R. M. Bourdon and J. S. Brinks 1987, 65:956-962.

Simulated Efficiency of Range Beef Production. II. Fertility
Traits
R. M. Bourdon and J. S. Brinks
J ANIM SCI 1987, 65:956-962.
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SIMULATED
EFFICIENCY
OF RANGE BEEF PRODUCTION.
I1. F E R T I L I T Y T R A I T S 1
R. M. B o u r d o n and J. S. Brinks 2
Colorado State University, F o r t Collins 80523
ABSTRACT
A modified version of the Texas A&M University Beef Cattle Production Model was used to
simulate the effects of changes in potential for age at puberty (AAP), potential for probability of
conception (PCA) and winter feed levels on biological and economic efficiency of beed production
in a northern plains, range environment. Two management systems were simulated: a weanling
system in which all calves except replacement heifers were custom fed in a feedlot immediately
post-weaning; and a yearling system in which calves were kept on the ranch through their second
summer, then custom-fed. Biological efficiency was defined as the ratio of TDN input to product
output, and economic efficiency was defined as the ratio of total dollar cost to 100 kg product
output. A simulated increase in AAP from 365 to 425 d resulted in slightly decreased economic
efficiency under a yearling system of management. An increase in PCA from .75 to .85 caused decreased biological efficiency under both weanling and yearling management systems, suggesting
biological inefficiencies associated with maintaining mature cows. Simulation results indicate that
optimal supplementation levels and corresponding levels of observed fertility depend on the value
of product derived from cull cows relative to the value of product derived from fed animals and on
the costs of developing replacement heifers relative to the costs of maintaining mature cows.
Decreased fertility causes change in the sources of products, not product loss per se. For this
reason, survivability may be a more important aspect of reproduction than fertility.
(Key Words: Beef Cattle, Simulation, Fertility, Reproduction.)
I ntroduction
otherwise constant g e n o t y p e and d e m o n s t r a t e d
decreased e c o n o m i c efficiency with decreased
fertility. When adjustments were m a d e for cow
slaughter, biological efficiency did not change,
however. Earlier age at p u b e r t y resulted in only
m i n o r i m p r o v e m e n t s in efficiency. A possible
explanation may have been the relatively y o u n g
age at puberty simulated.
The objectives o f this study were to use
c o m p u t e r simulation to: 1) compare the
life-cycle, herd-wide, biological and e c o n o m i c
efficiencies of g e n o t y p e s varying in inherent
fertility, specifically in age at puberty (AAP)
and m a x i m u m
probability o f c o n c e p t i o n
(PCA); 2) determine the relative i m p o r t a n c e of
fertility to p r o d u c t i o n efficiency and 3) study
the effects of m a n a g e m e n t and e c o n o m i c s and
their interaction on b o t h herd fertility and
p r o d u c t i o n efficiency.
Researchers have generally agreed that
improved reproductive performance, n a m e l y
b e t t e r fertility and increased survivability,
improves overall p r o d u c t i o n efficiency in beef
cattle. Dickerson (1974), using data on calving
difficulty f r o m Laster et al. (1973), suggested
the i m p o r t a n c e o f weaning rate to p r o d u c t i o n
efficiency when he defined net merit to include
n o t only growth rate, but also birth weight, an
indicator of calving difficulty and, therefore,
survivability. Using a linear p r o g r a m m i n g
model, Wilton and Morris (1976) r e p o r t e d
increased efficiency with increased weaning
rates, although trade-offs were evident b e t w e e n
weaning rate and cow size.
A l t h o u g h the value o f increased weaning rate
is fairly well established, less is k n o w n a b o u t
the i m p o r t a n c e of fertility. N o t t e r (1977)
simulated different levels o f fertility in an
Materials and Methods
The Original Model. The biological m o d e l
t Funding was provided by Colorado Agric. Exp.
Sta. Project No. 1-5607 and the Institute for Computational Studies at Colorado State Univ.
aAssistant Professor and Professor, respectively,
Dept. of Anim. Sci.
