estrus

_by

the

farmer,

half had their first observed heat less than 56

_days

after

calving

and the _average date was 61

_days

post-calving.

Among

the 593 cows that

_cycled

within 2 months of

_calving,

based on

_progesterone

assays, 44 % were not seen in heat

_by

the

farmer within 2 months

_{post-calving.}

Fur-thermore,

among the 285 anestrous cows,

I I % were

_judged

_by

the farmer to be in

estrus within 2 months of

_calving.

3.3. Heat detection

_protocol

Among

the 60

_surveyed

farms,

25 % used

mating

with a bull

_{exclusively (mainly}

pas-ture-bred),

23 % used artificial

insemina-tion

_exclusively

and 52 % used both. The

estrus detection

_protocol

varied

consider-ably

from farm to farm. The two main

_signs

of estrus considered

_by

the farmer were related to _{the immobilization response}

₍₅₁

%

of the

_farmers),

to

_physical

characteristics of the vulva

_{only (congestion,}

clear or

blood-tinged

mucous

_discharge)

₍₂₂

% of the

farm-ers),

to

_excitability

_{only (irritability,}

bel-lowing)

(3

% of the

_farmers),

or to

_physical

characteristics of the vulva and

_excitability

( 12

% of the

_farmers).

The

_remaining

12 %

of the farmers either considered those cows that mounted others to be in

_heat,

or

_they

did not _pay much attention to heat

detec-tion. The number of _persons from the farm involved in heat detection

_ranged

from zero

to five _{and the average} was 1.4.

The heat detection schedule also differed

greatly

from farm to farm. The number of

daily periods

devoted to heat detection

ranged

from zero to

_eight;

22 % of the farms

passed

less than two times a

_day

and the median was between 2 and 3. The hour of

the first

_daily

_{period ranged}

from 6 a.m. to

2 p.m.; the hour of the last

_period

_ranged

from 8 a.m. to _{10 p.m.}

_(which

also includes those farmers who had

_only

one

_period

of

observation).

The interval between the first

and last

_daily

observation

_{periods ranged}

from zero to 14

h,

and the median was 8 h.

The overall time

_spent

each

_day

in heat

detection is shown

_{in figure}

2. It

_ranged

from zero to more than 3 h and was on

aver-age 40 min.

Nearly

half of the farmers

(43 %)

spent less than _{30 min per}

_day

check-ing

the cows.

3.4. Estrus detection

_efficacy

detection

_figure

_3).

_Only

half as _many cows were seen in

heat,

and at later

_{periods, by}

the farmers who did not use artificial insem-ination

_compared

to farmers

_using

artificial insemination

_{systematically}

or from time to

time. This

_relationship

remained

significant

when

_adjusted

for

_confounding

factors.

Among

factors related to estrus

detec-tion _strategy such as the number of

obser-vation

_periods

or the number of _persons

involved,

the overall

_daily

time

_spent

for

heat detection was the

_only

factor

signifi-cantly

linked to the

calving-to-first-observed-heat interval

_(figure

_4).

The

rela-tionship

remained

_significant

when

_adjusted

for real anestrus status

_(determined

_by

pro-gesterone assays 2 months after

calving),

housing

type and main

_signs

of estrus

con-sidered

_by

the farmer

_{(table 0.}

The

_longer

the time _spent in heat

detection,

the smaller the interval from

_calving

to first observed

4. DISCUSSION

4.1. Farm characteristics

Farms followed _up were not chosen at

random,

the

_agreement

and the

participa-tion of the farmer

_being

_necessary to col-lect the data. Farms varied with _respect to

the level of

_{specialization}

in beef

cattle,

herd

size and

_{technological}

standards.

However,

they

were, on _average,

_larger

than most beef farms in

France;

53 % of the

_surveyed

farms

had more than 40 beef cows,

compared

to

10 % of all French beef farms in 1990

_[9].

_1.

Farmers were also _younger;

_only

1 _5 % were more than 50 _{years old in the}

_surveyed

farms

compared

to 56 % in all French beef farms

in 1990

_{[9] and, hence,}

_they

could be assumed to be

_technically

more trained than

the average.

Therefore,

caution is needed

in

_{extrapolating}

these data to all farms.

4.2. First estrus observation after

_calving

Estrus detection was

_performed

_by

the farmer under

_{regular farming}

conditions.

