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University of Nebraska - Lincoln
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Roman L. Hruska U.S. Meat Animal Research
Center
U.S. Department of Agriculture: Agricultural
Research Service, Lincoln, Nebraska
1993
Effects of Inbreeding and Heterosis in Hereford
Lines on Reproduction and Maternal Performance
Michael D. MacNeil
Fort Keogh Livestock and Range Research Laboratory
Delwyn D. Dearborn
University of Nebraska West Central Research and Extension Center
Larry V. Cundiff
Genetics and Breeding Research Unit, MARC
Chris A. Dinkel
South Dakota State University
Keith E. Gregory
Genetics and Breeding Research Unit, MARC
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MacNeil, Michael D.; Dearborn, Delwyn D.; Cundiff, Larry V.; Dinkel, Chris A.; and Gregory, Keith E., "Effects of Inbreeding and
Heterosis in Hereford Lines on Reproduction and Maternal Performance" (1993). Roman L. Hruska U.S. Meat Animal Research Center.
Paper 336.
http://digitalcommons.unl.edu/hruskareports/336
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Effects of Inbreeding and Heterosis in Hereford Lines on Reproduction and Maternal
Performance
Michael D. MacNeil,Delwyn D. Dearborn, Larry V. Cundiff, Chris A. Dinkel, and Keith E. Gregory'
Introduction
Two genetic mechanisms have been described as potential explanations for heterosis. The first mechanism is dominance. Dominance occurs when there are two differing
forms of a gene (alleles) at a given position (locus) on a pair
of chromosomes and where one of the pair of alleles masks
or over powers the effect of the second. Having two different alleles at a locus is referred to as heterozygosity and the
affected individual is heterozygous. Whether an individual
has one or two copies of a dominant allele makes little difference in its superiority over others having two copies of
the recessive allele. A higher degree of heterozygosity is
expected when one population carrying the dominant allele
in high frequency is crossed with a second population carrying the recessive allele in high frequency. Alternatively, heterosis may result from joint effects of genes at several loci.
This alternative mechanism is called epistasis.
Previous research documents reduced performance
resulting from the mating of closely related individuals
(inbreeding). Inbreeding generally reduces growth and
reproductive rates and delays maturity. This inbreeding
depression arises from increasing the frequency with which
two alleles at a locus are identical (homozygous) and again
coupled with dominant gene action. Thus, effects of
inbreeding and heterosis are of similar size but opposite in
direction, if dominance at individual loci causes both.
In this study, we used inbreeding and linecrossing of
Hereford cattle in an attempt to distinguish between these
two explanations for heterosis influencing maternal traits.
Answering this question sheds light on the amount of heterosis to be expected in composite breeding schemes.
Procedure
Scientists with the South Dakota Agricultural Experiment
Station created four inbred lines. Each inbred line started
from 1 bull and 15 cows. The same 4 bulls and 60 cows
were the basis for a contemporary control line. The control
line was maintained as a single herd. Mating bulls from
each inbred line with cows from the other inbred lines
resulted in production of linecross females. Mating inbred
bulls to control line cows produced topcross females.
Mating of related cows and bulls was avoided in producing
topcross females. Replacement females (control, inbred,
linecross, and topcross) were transported to MARC and
evaluated for reproductive and maternal performance over
an eight-yr period.
Results
Performance of females from the four lines as 2-yr-old
heifers and at all ages is shown in Table 1. The topcross
breed group can be used to separate effects of inbreeding
of sire and dam. In this study, the topcross breed group did
not differ in performance from either the linecross or control
breed groups.
If performance of inbred and control lines differs, then
effects due to inbreeding exist. Inbreeding depressed survival of calves from pregnancy testing to calving of first calf
heifers. Birth weights of calves from inbred cows were also
lighter than from control line cows. Except for pregnancy
rate, other comparisons of inbred and control line cows
were also negative. However, they were not large enough
to establish conclusively the existence of inbreeding effects.
Heterosis exists if performance by linecrosses differs
from that of the parental inbred lines. Survival rates for
calves from linecross females exceeded those from inbred
females both from pregnancy testing to calving and from
calving to weaning. Linecross cows also had heavier calves
than inbred cows, both at birth and at weaning.
Comparing effects of heterosis and inbreeding, we find
no differences in their size for any trait except birth weight.
For birth weight, inbreeding depression was larger than heterosis. This result may stem from the heavier than
expected birth weights of calves from control line cows.
Results of this study indicate that effects of inbreeding are
detrimental to reproduction and maternal performance in
cattle. Crossing inbred lines results in significant heterosis.
Performance levels of linecrosses apparently are restored
to the level of noninbred contemporaries.
Table 1-Levels of Inbreeding, reproductive traits of
two-year-old heifers and maternal performance of
Inbred, IInecross, topcross, and control line cows
Breedgroup
Inbred Linecross Topcross Control
Traits
Levelof inbreeding, percent
Individual
Sire
Dam
27
31
24
0
34
27
0
27
7
7
4
6
2-yr-old
Pregnant, percent
Prenatal survival,percent
Birthrate, percent
Postnatal survival,percent
Weaning rate, percent
76
85
66
70
46
79
97
78
90
70
70
97
68
80
55
76
100
77
83
65
72
400
75
429
76
432
82
431
Allages
Birthwt, Ib
Weaning wt, Ib
'MacNeil is a research geneticist, Fort Keogh Livestock and Range
Research Laboratory, Miles City, MT (formerly research animal scientist, Production Systems Research Unit, MARC); Dearborn is the associate district director, University of Nebraska West Central Research
and Extension Center, North Platte, NE; Cundiff is the research leader,
Genetics and Breeding Research Unit, MARC; Dinkel is a professor
emeritus, South Dakota State University, Brookings, SD; Gregory is a
research geneticist, Genetics and Breeding Research Unit, MARC.
167
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