www.elsevier.com / locate / livprodsci
Maternal heritability and repeatability for litter traits in rabbits
in a humid tropical environment
a ,
*
b cR.K. Rastogi
, S.D. Lukefahr , F.B. Lauckner
a
Department of Food Production, Faculty of Agriculture and Natural Sciences, University of the West Indies, St. Augustine,
Trinidad and Tobago
b
Department of Animal& Wildlife Sciences, Texas A&M University, Kingsville, TX 78363, USA c
Caribbean Agricultural Research& Development Institute, Trinidad and Tobago Received 14 July 1999; received in revised form 26 January 2000; accepted 11 February 2000
Abstract
Data on litter production traits of does, involving 1120 litters, was collected over a 10-year period (1984 to 1994) and analyzed to estimate heritability and repeatability. Locally adapted does and bucks of predominantly New Zealand White breeding were involved. Traits studied were total litter size born and born alive, and litter size and weight at 21 days, 28 days (weaning) and 84 days (marketing). A doe repeatability model was employed which included a fixed contemporary environmental group effect (year–season–doe parity) and litter size at the corresponding age as a linear covariate (for litter weight traits only), and random animal (additive maternal genetic), permanent (non-additive maternal genetic plus permanent environmental), service sire, and residual effects. Heritability (repeatability) estimates for total litter size born and born alive, and litter size at 21 days, 28 days, and 84 days were 0.09, 0.12, 0.06, 0.09, and 0.07 (0.30, 0.32, 0.26, 0.25, and 0.19), respectively. For litter weight at 21 days, 28 days, and 84 days, heritability (repeatability) estimates were 0.08, 0.09, and 0.02 (0.18, 0.19, and 0.09), respectively. In conclusion, litter production traits tended to be lowly heritable and lowly to moderately repeatable. 2000 Elsevier Science B.V. All rights reserved.
Keywords: Rabbit; Heritability; Maternal effects; Litter traits
1. Introduction especially needed from tropical rabbit populations in order to provide quality breeding stock to farmers.
Limited rabbit genetic studies have been con- Reports from Tanzania (Mgheni and Christensen,
ducted in tropical countries where climate, diet, 1985), Ghana (Lukefahr et al., 1992), Brazil (Ferraz
management, and stock sources can differ markedly and Eler, 1994), and Hawaii (Moura et al., 1997)
from those in temperate countries. Genetic parameter tend to indicate higher heritabilities for production
estimates (e.g. heritability and breeding value) are traits than is generally reported from temperate
environments. In tropical environments, rabbit popu-lations often have a heterogeneous history involving
*Corresponding author. Tel.: 11868-662-2002; fax: 1
1868-multiple breed introductions and crossings which
663-9686.
E-mail address: rajrastogi@yahoo.com (R.K. Rastogi). could explain the higher heritability values reported.
The objectives of this study were to estimate 14 days after service. If not pregnant, the doe was
maternal heritability and repeatability for several immediately rebred. On day 28 of gestation, the nest
litter production traits of the doe for application in box was presented. Cross-fostering of kits among
selection or culling programmes in an experimental does was not practised so that a doe’s own producing
herd of predominantly New Zealand White rabbits ability could be assessed. Does were culled for poor
raised in the humid-tropical environment of Trinidad fertility and productivity (i.e. failure to wean at least
(West Indies). a total of 12 kits during the first three consecutive
matings). Male replacements were selected for post-weaning growth until breeding age of 6 months,
2. Materials and methods whereas doe replacements were selected at weaning from dams with larger than average litters over the
2.1. Rabbit type, climate and housing first three kindlings and with below average
inter-kindling period. However, the amount of applicable
Data on doe performance traits were collected selection pressure was rather low due to small flock
over a 10-year period (1984 to 1994) from a small size. In selecting replacement breeding stock,
prefer-experimental rabbitry, established in March, 1983, at ence was given to all-white rabbits.
the UWI, Trinidad. Locally adapted rabbits of mixed
breeding (including contributions from New Zealand 2.3. Traits studied
White, Californian, Checkered Giant, etc.),
pur-chased from pet keepers and a few small rabbitries, Total litter size born and born alive, and litter size
served as the foundation stock. The herd was closed and weight at 21 days, 28 days (weaning), and 84
by the end of 1983 with 10 does and three bucks. days (marketing) were measured. Obviously, litter
However, the herd continued to expand to reach a traits measured on day 84 after kindling cannot be
total of 35 breeding does by the end of 1994. During strictly regarded as doe performance traits.
