Population analysis of a purebred Hereford and a multibreed synthetic beef cattle herd

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Population analysis of a purebred Hereford and a

multibreed synthetic beef cattle herd

Pf Arthur, M Makarechian, R Weingardt, Rt Berg

To cite this version:

Original

article

Population analysis

of

a

purebred

Hereford and

a

multibreed

synthetic

beef

cattle herd

PF

Arthur

M

Makarechian

2

R

_Weingardt

RT

_Berg

1

NSW

_{Agriculture, Agricultural}

Research

_{Centre, Rangie,}

NSW

_{2823, Australia;}

2

Department of

Animal

_{Science, University of Alberta,}

Edmonton, AB,

T6G

_2P5,

_Canada;

(Received

25

_{May 1994; accepted}

17 November

₁₉₉₄₎

Summary - Lifetime records of females born from 1966 to 1975 were used to estimate

and compare

population

parameters of a

_purebred

Hereford

(HE)

and a multibreed

synthetic

(SYl)

beef cattle herd raised under a

stringent culling policy whereby

heifers

and cows

failing

to wean a calf each _year were culled.

Population

size

averaged

118 _cows,

39 heifers and 155 cows, 56 heifers a _year for HE and

SY1, respectively.

The SY1 was a

multibreed

composite

breed group with an _average breed

composition

of 33%

Charolais,

33%

_Angus,

and 20%

Galloway,

and the remainder from other beef breeds. The 2 herds

were raised under the same _management. Nine life table statistics were studied:

age-specific survivorship, age-age-specific

survival _rate,

_mortality

rates

_(Qx),

_expected

herd

_life,

age-specific

birth _rate,

_{reproductive value,}

net

_reproductive

rate

_(Ro);

instantaneous rate

of

population

increase

_(r)

and

_generation

interval

_(T).

Differences were obtained between

the herds for the

_age-specific

life table

_statistics,

with SY1

_{having higher}

values

_(except

for lower

_Qx

_values)

than HE. SY1 had

higher

means than HE for Ro

_{(1.57 ±0.11}

versus

1.21 t _{0.15; p} <

_0.07)

and r

(0.09 f

0.01 versus 0.03 f

0.02;

_p <

_0.04),

_indicating

a faster

rate of

_{population growth}

in SY1. The value of T for SY1 was

higher

(p

<

_0.01)

than that for HE

_(5.09

f 0.11 versus 4.25 f 0.19

_years).

The results indicate that the same

management and

culling policy

may result in different life table

statistics,

which in this

study

was

_possibly

due to the influence of heterosis for calf survival in the multibreed

composite

SY1 herd. Over time the

_{stringent culling policy}

had the effect of

_reducing

_Ro,

r and T values to the

point

where herd size in the HE herd could not be maintained

(Ro

< 1 in the 1972 and later

_cohorts).

beef cattle

_/

_demography

_/

_longevity

_/

survival

_/

reproduction

Résumé - _{Analyse démographique} d’un troupeau de race pure Hereford et d’un

troupeau synthétique multiracial de bovins à viande. Les

_{enregistrements}

des carrières des

_femelles

nées de 1966 à 1975 sont utilisés pour estimer et comparer les

paramètres

démographiques

d’un troupeau de race _pure

Hereford

(HE)

et d’un _troupeau

synthétique

dans

laquelle

toute

_génisse

ou toute vache ne

_produisant

_pas un veau sevré _par an était

éliminée. La taille

_moyenne

instantanée de la

population

était de 118 vaches et 39

_génisses

dans le troupeau

HE,

de 155 vaches et 56

_génisses

dans le troupeau SY1. Le _troupeau

SYI était un ensemble composite

d’origine

multiraciale comprenant 33% de

Charolais,

33%

_d’Angus

et 20% de

_Galloway,

le reste _provenant d’autres races bovines à viande. Les

2 _troupeaux étaient conduits de manière

identique. Neuf paramètres

issus de

l’analyse

des carrières

_{des femelles}

sont étudiés :

la probabilité,

à la naissance, de survivre

_jusqu’à

un

_âge

donné ;

le

quotient

de survie Px

_{(probabilité,}

à un

_{âge donné,}

de survivre

_jusqu’à

la classe

d’âge

suivante);

l’espérance

de vie à un

_âge

donné

_(nombre

_moyen d’années restant à vivre

à une

_{femelle atteignant l’âge}

x) ;

le

_quotient

de

_fécondité

_{(probabilité}

_pour une

_femelle,

à un

_{âge donné,}

de

_produire

une

fille) ;

la contribution relative d’une

_femelle

d’un

_âge

donné

à la

_procréation

des

_{générations futures ;}

le taux net de

_reproduction

Ro

_(nombre

moyen

de

_filles

de

_{remplacement produites}

par une

femelle) ;

le taux instantané d’accroissement de la

_population

_(r)

et l’intervalle de

_génération

_(T).

