Use of exotic germplasm to enhance the performance of local maize
Ndoro, Oswell Farayi
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Exotic maize germplasm has been used minimally in most maize breeding programmes in Zimbabwe and sub-Saharan Africa (SSA). The major reasons for this include challenges in adaptation and the general tendency by breeders to shun the dilution of their elite breeding material. Breeders often prefer the easier and more predictable option of recycling their elite materials. This has resulted in the loss of genetic diversity, development of breeding bottlenecks and subsequent possibilities of stagnating or deteriorating yield gain. The cost and time constraints, the generally high expectations imposed by industry, and the huge capital outlays required to introduce exotic germplasm further discourages its inclusion in local breeding programmes. Traditionally in most breeding programmes in SSA, exotic germplasm is incorporated through introgression with backcrossing to produce modified local inbred lines. The modified local inbred lines with minor exotic components are then used in the production of three-way hybrids for commercialisation. Only minor modifications to the elite germplasm are accepted by most breeders. The usefulness of an inbred in any breeding programme is dependent upon its performance in combination with other inbreds. In this study, the usefulness of inbred lines was investigated through the production and evaluation of F1 single-cross hybrids and F1 three-way hybrids. The general aim of this study was to illustrate quicker and easier ways of identifying usable exotic inbreds in local breeding programmes. The yield per se performance of each hybrid in different stress environments was used as the major reference for selection. This study also challenged and allayed the breeders’ fears concerning the use of exotic germplasm by identifying superior marketable hybrids without going through the lengthy process of backcrossing. Furthermore, the study demonstrated the huge potential of local x exotic crosses as sources of multiple pedigree starts. All hybrids in this study were evaluated in varied stress environments approximating the local farmers’ conditions of low phosphorus, low nitrogen, random stress, high density and optimal conditions. Two hundred and fifty temperate inbreds with expired Plant Variety Protection (ex-PVP) certificates from the United States of America (USA) were crossed with three CIMMYT single-cross testers: CML539/CML442 (A tester), CML444/CZL068 (B tester), and CML312/CML444 (AB tester) to produce three-way hybrids which were evaluated over eight sites. The best inbreds, which were identified for enhancing yield from heterotic group A (SS group) were LH159, LH214, LH23HT, LH213 and MM402A. The best combiners from heterotic group B (NSS group) were HB8229, W8304, LH198, PJH40 and LH190, and from the unclassified lines were PHR58, WIL500, PHK35 and ICI441. The Griffing diallel model 1 method 4 mating scheme was used to evaluate 18 local inbred lines in a local x local diallel scheme and nine selected exotic inbred lines in an exotic x exotic diallel scheme. The North Carolina design II (NCII) was used to evaluate 18 selected local inbreds in combination with 12 selected exotic lines. Highest yielding crosses from the local x local diallel were, L3 x L6 and L4 x L14, and from the exotic x exotic diallel were E7 x E1 and E1 x E9. Inbred lines with the highest GCA from the diallels were L16, L4, E1 and E9. The best crosses from the NCII were N28 x N16 and N21 x N4. Local inbred lines N19, N28 and N30 and exotic inbred N3, N8 and N16 had the highest significant positive GCA effects. Inbred lines from Mexico lowland tropics and Kenya produced the best hybrids in combination with local lines, suggesting them as the most promising future sources of usable germplasm. The leveraging of local single-crosses using exotic tropical germplasm produced 1860 hybrids which were evaluated across eight sites. The outstanding combinations of (local x local) x local hybrids were DJH141028, DJH153523, DJH152318, DJH152580, DJH166030, DJH167263, and DJH168087. The best combinations of (local x local) x exotic hybrids were DJH161178, DJH152183, DJH152552, and DJH168068. The (local x local) x exotic crosses produced equally competitive hybrids as compared to the (local x local) x local. The highest heterosis was generated between the combination of IITA inbred lines and CIMMYT single-crosses, identifying IITA germplasm as the most promising source of tropical exotic germplasm. Overall, this study identified exotic inbred lines that can be used in local breeding programmes to produce hybrids directly and provides an initial step in a possibly bigger and even more comprehensive screening and evaluation programme that can be funded to create a database for the performance of exotic inbred lines in local breeding programmes.
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