Received November 14, 1986.
Accepted April 29, 1987.
used was a version of the Texas A&M Beef
Cattle P r o d u c t i o n Model (Sanders, 1977;
Sanders and Cartwright, 1979). Fertility was
simulated as a f u n c t i o n of t w o factors: the
probability that a female will c o m e into estrus
in a 30-d period and the probability that she
will conceive in a 30-d period given estrus.
956
J. Anim. Sci. 1987. 65:956-962
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FERTILITY TRAITS
Probability of estrus was determined by body
condition, weight change, time since calving in
cows, maturity in heifers and a fixed potential for cycling defined as PEA or maximum
probability of estrus. Probability of conception
was determined by b o d y condition, weight
change, time since calving (in cows) and a fixed
potential for conception known as PCA or
maximum probability of conception given
estrus. Both the original Texas model and our
version of it operated on classes of cattle as opposed to individual animals. Probabilities of
estrus and conception determined, therefore,
the fractions of groups cycling and conceiving
in a given period.
Changes in the Model. Model inputs were
altered to reflect a northern plains, range
environment. We changed the fertility-related
aspects of the original model by making the
potentials for fertility, PEA and PCA, variable,
and by adding a parameter for potential age at
puberty (AAP). Decreases in AAP were interpreted in the model as increases in degree o f
maturity and consequently increased the
probability that an animal would come into
estrus. Changes in other aspects of the biological
model were outlined in a companion paper
(Bourdon and Brinks, 1987a) and in the dissertation by Bourdon (1983).
Economic Analysis. The economic program
was identical to that described by Bourdon and
Brinks (1987a). Of the seven economic scenarios simulated, three were of particular interest in
this study: the base economic scenario and scenarios in which the ratios of prices paid for cull
cows and fed animals were varied upward or
downward by 25%. Differences in pregnancy
rates change the proportions of product derived
from cows and fed animals. We expected,
therefore, that price relationships and fertility
levels would interact in their effect on overall
production efficiency. Base price ratios (cull
cow/slaughter steer) used were .635 for live
weight at weaning (LWW), .839 for e m p t y b o d y
weight at slaughter (EBW) and .675 for fat-free
weight at slaughter (FFW). Biological efficiency
was defined as the ratio of TDN input to product output, and economic efficiency was defined
as the ratio of dollar cost to 100 kg product
output.
Herd Management. Two spring-calving management systems were simulated. In the weanling system, all steers and heifers not needed for
replacements were placed in a custom feedlot at
weaning. In the yearling system, these animals
957
were wintered on the ranch, pastured the next
summer and placed in a custom feedlot the
following fall. Ownership of cattle through
slaughter was retained in both systems.
To compare fairly genotypes with varying
requirements for winter supplement, winter
feed levels were adjusted so as to maximize
economic efficiency o f fat-free weight (FFW)
production in the base economic scenario.
The breeding season for yearling heifers
began 1 mo before that for cows. All nonpregnant cows were culled at weaning and an agedependent proportion of pregnant cows was
culled for unsoundness. Culling of pregnant
cows for other reasons was not incorporated in
the model. Enough replacements were kept so
that approximately 20% more pregnant replacements were available than were actually required. Other assumptions of the model that
are of less specific interest in this study were
described by Bourdon and Brinks (1987a).
Results and Discussion
Simulated performance, production and efficiency data for three genotypes are listed in
tables 1, 2 and 3, respectively. The base genotype (genotype 1 described by Bourdon and
Brinks, 1987a) was characterized by a mature
weight potential (WMA) of 525 kg, potential
for milk production (PMA) of 12 kg/d, potential
age at puberty (AAP) of 365 d and maximum
probability of conception (PCA) of .75. Other
genotypes simulated were a late puberty genotype (AAP = 425 d) and a more fertile genotype (PCA = .85).