On observe des

_différences

entre

les

_{2 troupeaux}

sur les

paramètres démographiques spécifiques

de

l’âge,

le _troupeau SYI

présentant

des valeurs

plus

élevées que le troupeau HE. Le troupeau SY1

présente

des valeurs moyennes

plus

élevées que le troupeau HE _pour le taux net de

_reproduction

Ro

!1, 57 ±

0, 11 contre 1,21 t 0, 15; p <

_{0, 07)}

et pour le taux instantané d’accroissement

r

(0, 09 !

_{0, Ol}

contre

_{0, 03 ! 0,02;}

_{p <}

_{0, 04),}

ce _qui

_indique

un taux d’accroissement de la

_{population plus}

élevé dans le _troupeau SYl. L’intervalle de

_génération

T est

_plus

élevé

_(p

<

_{0, Ol)}

dans le _troupeau SYI _que dans le troupeau HE

_{!5,09 ±0,11}

contre

4,25

t 0,19

années).

Ces résultats montrent

_qu’une

même conduite et

_qu’une

même

politique

de

_réforme

peuvent se traduire _par des

_{paramètres démographiques différents,}

phénomène

résultant sans doute dans cette étude d’un

_effet

d’hétérosis sur la survie des

veaux dans le _troupeau

_{synthétique d’origine}

multiraciale SYl. Sur la

_durée,

la

_politique

rigoureuse de

réforme

entraîne une réduction des

paramètres Ro,

r et T

_jusqu’à

un _point

où la taille du troupeau HE ne _peut

_plus

être maintenue

_(Ro

< 1 en 1972 et dans les cohortes

_{ultérieures).}

bovin à viande

_/

démographie

/

longévité

/

survie

_/

reproduction

INTRODUCTION

Demographic analyses

are used

extensively

in

humans,

wildlife and fisheries to

characterize

populations. They

involve estimation of

_parameters

such as

reproduc-tive and

_mortality

_rates,

_growth

in numbers and

_biomass,

_{age structure} and other vital statistics of the

_population.

Research

reports

in domestic animals

_using

simi-lar

_analyses

were reviewed

_by

Vu Tien

_Khang

_(1983).

A

_{comprehensive population}

analysis

of a commercial beef cattle herd was made

_by

Schons et al

₍₁₉₈₅₎

and some

possible

uses of the various

parameter

estimates discussed. In

_livestock,

results of

such

_analyses

have been used to formulate

_strategies

for

_culling

and

_replacement

(Turner

et

_al,

_1959;

_{Hickey, 1960;}

Basu and

_{Ghai, 1980;}

Greer et

_al,

_1980),

_organize

breeding

schemes

_(Wiener,

_1961;

_Lauvergne

et

_{al, 1973; Martin, 1975;}

Basu and

Ghai,

1980)

and as a check on

_management

_practices

(Nadkarni

et

al,

1983).

A

similar

_analysis

was used

_by

Ahmad et al

₍₁₉₉₂₎

to characterize a herd of

_dairy

The

_goal

of faster

_{genetic improvement}

in

_{operations producing}

seed stock dictates that the

generation

interval should be

shortened,

and hence intense selection

_and/or

_stringent

_culling

practices

are

_{usually employed.}

Arthur et al

(1992)

reported

the reasons for

_disposal

of cows from a

purebred

Hereford and 2 multibreed

synthetic

beef cattle herds

_managed

at the same location and under a

_stringent

culling policy.

The

_longevity

and lifetime

_productivity

of the cows were

reported by

Arthur et al

_(1993).

There is _very little information available for beef cattle on the

effect,

over

_time,

of such a

stringent

culling policy

on

_population

_parameters

and on the

sustainability

of herd numbers. The

objective

of this

study

was to

construct

age-specific

and overall life tables to characterize and compare a

_purebred

Hereford

and a multibreed

synthetic

beef cattle herd under a

_stringent

culling

_system.

