Genetic studies of secondary traits and yield heterosis in maize under high and low nitrogen conditions, incorporating farmer perceptions and preferences for varieties.
In Tanzania, 85% of the population depend on maize for food security; it provides 60% of their dietary calories and about 50% of their protein. It is not only a source of food security, but is also a cash crop on which they depend as a source of income. Small-scale farmers contribute over 80% of Tanzania’s total maize production. Of this, 50% is produced in the Southern Highlands agro-ecological zone, where the crop is grown under low-fertiliser input conditions — especially nitrogen (N), which compromises grain yield. Other constraints include the use of unimproved varieties and lack of access to other technologies. Any threat to maize production in this zone may impact negatively on food security and therefore the livelihoods of all Tanzanians. The major objectives of the present study were to document farmers’ perceptions of their preferred elite varieties and their production constraints and to determine the genetic effects and heterosis of maize hybrids among local (Tanzanian) and exotic inbred lines under low and high N conditions. Relationships among secondary traits in these hybrids were also studied under both N conditions. This knowledge is very important for developing strategies for breeding maize cultivars that are high-yielding, early-maturing and adapted to the intermediate altitudes of Tanzania. Prior to undertaking the genetic studies, formal questionnaires and informal surveys, such as focused group discussions were conducted in eight maize farming communities in the Mbeya region, involving 214 randomly selected farmers. In order to determine the genetic effects, six generations that included parents P1 and P2, F1 and F2, and backcross progenies BCP1 and BCP2, were generated from two of the crosses with contrasting inbred parents for the traits under the study. These crosses included divergent parents for leaf chlorophyll concentration and one cross of each selected for plant and ear heights, number of leaves above the ear and number of kernels per ear. The six generations of each cross were evaluated in a randomized complete block design — with two replications under high and low N conditions (120 and 60 kg N ha-1, respectively) — in separate trials that were conducted side by side at one location for two seasons. To determine the genetic effects governing the traits, generation mean analyses were performed with SAS computer software on the data collected under both N conditions. Forty-eight F1 hybrids were developed from local and exotic inbred lines from CIMMYT (Southern Africa) and IITA (West Africa) to form six hybrid sets. The experimental hybrids, plus six check varieties, were evaluated in a 9 x 6 alpha lattice design, with two replications under both N conditions. Heterosis for yield was measured under both conditions. Relative effects of low N on yield and other traits were calculated. Multiple trait linear regression, correlations, and path coefficient analyses were performed to determine the associations among secondary traits and grain yield. The survey revealed a marked variation among the farmers in background training and experience, access to new technology and preferences for maize cultivars. They required maize cultivars that: • matured in two months (extra early); • had fast kernel dry-down rates; • stayed green longer upon maturity; • were drought tolerant; • had large cobs; and • had high single plant yield that was used to select seed for the next planting. In addition to outside influences helping to develop elite varieties that incorporate these traits, any improvement made to the farmers’ socio-economic situation through poverty reduction might also impact positively on yield. The study established that different genes govern leaf chlorophyll concentration character after mid-silking through to physiological maturity. The ratio of fixable (additive and additive x additive) to non-fixable (dominance, additive x dominance and dominance x dominance) genetic effects was 74% to 26% under HN and 35% to 65% under LN in the cross T20 x C58. In sharp contrast, the non-additive genetic effects were predominant in the cross T20 x NG8, whose ratio of fixable to non-fixable effects was 37% to 63% under HN and 20% to 80% under LN. It is suggested that the fixable genetic effects with N regimes is genotype specific. Additive genetic effects prevailed during early grain filling stages, whereas non-additive gene effects were preponderant during the later grain filling stages for leaf chlorophyll concentration. It was found that the nitrogen conditions in which the trait and crosses were evaluated influenced a number of factors, including genetic effects on the plant height, ear height, number of leaves above the ear and the number of kernels per ear. Non-fixable genetic effects