Results for four supplementation strategies
for genotype 1 are shown in tables 1, 2 and 3
also. In the base strategy, supplementation was
optimized independently for each genotype
based on efficiency of FFW production from
the weanling management system using standard
price relationships. In other strategies, supplementation was optimized for efficiency of FFW
production from: the weanling system with cow/
calf price ratios decreased 25%; the yearling system with standard price relationships; the yearling system with cow/calf ratios decreased 25%.
Age at Puberty. Simuiated reproductive performance of the genotype with genetic potential
for later age at puberty was predictable. Compared with genotype 1, mean age at puberty
was much older, pregnancy rate for heifers
much lower, pregnancy rate for cows somewhat
lower, replacement rate higher, average calving
date later and corresponding weaning weights
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958
BOURDON AND BRINKS
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slightly lower (table 1). Because they were older
and larger at first calving, late-puberty heifers
experienced somewhat less dystocia and calving
loss. Weaning rate was not appreciably affected
by age at puberty because sufficient numbers
of pregnant replacements were retained in each
case.
In the weanling system, greater numbers
of yearling females were required to produce
enough pregnant replacements of the latepuberty genotype. As a result, herd size was reduced (table 2) and large differences were
apparent in the relative amounts of products
from base and late puberty types 9 Late-puberty
cattle produced many fewer heifers to be fed
out after weaning, many more heifers to be
carried over and slaughtered as long yearlings,
and more cull cows. Differences in relative
amounts of products were much smaller in the
yearling system because all heifers were wintered.
Production efficiency (table 3) generally
increased (efficiency ratios decreased) with
increased AAP in the weanling system and
decreased with increased AAP in the yearling
system 9 The heavier slaughter weights produced
in these simulations by the yearling system of
management caused this system to be favored
over the weanling system (Bourdon and Brinks,
1987a). Because almost all weaned heifers of
the late-puberty genotype were required as potential replacements, heifers of this genotype
were managed as yearlings even when the management system simulated was ostensibly a
weanling system. We conclude, therefore, that
the advantage of late puberty in the weanling
system reflected advantages of a management
system and not of late puberty.
The fact that efficiencies decreased with increased AAP in the yearling system suggests
that earlier puberty is indeed beneficial, at least
at the levels of AAP simulated. Differences in
efficiency for different potentials for age at
puberty were small, however.
Probability of Conception. When maximum
probability of conception was increased from
.75 to .85, simulated pregnancy rates for both
replacements and cows increased and replacement rate decreased (table 1). The simulated
mean calving date was slightly later due to the
smaller number of heifers bred to calve early in
the season. Weaning rate remained essentially
constant.
Fewer replacements were required with
increased PCA. As a result, herd size (table 2)
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FERTILITY TRAITS
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FERTILITY TRAITS
under the weanling management system was
increased for the more-fertile genotype. As in
the AAP comparison, changes in PCA caused
differences in relative amounts of products
from base and more fertile genotypes. In both
management systems, the genotype with greater
PCA yielded more product from slaughter
heifers and less product from cull cows.
Biological efficiency decreased with increased
fertility (table 3). Economic efficiency generally
decreased with increased fertility when the
ratio of prices paid for cull cows and fed
product was either "standard" or greater. Only
when fed animals were especially valuable
relative to cull cows did increased fertility
translate into improved efficiency. While these
economic results may simply be artifacts of the
price ratios and production costs used in this
simulation, the decrease in biological efficiency
with increased fertility suggests another explanation.
Because all nonpregnant cows were culled in
this simulation, genetic potential for fertility
directly affected replacement rates and the age
structure of the herd. Increased fertility resulted
in an older average cow age. We suspect that
mature cows are less biologically efficient than
first-calf heifers due to a greater proportion of
dietary energy required for maintenance,
pregnancy, lactation and regaining of weight.
This contention and our results are in general
agreement with results of Taylor et al. (1985),
which predict biological inefficiency of maintaining mature cows. The conclusion we draw is
that from the standpoint of purely biological
efficiency, cow herds should be kept young.
Longevity in itself may not be especially
important. This subject is discussed further by
Bourdon and Brinks (1987b).