MATERIALS AND METHODS Herd

_management

and

_breeding

plan

The data used for the

_study

were from the

University

of Alberta ranch at

Kinsella,

located 150 km south-east of

_Edmonton,

_Alberta,

Canada. Two main

_breeding

populations

were established in

1960,

namely

the

purebred

Hereford

(HE)

and

the Beef

_Synthetic.

The

_history

of the ranch and the formation of the

breeding

populations

have been

reported

in detail

_{by Berg}

_(1980).

The Beef

_Synthetic

population

was renamed Beef

Synthetic

#1

(SY1)

in

1982,

after another

synthetic

group

composed

of beef breeds was

developed.

To

satisfy

the criteria of

relatively

stable herd

numbers,

consistent

_management,

detailed identification and

production

records

_required

for such

analyses,

records on females born at the Kinsella ranch from 1966

_through

to 1975 and followed till

_disposal

were used. The _average herd

size and standard deviation of the cow herd was 155 ! 6.7 for SY1 and 118 ! 3.4 for HE. The

corresponding

values for the heifers were 56 ! 3.0 for SY1 and 39 ±3.5 for HE. All the females had left the herd at the time of data

_analyses.

The Beef

_{Synthetic #1}

_(SY1)

_population

is a multibreed

_composite

_group with

mainly Charolais,

Angus

and

_Galloway

_breeding.

The average breed

composition

of the SY1 females is

presented

in table I. The

_management

and

_breeding

_plan

of the herds were described in detail

_by

_Berg

et al

_(1990).

In _summary, the 2 herds

were treated as

similarly

as

_possible.

The

_breeding

herds were on the _{range year}

round and

_dependent

on natural

_grazing,

_except

for 3-4 months is winter when

supplementary

feed was

provided.

The level of

supplementary

feed

depended

on

the

pasture

conditions and

_severity

of the winter. Selection of sires was within each

herd and was based on _pre- and

_{postweaning gain.}

On a few

occasions,

Hereford

bulls from outside the HE herd were used for

breeding.

Sires were selected for

breeding

as

_yearlings

and about

25%

of these bulls were

_again

used in the

_following

year. All sound heifers were

_exposed

to bulls as

_yearlings

to calve as 2 year olds.

Cows and heifers were

_exposed

to bulls for 60 d in the

_breeding

season which was

July/August

each _year.

_Breeding

occurred in

_single-sire

_groups of about 25 cows.

To

_prevent

_reproductive

failure

_resulting

from _poor

_{serving capacity}

of a

particular

bull,

mating

groups were monitored

during

the first half of the

breeding

season.

Any

bull found to have poor

serving

capacity

was

replaced

with a _proven older

and _{2 year} olds were calved

_separately,

_closely

_supervised

and remained

_separated

until

_breeding

commenced. Calves remained with their dams until

_weaning

in

_early

October each _year. Heifers and cows

_failing

to wean a calf each _year were culled.

Heifers and cows were also culled for unsoundness and defects such as bad

udders,

leg

and feet

problems,

etc. The

_frequencies

of the various reasons for

_disposal

for

the herds have been

_{reported by}

Arthur et al

_(1992).

The lifetime

_productivity

of these cows has also been

_reported

_by

Arthur et al

_(1993).

Demographic analyses

Life table statistics were

_computed

for the 2 herds in this

_study.

The cohort method of life table construction was used. This method follows an actual cohort

(birth

_year

group)

from birth to the end of the last member’s life

_(Caughley,

_{1966, 1967; Mertz,}

1970).

Data on 10 full cohorts

₍₁₉₆₆

_through

_1975;

all animals had left the

_herd)

were used in the construction of the life tables for each herd. The model utilized

females

_only

and involved annual seasonal

_breeding

and

_overlapping

_generations.

The time reference was

immediately

_postpartum,

with birth considered age 0 and

time interval

_being

_{1 year.}

_Leaving

the herd for _any reason was

_equated

with

Six

_age-specific

life table

statistics

and 3 overall life table

statistics

were

com-puted

(Caughley,

1966,

1967; Mertz, 1970;

Pianka and

Parker,

1975).

