Doctoral Degrees (Animal, Wildlife and Grassland Sciences)

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Browsing Doctoral Degrees (Animal, Wildlife and Grassland Sciences) by Subject "Africaner cattle -- Breeding"

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    Genotype x environment interaction in Afrikaner cattle
    (University of the Free State, 2005-05) Beffa, Luigi Mario; Van Wyk, J. B.; Erasmus, G. J.
    English: 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 importance 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-crossover 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 –0.57). 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 sires’ 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 sires’ 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.