Doctoral Degrees (Plant Sciences)

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Browsing Doctoral Degrees (Plant Sciences) by Advisor "Cairns, J. E."

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    Evaluation of genetic gain and diversity in CIMMYT Southern Africa hybrids and open pollinated varieties tested in regional trials from 2000 to 2010
    (University of the Free State, 2014-06) Masuka, Benhildah Pamhidzai; Labuschagne, M. T.; Cairns, J. E.; van Biljon, A.
    English: Maize is a staple food crop in ESA, making it a priority in terms of production. Farmers face challenges in production when maize crops fail in the face of drought, limited or no access to fertiliser or lack of capital to purchase the fertiliser and when there are disease outbreaks. CIMMYT embarked on a drought breeding programme in 1975 that spread to Southern Africa in the 1990s. Since 1997 CIMMYT ESA has focused on producing drought tolerant varieties but incorporated into the selection screening for high yielders under optimal conditions and low N stress tolerance and MSV, GLS and ET disease resistance including. A study to evaluate 67 hybrid best yielders selected from an 11 year period covering 2000 to 2010 for genetic gain in grain yield, random and managed drought stress tolerance, low N tolerance and disease (MSV and ET) resistance was conducted from the 2011/12 main season to the 2013 off season. Parental lines of the 67 hybrids were evaluated for genetic diversity. Sixtyseven CIMMYT ESA hybrids released from 2000 to 2010, checked by three commercial hybrids, were evaluated for genetic gain in grain yield and secondary traits in a multi-site alphalattice designed experiment over two seasons. The hybrids were evaluated across 14 optimal sites in Zimbabwe, Zambia, Malawi, Kenya and Uganda; seven random drought sites in Chiredzi and Kadoma (Zimbabwe) and Potchefstroom in South Africa; four managed drought sites in Chiredzi, Zimbabwe and Kiboko, Kenya; five low N sites in Harare, Zimbabwe and Kiboko, Kenya and three MSV sites all in Harare, Zimbabwe. Significant gain in grain yield and stress tolerance was realised in the CIMMYT ESA maize breeding programme under optimal and stress conditions. Genetic gain in grain yield of 109.4 kg ha-1 year-1 (1.4% year-1), 20.9 kg ha-1 year-1 (0.62% year- 1), 141.3 kg ha-1 year-1 (2.2% year-1), 22.7 kg ha-1 year-1 (0.85% year-1) and 32.5 kg ha-1 year-1 (0.85% year-1) were realised under optimal, low N, MSV, random drought and managed drought stress respectively. This gain in grain yield was associated with phenological and physiological changes. The season length showed a tendency to increase across all environments at rates ranging from 0.048 d year-1 to 0.134 d year-1 but did not significantly change. The season length of the new hybrids was still within the length of the local checks, meaning that the new hybrids still fit in the normal season. The new hybrids had shorter ASI under all environments that insured better pollination and grain set. An increase in the number of ears per plant was recorded across all environments and this positively contributes towards increased grain yield and reduced barrenness. New hybrids were shorter in all environments except managed drought stress. A decrease in plant height indicate improved dry matter partitioning to the ear for ear development, silking, pollination, grain set and grain development and filling. Under random and -managed drought, low N and biotic stress newer hybrids were taller showing normal growth under stress while the old hybrids were stunted. Senescence was lower in new hybrids across all environments except managed drought. There was no change in the number of days to mid-anthesis. MSV and ET resistance or tolerance was higher in the new hybrids that had low disease scores and yielded better than the old hybrid. Overall the genetic gain in yield, drought stress tolerance, low N stress tolerance and biotic (MSV and ET) tolerance over the 11 years from 2000 to 2010 was high. The diversity of the parental lines was relatively low with genetic distances ranging from 0.006 to 0.4005 with most distances ranging from 0.2000 to 0.4000. Considering the 54 parental lines were from the CIMMYT Southern Africa breeding programme only, and that the lines are a subsample of the breeding programme being parents of a sample (only 67) of the produced CIMMYT hybrids, the diversity was narrow because the lines are from the same geographical origin selected in a defined system for adaptation to the local environment and needs. A few lines (four) out of the 54 were used more frequently in most of the hybrids. The same lines were parental lines for some of the other 50 lines. Despite the wide range of lines, using a few lines frequently in formation of most of the hybrids can potentially narrow down the diversity of hybrids in the region if the hybrids as the best performers, are disseminated to farmers. This will pose a risk in the event of outbreaks of new pests and disease. The CIMMYT ESA maize breeding programme for drought tolerance realised significant gains in yield and stress tolerance providing farmers with better yielding varieties that are more resilient to stresses.