The

age-specific

life table statistics

_computed

were as follows:

_survivorship

(probability

at

birth of an animal

_surviving

to a

particular

_age,

Lx);

survival rate

_(probability

at a

_particular

_age of

_surviving

to the _{next age,}

_Px);

_mortality

rate

_(probability

at a

particular

age of

dying

before the next _age,

_Qx

= 1 &mdash;

Px);

expected

herd life

(ad-ditional number of _years an animal of a

_particular

_age is

_expected

to remain in the

herd,

Ex);

birth rate

_(probability

of a cow of a

_particular

_age

_producing

a live female

calf,

Mx);

and

_reproductive

value

_(relative

contribution of an animal of a

particular

age to future

generations,

Vx).

The overall life table statistics

_computed

were net

reproductive

rate

_(expected

number of

_{daughters produced by}

each

_animal,

_Ro),

instantaneous rate of

_population

increase

_(a

measure of herd number increase or

decrease,

r)

and

generation

interval in years. The

_{computational}

formulae for these life table statistics have been summarised

_by

Schons et al

_(1985).

In

_many

stud-ies

_populations

are characterized

_{by constructing}

a life table

_pooled

across cohorts

(Krehbiel

et

_{al, 1962;}

Greer et

_{al, 1980; Melton,}

_1983).

Another method to cancel

out _any differences between cohort and distortions due to small numbers of animals

at _{older ages} is to _average each life table statistic over all cohorts and at each _age, if

_age-specific

_(Schons

et

_al,

_1985).

The

_age-specific

_survivorship

_(Lx)

forms the

basis of all the life table statistics.

_{Preliminary analysis}

of Lx curves indicated that

using

the

_pooled

or the _average life table method described the herds in a similar manner, hence

only

the average life table method was used for the

computation

of all the life table statistics. The Lee-Desu D statistic

_(Lee

and

_Desu,

₁₉₇₂₎

was

computed,

using

the Survival

_procedure

in SPSS

_(1990),

to compare the

_age-specific

survivorship

of the two herds. The D statistic is based on a score that compares

the Lx values or another statistic between herds and tests the null

_hypothesis

that the herds are

_samples

from the same survival distribution. Differences between the 2 herds in the overall life table statistics

_(Ro,

T and

_r)

were tested

_using

a t-test.

Within each

_{herd, simple}

linear

_regression

_analysis

was done for each of the overall

life table statistics to examine the nature of the

_slope

_(Steel

and

_Torrie,

_1980).

RESULTS AND DISCUSSION

Comparison

of the

survivorship

(Lx)

values of the 2 herds

_using

the Lee-Desu D statistic showed

_significant

difference between the herds

_(D

=

3.98;

p <

_0.05)

leading

to the

_rejection

of the null

_hypothesis

that the survival distributions of the

2 herds are the same. The survival

_patterns

(L!)

for HE and SY1 were similar up

to _{age 2 years,} after which there was a faster rate of the decline in HE

_compared

to

SY1

_(fig

_2).

A detailed discussion on the L! curves and the reasons for

_disposal

of

the females of the 2 herds have been

_{reported previously}

_(Arthur

et

_al,

_1992,

_1993).

The

_{probabilities}

at a

_particular

_age of

_surviving

to the next _age

_(Px)

are

_presented

in table II.

There

was a

high probability

for survival from birth to age 1 year and

from _age 1 to _age 2 _years

_(greater

than

_0.9).

This was due to the facts that

_firstly

there were _very few deaths

_prior

to age 2 years

and, secondly,

all available females entered the

_breeding

herd as

_{replacements.}

From _age 2 _years, causes other than

death,

viz

_culling

for

_reproductive

_failure,

_calving

_problems,

calf

survival

and

udder

values after age 1 year. In

general,

SY1 had

higher

survival

probabilities

than HE

after age 1 year. The

_{age-specific mortality}

rate

_(Qx)

is a mirror

_image

(1 -

Px)

of the

_age-specific

survival rate

_(Px)

and the results are

opposite

those of the Px

statistic discussed.

SY1 had

_higher

values for

expected

herd life

_(Ex)

at all _ages

_compared

to HE

(fig

3),

indicating

that at any

particular

age SY1 females were

expected

to live

longer

than HE females. The

_age-specific

birth rate

_(probability

of a cow of a

particular

age

producing

a live female

calf,

Mx)

is a _very

_important

statistic in

beef cattle

_production,

because it influences the number of

replacement

heifers

available, population

growth

and

generation

interval. Mx is

_dependent

on the sex

ratio _{of progeny.} At older _{ages there} were fewer cows

_present

in the

herd,

hence

the likelihood that the sex ratio of _progeny of cows at these ages was not 1:1 was

high.