The 60 farmers used their habitual heat

detection

_method,

in terms of

_signs

observed and detection schedule. We observed a wide range of more or less

complicated

methods

used

_by

the

farmers,

ranging

from those who

fulfilled a

_heavy

and

_demanding

schedule,

to

those who did not _{pay much} attention to

heat detection but noted the heat that

_they

happened

to observe when

_they

fed or

cleaned the cows.

Given these

facts,

the interval from

calv-ing

to first observed heat

_ranged

from 9

_days

to 6 months

_(recordings

were _up to

6

_months),

and

_averaged

2 months. This

range is in

agreement

with Garverick and Smith’s _report in

_dairy

cattle

_[8],

even

though

estrus is

_usually

said to be

_delayed

in

beef

cattle,

compared

to

_dairy

cattle

[8].

Twenty-nine

percent

of the cows were not

seen in heat after

_calving.

However, the

cullings

which occurred after 2 months

post-calving

were not recorded. There is no doubt that some cows were culled

during

this

period,

whereas

_they

could have been in heat if not removed from the

herd;

there-fore,

some cows should have been

consid-ered in heat

_during

the

_study.

Thus,

the

per-centage of cows not seen in heat _{may be} overestimated.

_By

first

_considering

an annual

_culling

rate of 18

%,

previously

reported

in the same kind of herds

_[5],

and

second a uniform distribution of

_culling

over the _year, which is

_exaggerated

because

culling usually

occurs

_just

after

_calving

or after

_weaning,

we would obtain an estimated

culling

rate of 6 %

(one

third of 18

_%)

between 2 and 6 months

_post-calving

(one

third of the

_year).

In that case,

only

24 %

instead of 29 % of the non-culled cows

would not have been seen in heat in the

study.

This, however,

does not

_strongly

modify

the result.

Reproductive efficiency requires

that

cows exhibit estrus

_cycles

_shortly

after

calv-ing.

For

_reproduction

_management

effi-ciency,

it is recommended that cows should be seen in heat within 60

_days

after

_calving

[8].

Only

40 % of the cows in our

_study

ful-filled this condition and the

_percentage

was

still poor

(56 %)

according

to

_Cupps’

rec-ommendation within 80

_days

[3].

These _poor

results, however,

should be relativized since the

_study

was focused on the fall and winter

periods,

when

_calvings

are

_usually

followed

by

a

_longer

anestrous

_period

than

_calvings

occurring

in the

_spring.

4.3. Estrus observation versus ovarian

_cyclicity

The

_comparison

between estrus detec-tion and ovarian

_cyclicity,

determined

_by

progesterone

assays, showed that 44 % of the

_cycling

cows were not seen in heat

_by

the farmer

_by

2 months _post-partum. Silent

ovulation and

_quality

of heat detection must

ovulation is not associated with estrous

behavior

1

J, and

can reach 50 % for the

first _post-partum ovulation in

_dairy

_{[ 15]}

and

suckling

cow

_[19].

This

_phenomenon

_may

explain

a

_large

_part of the ’missed estrus’

because the time when the

_progesterone

assay was

_{performed corresponds}

to the

peak

(median

and

_mode)

of estrus detection

(figure

1); however,

given

the

_{study design,}

there is no

_precise

_way to evaluate this effect.

_{Many inhibitory}

influences are

reported

to determine silent ovulation

_{I I [.}

most of which are environmental and social factors. The second factor

_explaining

that

nearly

half of the

_cycling

cows were not

seen in heat is the

_sensitivity

of heat

detec-tion

_by

the farmer. This

_aspect

is discussed

in section 4.4.

It also

_appeared

that 11 % of the

non-cycling

cows, based on _{progesterone assays}

performed

2 months after

_calving,

were

reported

to be in estrus

_by

the farmer. This

indicates lack of

_specificity

of the heat

diag-nosis which was

_reported

to be

_equal

to

87 °Io in

_dairy

_{cattle 13].}

_Specificity

prob-lems are

_higher

if farmers consider

behav-ioral

_signs

of estrus other than immobiliza-tion _responses

_{[ I}

_such

as

_mounting

_activity

or

_{irritability.}

This is

_particularly

the case

in tie

_housing

_[21].

Another

_possibility

is that some cows

_might

have

_cycled

for a

while,

_expressed

estrus, and then moved to

a second anestrous

_period;

this situation has been

_reported

for

_dairy

cows

_{by Opsomer}

et al.

[20]

but may be rare.

