A line x environment interaction study with grade Afrikaner cattle was established at
Matopos Research Station, Zimbabwe, in 1956. From a grade pool of 200 cows, two
selection lines of 100 cows were reared in different management environments. The
non-supplemented (NS) line relied on the range throughout the year and was mated to
calve with the onset of the rains (December to February). The supplemented (S) line
was offered protein-rich supplements during the dry season and mated to calve prior
to the onset of the rains (October to December). Bull selection based on weight at
weaning was to develop lines adapted to their respective environments.
After approximately 2 generations of selection, lines were sub-divided into 4 sub-lines,
consisting of 75 cows each. One sub-line remained within each environment as
a control; the remaining sub-lines were interchanged between environments. Bulls
were selected within control lines, while replacement heifers were generated within
sub-line. Heifers were selected on weight at mating and replacement rates were
dictated by the sub-line with the least number of suitable animals (>265kg). Data
collected over a 40-year period representing 6 generations of selection were analyzed
to determine a) genetic and environmental factors influencing calf growth and
survival and cow fertility, b) direct and correlated responses to selection and c)
presence and importance of genotype x environment interactions.
Productivity in the S environment, in terms of calf growth and cow fertility, was
superior compared with the NS environment. While there were no differences in birth
weight, calves born early in the S environment were estimated to be 90kg (33%)
heavier at 18 months than calves born late in the NS environment. The performance
of calf growth from birth to 18 months in this study has emphasized the importanc e of
matching animal physiological status with seasonal changes in the sub-tropics. Given
the significant and consistent detrimental effects of previous lactation status of the
cow on fertility and calf growth, it is recommended that this effect be considered in
routine evaluations.
The high generation intervals (6.7 and 5.5 years pre- and post-crossover) reflect the
cow age at first calving (4 and 3 years respectively for the two periods). Direct
cumulative selection differentials for weaning weight were very low (0.08 and 0.05
standard deviation units/generation for the S and NS lines respectively). The
correlated cumulative selection differentials for pre-weaning weight gain were
negative for the pre -crossover period and positive for the post-crossove r period, and
together with the low direct cumulative selection differentials suggest that significant
attention was paid to secondary characteristics. Consequently, selection responses
were low (2.35+0.045 and 2.22+0.047kg/generation for weaning weight for the S and
NS lines respectively). All correlated direct genetic trends were positive and linear,
while maternal genetic trends were very low and negative for the S line.
Direct and maternal (in parenthesis) heritability estimates for weights at birth,
weaning and 18-months were 0.39+0.03 (0.14+0.02), 0.19+0.03 (0.21+0.04) and
0.36+0.06 (0.15+0.04) respectively. Apart from the non-significant direct-maternal
genetic correlation for birth weight, other correlations were all moderately negative (-0.35
to â0.57). With the exception of correlations with birth weight which were
variable and often close to zero, direct and maternal genetic correlations among traits
were high, particularly for traits measured at ages close to each other
The average incidence of calving success (CS: including abortions at term and
stillbirths) was 68%. Heritability and repeatability (in parenthesis) estimates for CS
and calving date (CD: with a 21 day penalty for censored records) were 0.08+0.02
(0.10+0.02) and 0.09+0.02 (0.17+0.02) respectively. Service sire was important for
both traits, but only accounted for <2% of the total phenotypic variation. Favourable
genetic trends were shown for both traits (1.2+0.14% and 0.8+0.13%/generation for
CS for the S and NS lines respectively, and -0.8+0.09 days/generation for both lines
for CD). The genetic correlation between CS and CD was high (-0.95) and the
correlation of siresâ estimated breeding values (EBV) indicated that a one-day
improvement in CD EBV is associated with a 1% increase in CS EBV. However,
correlation estimates of siresâ EBV between measures of fertility and growth were all
unfavourable, but not-significantly different from zero.
There were no indications of line x environment interaction for all the growth traits.
A significant interaction was, however, manifested for CS and was due to the
markedly poorer performance of the S line cows in the NS environment. While no
sire x environment interactions were manifested, significant sire x year interactions
were found for all the growth traits and for CS, which serves to highlight the
tremendous seasonal differences experienced in the sub-tropics. Therefore there is a
need to consider selection for genotypic stability, particularly for the more extensive
farming environments.
This study revealed that concomitant genetic improvements in both growth and
fertility were attainable. It is recommended that fertility and calf survival be included
in routine breed evaluations. There is however an urgent need to implement data
collection strategies to ensure that all calving activities are comprehensively recorded,
not least the adequate definition of contemporary groups.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-07042006-082026 |
| Date | 04 July 2006 |
| Creators | Beffa, Luigi Mario |
| Contributors | Prof JB van Wyk, Prof GJ Erasmus |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-07042006-082026/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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