This deviation from the

_expected

1:1 sex ratio contributed to the

relatively

defined as the ratio of the

expected

size of a herd

_(at

some future

_time)

founded

_by

a cow or group of cows of a

_particular

_age to the

_expected

size of a herd founded

simultaneously by

a heifer calf or _group of heifer calves

_aged

0

_(MacArthur

and

Wilson,

1967).

The Vx values for both herds

_peaked

at _{age 2 years} and

_gradually

dropped

off with SY1

_{having higher}

values than HE at all _ages

_{(table II).}

The rise in the V! values _up to _{age 2 years} was due to the lack of

_reproduction

until 2 years

of _age

_(Mx

values are zero for _ages 0 and 1

_year).

The overall life table statistics for the 2 herds are

_presented

in table III.

The means for net

_reproductive

rate

_(Ro)

were

_greater

than

_unity

and those for

instantaneous rate of

_population

increase

_(r)

were

_greater

than zero for both herds.

These

_results,

which

_represent

the _average for the entire

_{study period,}

indicate that

cows in both herds were more than

replacing

themselves and that herd numbers were

_increasing.

The means for these statistics were

_higher

in SY1 than is HE. For natural

_populations

r can be used as a measure of a

population’s capacity

Alternatively,

the environment in which a

_population

has its

_highest

r is the

_optimal

environment for that

_population

_{(Mertz, 1970).}

Because of human influence on

livestock

_management

for

_production

_efficiency,

the

application

of r as a measure of

a

_{population’s capacity}

for sustained

_change

in numbers is limited. Herd differences

in

_generation

interval

_(T)

also followed a

_pattern

similar to Ro and r with SY1

having higher

means than HE.

Examination of the trend in the overall life table statistics indicated that the

means of these statistics had been

decreasing

with time. For Ro both herds had

a

_{negative regression}

coefficient

_although

the coefficient for SY1 was smaller than

that for HE

_{(fig 4).}

For HE the

predicted

Ro values were less than

_unity

for the

1972 and later

cohorts,

indicating

that those cohorts were not

_replacing

themselves. The

pattern

for r was similar to Ro and hence the results are not

_presented.

For

_T,

the

_regressions

for SY1

_(T

= 5.73 -

O.11Y)

and for HE

(T

= 4.84 -

0.108Y)

_were

significant

and both herds had

_{negative regression}

coefficients. Y in the

_equations

represents

year

cohorts,

with the 1966 cohorts

having

a value of 1. The

_declining

trend in the overall life table statistics was

_probably

the result of the

stringent

culling policy.

The contribution of

_inbreeding

to this

_decline,

if any, would be

minimal since efforts were made to

_prevent

_inbreeding

in both

_{herds, through}

the

selection of

_breeding

bulls,

as well as

_through

the occasional use of Hereford bulls

from outside the HE herd. The breed

_composition

of the multibreed SY1 herd

stabilized around 1970

_(table

_I).

The trends in the overall life table statistics were

also evaluated

_{using only}

the 1970 to 1975 cohorts for both

_herds,

to reflect the

period

of stable breed

_composition

of SY1. For these cohorts the

_regressions

for SY1

were not

_significant

_(Ro

= 1.36-0.004Y and T =

5.22-0.103Y)

while those for HE

were

_significant

(Ro

= 1.34-0.103Y and T =

5.16-0.286Y).

These results indicate

that after the breed

_composition

of the SY1 herd had stabilized in 1970 its Ro and

T statistics also

stabilized,

while those for the HE herd continued to decrease. If this

declining

trend needs to be

_{arrested, strategies}

which aim at

_increasing

the survival rates of 2 and 3 year old cows should be

considered,

since survival rates

(Px)

dropped

at ages 2 and 3 _years relative to ages 1-2 and 4-5 years. In this

study,

2 and 3 year old cows are first and second

calvers;

53 and

65%

of mortalities in first

to

produce

a

_calf,

Arthur et

_al,

_1992).