Nonethe-the period of this study, Nonethe-the herd was opened a few less, our interest was in determining the degree of
times to introduce bucks from outside sources (four maternal influence. Of the recorded litters, those
in 1985, two in 1987 and one in 1988) in order to which did not survive to weaning (,4%) were
keep the level of inbreeding low (Rastogi and Heyer, eliminated from the data set. Descriptive statistics for
1992). all traits studied are presented in Table 1. Table 2
The Trinidad climate is humid-tropical with aver- shows the number of animals included and the
age relative humidity varying between 80 and 85%. structure of the data used in the genetic analysis. It
Daily temperature inside the rabbitry varied between should be noted that the number of base animals and
25 and 368C. There is a major dry season from dams are different because missing records were
January to May and a short one in October. The deleted from the data set.
mean annual rainfall is 1700 mm. Animals were
housed in a well-ventilated building with all-wire 2.4. Statistical procedures
cages and had access to automatic waterers and
feeders. A repeatability animal model was employed to
estimate genetic and environmental variances for
2.2. Breeding management litter production traits of the doe by AIREML
(average information restricted maximum likelihood)
An 11- to 14-day remating schedule was followed. as developed by Johnson and Thompson (1995). The
A group rotation breeding scheme was practised model terms included the fixed effect of year–
whereby each doe was randomly assigned to the season–doe parity to represent contemporary
en-same buck for the entire period of her reproductive vironmental subgroups, random doe (additive
mater-life. This scheme used together with pedigree in- nal genetic), permanent maternal (non-additive
ma-formation helped to avoid matings among closely ternal genetic plus permanent environmental),
Table 1
Means, standard deviation and range for maternal performance traits analysed
Trait Mean S.D. Range
Min. Max.
Total litter size born 5.3 1.52 1 12
Litter size born alive 5.2 1.55 1 12
Litter size at 21 days 4.6 1.48 1 9
Litter size at 28 days (weaning) 4.6 1.48 1 9
Litter size at 84 days (marketing) 4.2 1.52 1 9
Litter wt. at 21 days, g 958 168 120 2410
Litter wt. at 28 days, g 1304 222 180 2870
Litter wt. at 84 days, g 6455 846 1100 17200
maternal environmental and possible genetic contri- AIREML analyses, however standard errors for
bution of the litter) effects. For litter weight traits, repeatability could not be directly obtained from the
the model also included litter size at the corre- analysis.
sponding age as a linear covariate. Within each year,
two seasons were considered (dry5January to May,
wet5June to December). There were three doe 3. Results and discussion
parity classes (first, second through fifth, and sixth
through 12th). Additive maternal genetic, non-addi- 3.1. Heritability estimates for maternal
tive maternal genetic plus permanent environmental, performance traits
service sire of the litter, and residual effects were
assumed to be normally and independently distribut- In general, maternal heritabilities for all litter traits
2 2 2
ed with mean 0 and variances As a, Is p, Is s and investigated were low and non-significantly different
2 2
Is e, respectively. from zero (0.02#h #0.12, Table 3). For litter size
Maternal heritability was computed by expressing at various ages, heritability estimates varied between
2
s a as a proportion of the total phenotypic variance 0.06 for litter size at 21 days and 0.12 for litter size
2 2 2 2
(s t5s a1sp 1se ). To estimate repeatability, born alive. At present, consensus exists that
variance components due to additive maternal ge- heritability of litter size in rabbits (and other prolific
netic and permanent effects were summed together species) is generally around 0.1 (Randi and
Scos-2 2
(s a1sp ) and expressed as a proportion of the siroli, 1980; Johnson et al., 1988; Afifi et al., 1992;
total phenotypic variance. Standard errors for ratios Baselga et al., 1992; Ferraz et al., 1992; Panella et
2 2 2
of sa and s p to st were computed from al., 1992; Blasco et al., 1993; Ayyat et al., 1995;
Table 2
Number of animals and records for maternal performance traits