Another

_explanation

for the difference

between the _{progesterone assay} and the

observation of heat may be the

quality

of the _{progesterone assay,}

_especially

the

sen-sitivity

of the milk _assay

_compared

with the blood _{assay. The} cows with the milk _assay

were at five times _greater risk of real

anestrus than were cows with the blood

assay

[6]. However,

these assays were

com-parable

in a

_preliminary

_study

_!24].

The

main reason _{for the difference may be} that some farmers with

_heavy

anestrus

_problems

preferred

the milk

_sampling

which was per-formed for all their cows. This _may have

built an artificial

_relationship

between the

type of

_{sampling and}

the anestrus risk.

4.4. Heat detection

_protocol

The heat detection

_protocol

used

_by

the

farmer differed

_greatly

from farm to farm. For

_example,

a _quarter of the farmers had

four or more estrus detection

_periods

a

_day

and _spent more than _{45 min per}

_day;

on the other hand, some farmers did not schedule

any

specific

time for heat detection;

they

just

looked at the cows

_{during cleaning}

or

feeding periods.

Allrich

_[1]

_presents the

usual recommendations for estrus detection:

&dquo;to

_perform

estrus observations three times per

day

for 20 to 30 min at each

observa-tion

_{period&dquo;.}

_Considering

these

recommen-dations,

it

_appeared

that

_only

50 °!o of the

surveyed

farmers visited their cows a

suffi-cient number of times each

_day,

and

_only

17 °l° of them

_spent

_enough

time

_observing.

This can

_{explain partly why nearly}

half of the

_cycling

cows were not seen in heat

_by

the farmers.

The

_{breeding policy appeared}

to be of

importance

in

_explaining

the heat detection

efficacy.

Farmers who did not use artificial

insemination did not _pay a lot of attention to

heat

detection,

and hence did not detect heat

successfully.

An easier _way for them to

_get

rid of the task of heat detection was to use a

bull, but this method cannot be

_applied

eas-ily

when the

_breeding

_period

is

_going

on

during

the

_{housing period, especially}

in tie

housing.

This observation,

_though

obvious,

is nevertheless

_important

to take into con-sideration for

_continuing

education and

extension _{purposes. It} means that

_planning

the

_calvings

earlier in winter does force

tra-ditional farmers to revolutionize their

opin-ions and

_improve

their

_practices

in heat

detection.

_Although

the

_{breeding policy}

was linked to the heat detection

_efficacy,

it was

not accounted for in the multivariate model.

First, the

_breeding

_policy

is

_highly

related to

the heat detection method:

_taking

into

multicollinearity

problems.

Second,

the

breeding

policy

induces a more or less

strong

need for

_good

heat

_detection,

but

choosing

between AI or

_bull-mating

does

not

_represent

a technical answer to better heat detection.

However,

if one includes the

breeding

system in the

model,

the factors relative to the heat detection method do not

remain

_{significant. Apart}

from the statistical

problems exposed

above,

this may reflect that

_practices

differ between farms

practis-ing

AI versus farms not

_practising

AI,

the factors of interest in our

_study

_being

_only

part of them.

The interval from

_calving

to the first observed heat is influenced

_by

three main factors:

_cycling

status of the cow

_(related

to true

_anestrus),

estrus behavior

_(related

to

silent

_ovulation)

and heat detection

_quality

[1].

In order to

_study

the last

factor,

we

adjusted

for the

_cycling

status of the cow in

the

_analysis,

based on _{progesterone assays}

2 months after

_calving.

The second one

(silent heats)

could not be taken into account, and it is therefore

_possible

that this decreased the

_ability

of the

_study

to

iden-tify

the factors that influenced the

_quality

of heat detection. Factors such as

_parity,

calving

assistance,

suckling

and nutrition were not taken into account because their role in heat detection is

_thought

to be due

to their effect on true anestrus. This was

already

considered in the

_analysis

_(a

model

including

these variables instead of the

estrus status gave

exactly

the same

_results).

Furthermore,

the

_analysis

was also

_adjusted

for

_housing

_type,

which can affect the ease of heat detection

[11],

and for the

_signs

of

estrus considered

_by

the

farmer,

in order to

keep

their effect on heat detection constant.

Herd was not accounted for in the model.

Taking

it as a random effect in the Cox

pro-portional

model is not

_possible

with usual statistical

software,

and

_considering

it as a fixed effect

_requires

59

_dummy

variables that should have decreased the power of the

analysis

strongly.

No interaction was

ana-lyzed

because we did not

_identify

_any

plau-sible

_hypotheses

in the studied data.