The

_stringency

of

_culling

for

_reproductive

failure can be relaxed for these _{2 ages,} or cows at these _ages could be

_provided

with a

higher

level of

_husbandry,

such as

_improved

_nutrition,

to increase their

_reproductive

rates.

There were no

_significant

differences between the herds in the

_percentages

of cows

disposed

(referred

to as

_mortality

in this

_study)

for _any of the

_major

reasons for

disposal reported by

Arthur et al

_(1992),

_except

for calf survival

_problems

_(perinatal

and

_preweaning).

The value of

17.1%

of all cows

_disposed

from the HE herd was

due to the fact that

_they

_gave birth to stillborn calves or that their calves did not

survive to

_weaning.

This _{compares to}

9.1%

for the SY1 herd. In the execution of the

_{culling policy}

no cow or herd was

_{given preferential}

_treatment,

hence any cow or heifer which

produced

a stillborn calf or failed to wean its calf was culled. The

HE and SY1 herds were raised at the same location and under similar

_management

and

_{culling policy}

hence herd differences were

_likely

due to the

_{genetic make-up}

of the females. The

_positive

effect of heterosis on calf survival has been

_reported

in

beef cattle

_(Cundiff

et

_{al, 1974;}

_Spelbring

et

_al,

_1977).

It is thus

_likely

that the calf survival in the SY1

herd,

being

a multibreed

_composite,

was

_positively

influenced

Population

analysis

of a commercial herd of

_Angus

cattle in

_Wyoming

was

reported

by

Schons et al

₍₁₉₈₅₎

_using

similar

_biological

model and

_{computational}

formulae. The Lx curves of herds in this

_study

_{(Kinsella herds)}

and that of the

Wyoming

herd

_(Schons

et

_al,

₁₉₈₅₎

are

_presented

in

figure

2. The most noticeable difference between the Lx curves of the Kinsella herds and that of the

_Wyoming

herd occurred _{at ages 1} and 2 years, where the Lx values for the Kinsella herd were

almost double those of the

_Wyoming

herd. This is attributable to differences in

replacement

heifer

_policy

at the 2 locations. In the Kinsella herds all sound heifers

were included in the

_breeding

herd and

_exposed

to bulls as

_yearlings,

hence an Lx of over 0.9 at _{1 year} of _{age. In} the

_{Wyoming herd,}

not all heifers entered the

_breeding

herds.

_Replacement

heifers were selected based on

_weaning

_{weight, yearling}

_weight,

dams

_record,

and in some _years on information on their sires. Hence

just

over

50%

of the heifers

_(Lx

of

_0.52)

were used as

_{replacements.}

Another difference between

the Kinsella and

_Wyoming

herds is in the

_slope

of the Lx curves after _{age 2 years,}

although

the curves were all smooth with no

_{major disruptions.}

The

slopes

of the Kinsella herds are

_sharper

than that of the

_Wyoming

herd. This is

_likely

due to

differences in the

_culling

_policies

at the 2 locations.

_Culling

was _very

_stringent

in

the Kinsella

_herds;

_any cow

failing

to wean a calf each _year was culled. Cows were

also culled for caesarian

_sections,

bad

_udders,

and

_leg

and feet

_problems

_(Arthur

et

al,

1992).

In the

_Wyoming

herd cows were culled for some of the reasons as in the

Kinsella

herds,

but

_culling

was not as

_stringent.

The Lx curve

reported by

Krehbiel et al

_(1962),

_using

data

_(1939-1961)

from the

_Virginia

Beef Cattle

_Improvement

Association

_(VBCIA),

was similar to that of the

Wyoming

herd

except

for its

slightly

steeper

slope.

Px values for the Kinsella herds were

_higher

than that of the

Wyoming

herd _{at age} 1 _year, similar at _age 2 _years

_generally

lower at

_subsequent

ages, due to the same reasons discussed for differences in Lx values between the 2 locations.

Ex curves from the

_Wyoming

_(Schons

et

_al,

₁₉₈₅₎

and VBCIA

_(Krehbiel

et

_al,

1962)

herds

_peaked

at _age 1 _year, whereas those of the Kinsella herds were

_highest

at age 0 but

_{peaked again}

_{at ages} 4 and 5 _years

_{(fig 3).}

_Although

the Ex values

reported by

Greer et al

₍₁₉₈₀₎

for a herd of

mostly

Hereford cattle in Montana did

not include ages 0 and 1 years, the values for ages 2 to 10 _years

_provided

did not show

any

peak

beyond

the

highest

value at age 2 years. The difference in the direction

of the Ex curves between ages 0 and 1 years for the Kinsella herds

(decreasing)

compared

with those of the other studies

_(increasing)

is

_probably

due to differences

in

_replacement

heifer

_policy.