Trait No. of No. of base No. of No. of
a a
records animals sires dams
Total litter size born 1118 80 55 121
Litter size born alive 1120 80 55 121
Litter size at 21 days 1053 79 55 120
Litter size at 28 days (weaning) 1051 79 55 121
Litter size at 84 days (marketing) 1036 79 55 121
Litter wt. at 21 days 1052 79 55 121
Litter wt. at 28 days 1051 79 55 121
Litter wt. at 84 days 1036 79 55 121
a
Table 3
Estimates of heritability, permanent effects, and repeatability for maternal performance traits
a
Trait Statistic
2 2 2
h6S.E. p6S.E. R ss6S.E. sp
Total litter size born 0.0960.07 0.2260.06** 0.30 0.0260.02 1.85
Litter size born alive 0.1260.07 0.2160.07* 0.32 0.0160.01 1.90
Litter size at 21 days 0.0660.07 0.2060.07* 0.26 0.0260.02 1.75
Litter size at 28 days (weaning) 0.0960.07 0.1660.07* 0.25 0.0160.02 1.73
Litter size at 84 days (marketing) 0.0760.06 0.1260.06* 0.19 0.0360.02 1.72
b
h , heritability (additive genetic variance as a proportion of phenotypic variance); p , permanent effects (non-additive genetic plus
2 2 2
permanent environmental effects variance as a proportion of phenotypic variance); R, repeatability (sum of h and p );ss , variance due to
random effect of service sire;sp, phenotypic standard deviation.
b
Adjusted for linear covariate of litter size at the corresponding age. *P,0.05; **P,0.01.
Blasco, 1996; Lukefahr et al., 1996a; Lukefahr and tween direct and maternal genetic effects for
post-Hamilton, 1997; Rochambeau, 1997). Low heritabili- weaning average daily gain in New Zealand White
ty of litter size is further reflected in low response to rabbits.
direct selection (¯0.11 rabbits per litter per gene- In other tropical environments, for example, in
ration; Baselga et al., 1992; Poujardieu et al., 1994; Tanzania, Mgheni and Christensen (1985) reported
Rochambeau et al., 1994; Gomez et al., 1996). realised heritability of 0.19 for 112-day body weight
However, other studies (Mgheni and Christensen, from a two-way selection experiment for litter size;
1985; Khalil et al., 1987, 1988; Krogmeier et al., in Ghana, heritability for individual fryer body
1994) reported moderate to high heritability values, weight at 90 days was 0.42 (Lukefahr et al., 1992);
but these estimates were associated with rather high and in Hawaii, heritability for 56 to 84 days average
standard errors. daily gain was 0.48 (Moura et al., 1997). In Brazil,
Maternal heritability values for litter weight at 21 Ferraz and Eler (1994) reported direct heritabilities
days, 28 days, and 84 days were 0.08, 0.09, and 0.02, of 0.26 and 0.17 for individual body weight at 11
respectively. These estimates, except that for litter weeks of age in New Zealand White and Californian
weight at 84 days, correspond well with those of fryers, whereas maternal heritabilities were 0.00 and
Randi and Scossiroli (1980), Afifi et al. (1992), 0.08, respectively. Unfortunately, in the present
Lukefahr et al. (1996b), and Lukefahr and Hamilton experiment, individual fryer 84-day body weight
(1997). Other researchers have reported a broad records were not available to provide an estimate of
range of heritability estimates (0.03 to 0.57) for litter direct heritability.
weight at weaning (Garcia et al., 1980; Khalil et al.,
1987; Johnson et al., 1988; Moura et al., 1991; 3.2. Repeatability estimates for maternal
Ferraz et al., 1992; Panella et al., 1992; Krogmeier et performance traits
al., 1994; Ayyat et al., 1995). Litter weight at 84
days, having the lowest maternal heritability of 0.02, Repeatabilities for litter size at various ages were
may best be explained by the expected increasing generally moderate, ranging from 0.19 for litter size
importance of the genetic contribution of the progeny at 84 days to 0.32 for litter size born alive (Table 3).
in conjunction with the diminishing influence of the There was a slight tendency for repeatability of litter
dam. It is also possible that a negative covariance size to decline with age of the litter. Repeatability
between direct and maternal genetic effects may values for litter size in this study tended to be close
explain the low heritability. McNitt and Lukefahr to or above the maximum of the range of values
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