Management and Economics. Simulated
results for animals of genotype 1 fed different
levels of winter supplement are listed in tables
1, 2 and 3. When supplementation was increased to a level which optimized efficiency of
FFW production in the weanling system when
calves were relatively valuable (supplementation
strategy II in the tables), the replacement rate
decreased and the pregnancy rate for cows increased (table 1). More slaughter heifers and
fewer cull cows were produced (table 2). Thus,
when calves were more valuable relative to cows,
it was more economically efficient to increase
supplement so as to produce more slaughter
calves. Optimal pregnancy rates in these simulations were only 80 and 83%, however, indi-
961
cating considerable trade-offs between pregnancy rate and feed costs. Lishman et al. (1984)
reported even lower optimal pregnancy rates.
When supplementation levels were similarly
increased in the yearling system (supplementation strategies III and IV in the tables), results
were much the same. Optimal pregnancy rates
increased from 83 to 86%. The higher pregnancy rates for this management system reflect
heavier slaughter weights and, therefore, a
greater proportion of total income derived from
fed animals.
These simulation results suggest that optimal
supplementation levels and corresponding levels
of observed fertility depend on the relative
contributions of products to total income and
the costs of producing those products. Specifically, they depend on the value of product
derived from cull cows relative to value of
product derived from fed animals, and on
the costs of developing replacements relative to
the costs of maintaining mature cows. Managing
for high fertility should be appropriate when
product from fed animals is responsible for a
relatively large proportion of total income and
when replacement costs are high relative to
costs of maintaining mature cows.
Conclusions
The question of the importance of fertility
traits is complicated. Optimal pregnancy rates
are not the highest pregnancy rates, but are a
function of production costs and prices received
for cows and young animals. In these simulations, younger age at puberty was favored, but
the potentially beneficial effects of earlier puberty in heifers and increased potential for fertility in cows were countered by the apparent
inefficiency of maintaining mature animals.
Increases in observed fertility did not, therefore, result in substantial improvements in production efficiency. This is not to say, however,
that there is little to be gained from improving
genetic potential for fertility traits. Highly fertile cattle are capable of functioning on less
feed and under s~ess. A more flexible model,
one that simulates a dynamic, sometimes stressful environment, will be required to evaluate
the importance of this ability.
These simulations illustrate an important
point about fertility, however, and one that is
often overlooked. If females are pregnancy
tested and open cows culled, weaning rates
need not decline with decreasing pregnancy
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962
BOURDON AND BRINKS
rates so long as p r e g n a n c y rates are high e n o u g h
t o p r o v i d e s u f f i c i e n t r e p l a c e m e n t s . ( P e r c e n t calf
crop was essentially c o n s t a n t in t h e s e simulations.) D e c r e a s e d p r e g n a n c y rates i m p l y n o t a
loss o f p r o d u c t , b u t a c h a n g e in t h e s o u r c e o f
p r o d u c t s - f r o m s l a u g h t e r heifers t o cull cows.
In this light, it can b e m i s l e a d i n g to express
t r a i t s o n a p e r c o w e x p o s e d basis. S u c h p r a c t i c e
a t t a c h e s u n w a r r a n t e d i m p o r t a n c e t o fertility.
W e a n i n g r a t e is i m p o r t a n t to p r o d u c t i o n efficiency, a n d e f f o r t s s h o u l d b e m a d e t o increase
w e a n i n g rates e c o n o m i c a l l y . T h e k e y traits t o
i m p r o v e m a y n o t b e f e r t i l i t y traits, h o w e v e r ,
b u t survivability traits.
Literature Cited
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and management on beef production efficiency.
Ph.D. Dissertation. Colorado State Univ., Fort
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Bourdon, R. M. and J. S.' Brinks. 1987a. Simulated
efficiency of range beef production. I. Growth
and milk production. J. Anim. Sci. 65:943.
Bourdon, R M. and J. S. Brinks. 1987b. Simulated
efficiency of range beef production. I11. Culling
strategies, nontraditional management systems.
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