All heifers were used as

_replacements

in the Kinsella herds versus the selection of a

_proportion

of heifers which was used as

_replacement

in the other studies. In the Kinsella herds females were

_expected

to wean a calf each year.

Culling

for

reproductive

failure and calf survival

problems

accounted for

63%

of all mortalities

_(Arthur

et

_al,

_1992).

There was therefore an indirect

_reproduction

related automatic selection with age. This automatic selection could be

responsible

for the rise in the Ex curves after first

_calving

(age 2.years)

and

_reaching

a

_peak

at _ages 4 and _{5 years,} which is

_unique

to the Kinsella herds. Mx values for the Kinsella herds

_(overall

average of 0.46 for HE and 0.48 for

SY1)

were

generally

higher

than those of the

_Wyoming

herd

_(overall

_average of

_0.42).

Mx is a function

of sex

ratio,

fecundity

and foetal

mortality.

In the Kinsella herds _any females which

for females with

_{high reproductive}

_efficiency.

This could account for the

_higher

Mx value in the Kinsella herds

_compared

to the

_Wyoming

_herd,

where

_culling

for

reproductive

failure was not as

_stringent.

Vx values in the

_Wyoming

herd are

higher

than those of the Kinsella

_{herds, probably}

due to differences in

_{culling policy}

and

_replacement

heifer

_management

as discussed for Lx. Means of overall Ro and r

values for the SY1 herd were

_{higher than,}

whereas those of the HE herd were similar

to,

those of the

_Wyoming

herd. Generation intervals

_(T)

for the Kinsella herds were

lower than that of the

Wyoming

herd. The value of T for most beef cattle herds is between 4.5 and 6 _years

_(Neumann

and

_Snapp,

₁₉₆₉₎

hence the Kinsella herds are

at the lower end of the range, due to the

_stringent

_{culling policy.}

In addition to the

_{reproduction-related}

automatic selection with age, the

strin-gent

culling

policy

had the effect of

_reducing

T. In situations where

_genetic

_progress is

_being

made for _any

_trait,

this reduction in T may be

beneficial,

as a lower T in-creases the rate of

_{genetic gain.}

In these

herds,

direct selection of bulls for _pre- and

postweaning gain

was

_occurring,

which resulted in

_{positive genetic gain}

_(Sharma

et

_al,

_1985).

The rate of this

genetic

progress was thus enhanced

_by

the reduction

in T for the females. It is however recommended that in

_using

such a

_stringent

culling

policy

careful

_monitoring

should be exercised to ensure that cows are able

to

_replace

themselves

_{(Ro §}

₁₎

so that herd numbers de not decrease

_{(r > 0).}

CONCLUSION

The results of this

_study

and those of other studies in beef cattle indicate that

life table statistics can differ _among herds where

_management

and

_culling

_policies

differ.

_However,

this

_study

also shows that the same

_management

and

_culling

_policy

may result in different life table statistics. This is

probably

due to the influence of heterosis for calf survival in the multibreed

_composite

SY1 herd. The

_stringent

culling policy

had the effect of

_reducing

the net

_reproductive

_rate,

instantaneous rate

of

population

increase and

_generation

interval in both herds over time.

_However,

in

the SY1 herd the trend stabilized in 1970 and the

_ability

to maintain herd numbers

was

_retained,

while in the HE herd the trend continued to the

_point

where herd size could not be maintained

_(Ro

< 1 for 1972 and later

_cohorts,

_fig

_4).

If the decline in

these overall life table statistics needs to be

stopped,

it is

_suggested

that

_strategies

that aim at

_{reducing mortality}

rates in 2 and 3 _years olds should be considered.

ACKNOWLEDGMENTS

Assistance

_{provided by}

G Minchau and the staff at the

University

ranch is

_{acknowledged.}

Financial _support

_{provided by}

the Natural Sciences and

_Engineering

Research

Council,

Agriculture

Canada and the Alberta Cattle Commission is

_{acknowledged.}

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