_Among

factors related to heat detection

_protocol,

only

the overall

_daily

time _spent in heat detection

_by

the farmer was

_{significantly}

linked to the date of first observed heat after

calving.

The best heat detection occurred

when farmers

_spent

at least 1 h each

_day

observing

the cows. This fits in well with Allrich’ recommendations

_{(three times,}

20-30 min each

_{time) [1J.}

We did not find a

_relationship

with the

number of observations

_performed

each

_day,

even if the short duration of cattle estrus

suggests that three observations or more are

needed each

_day

to avoid

_missing

estrus

_[1].

_{] .}

McDougall

and

_Hampson

_[18]

_reported

the same lack of

_relationship

with the number of observations. No effect of the number of

persons involved in the heat

detection,

nor of

the hours of first and last

_daily

observations was found.

_Many

environmental and social factors influence estrus

_expression,

which

ranges from

long

and

well-expressed

estrus to silent ovulation

_[I];

these factors differ from farm to farm. There is hence an inter-action between estrus

_expression

and heat detection

_protocol

which

_greatly

modifies

their effects on heat detection. No definite recommendation can be made for heat

detec-tion schedule and method. As Allrich

_reports

[ 1 ]:

&dquo;so many factors

_change

from one

live-stock

_enterprise

to the next that cases have

to be looked at

_{individually.}

This is an

important

concept

to remember because

techniques

that work well for one _person

will fail for his or her

_neighbor

down the road&dquo;. Hence it is not

_surprising

to find

con-tradictory

results

_[4,

_18J

that

_depend

on the

study

design

and on the number of farms

and observers involved.

4.5. Method

From a

_{methodological}

_point

of

_view,

two _aspects must be discussed. The first is

that some cows

_might

be lost to

_follow-up

previ-ously,

less than 6 % could have been in this

situation. It was

_possible

to

_study

the

poten-tial bias that these

_{lost-to-follow-up}

cows could induce in the

_{analysis by}

_considering

what the survival curves should look like if

all the culled cows

_belonged

to one

partic-ular

_category

of the studied variables. In

fact,

it

_appeared

that it would not have

sig-nificantly

changed

the results. A similar case is that of the undetected bull-bred pregnan-cies

_concerning

those cows that were not

seen in heat

_by

the farmer. These data do

not reflect the actual numbers of bull-bred cows at risk of first observed heat. This could

_explain

_why

the curve remains

hori-zontal after 3 months

_post-calving

in ’no AI’ farms

_(figure

_3).

The second

_aspect

of the

_methodology

is the choice of the unit of interest in the

analysis.

One may suggest that the heat

detection

_protocol

is related to farm

man-agement

and hence to farm

_level;

it

would,

perhaps,

have been

_preferable

to

_study

that

question

at the farm level instead of the cow

level.

_However,

there is no

_good

farm-level

outcome that

_properly

reflects the interval from

_calving

to first observed heat of the different cows in the herd. One _way to

per-form such an

_analysis

is to consider _the

per-centage of cows that were seen in heat in

the herd at a

_given

date after

_calving.

_But,

this leads to an

_important

loss of information about the actual individual intervals.

Nev-ertheless,

a

_{complementary}

farm-level

_study

was

_performed

and showed the same

rela-tionship

between heat detection schedule and heat detection.

However,

this

relation-ship

was not

_significant,

because the

_sample

size of 60 farms did not allow sufficient

sta-tistical power for the

_analysis.

This field

_{study clearly}

illustrates that

part of the farmers did not

_adapt

their

repro-duction _management

_practices

_adequately

to an earlier

_period

of

_calving

that

_implies

a winter

_period

of

_breeding,

_during

the

hous-ing period.

The _consequence is that 44 %

of

_cycling

cows were not seen in heat

_by

2 months

_post-partum.

_{In many} cases, the

heat detection

_protocol

does not match the

required

standards for

_optimal

heat

detec-tion. There is room for

_improvement,

based on heat detection

_efficacy

and

_according

to estrous behavior in the farm. Some of the

farmers have better

results,

mostly

those who were forced to

_improve

heat detection

efficacy

in order to use AI.

_Among

factors related to heat detection

_protocol,

the

_daily

time _spent for heat detection appears to be

important.

These results need to be consid-ered when extension is

_planned.

ACKNOWLEDGMENTS

The authors are

_grateful

to the veterinarians,

inseminators, technicians, teachers and

_colleagues

who contributed to the

_{study design}

and the data

collection, and to the farmers for their valuable

participation

and active collaboration.

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