predominated for plant height in the cross T20 x NG8, with negligible epistasis under both N conditions. For ear height in the same cross, the dominance effects had 90% of the non-fixable effects under HN, but both fixable and non-fixable effects were almost equal under LN. Fixable genetic effects controlled the number of leaves above the ear in the cross NG2 x C3 under both N conditions, although the frequency distribution curves were discontinuous and discrete, which suggested the predominance of a few major genes operating in an additive manner. The number of kernels per ear was controlled by non-fixable effects under HN, while the fixable gene effects conditioned this trait under LN. About 20% additive x additive effects and 30% dominance x dominance effects prevailed for the same trait under high and low N, respectively. Recurrent selection is suggested to improve the cross where it was found that fixable genetic effects prevailed, whereas reciprocal recurrent selection may be effective where both fixable and non-fixable genetic effects were about equal. For the hybrid-oriented programmes, it was found that selection, accompanied by inbreeding, could be used, firstly to exploit the additive genetic effects, and secondly, to do the same to non-additive effects such as dominance and positive epistasis, through the creation of hybrids between the inbred progenies. Results from the heterosis study indicated that grain yield could be improved by 30%, if recommended fertiliser rates were used. Maize farmers lose about 20% to 30% of grain yield by applying less N. Generally, hybrids that involved one or both local inbred parents exhibited a higher tolerance towards low N for yield than exotic x exotic inbred combinations. The study revealed three types of hybrids with general adaptation and specific adaptation to different N conditions. Four hybrids displayed high yield across N conditions and two exhibited high yield under low N. The other two hybrids were outstanding under high N. Application of low N fertiliser rate did not affect final calendar physiological maturity and kernel moisture content at the harvest of hybrids. However, low N reduced flowering dates by four days, an average of 18% to 25% for leaf chlorophyll concentration, quickened kernel dry-down by a proportion of about one to nine, and increased duration of grain filling by two to four days. Hybrid combinations involving West Africa inbred lines possessed genes for early maturity. Early- and high-yielding hybrids to be grown under low N conditions may therefore be developed if inbred lines with high grain yield potential are crossed with inbreds from West Africa. Generally, hybrid sets displayed higher standard heterosis under LN, compared with HN conditions, suggesting that the use of hybrids in low N input environments may improve maize grain yields and therefore have a positive impact on household food and cash security. The results demonstrated that yield can be improved under LN by increasing the number of ears per plant, whereas under HN, the yield could be raised by increasing the number of kernels per ear. Three factors — plant and ear heights, plant stand at harvest and the number of kernels per ear — showed a consistent, significant positive correlation with yield across N regimes and seasons. The grain fill duration and mean leaf chlorophyll concentration were strongly positively correlated but in most cases they correlated negatively with other traits. Strongly positively correlated traits suggested indirect selection and a separate selection programme to that of negatively associated traits. Direct effects to grain yield were higher under LN compared with HN for kernels per ear and plant height. The indirect effects to grain yield were higher under HN than LN. Under HN, grain yield was significantly reduced by plant stand at harvest via ears per plant (-0.418), followed by mean chlorophyll concentration through 50% anthesis date (-0.376), whereas yield could be raised by plant height via 50% anthesis date (0.294), then by kernels per ear via plant stand at harvest (0.229). Furthermore, it would be simple to raise dry matter accumulation, as the vegetative phase could be reduced and grain filling duration increased due to the fact that leaf chlorophyll concentration and duration of grain fill correlated negatively with flowering dates. It was established that breeding, both for high grain yield potential and earlier-maturing maize cultivars for production under low soil N in tropical maize, was possible — and that varieties that performed above average, under both N conditions, could be developed. The identified significant relative yield loss of about 30% that was observed when maize hybrids were grown under low soil N may justify the need for development of maize hybrids that are adapted to low soil N conditions. For the new hybrids to meet farmers’ requirements in the Southern Highland Zone of Tanzania, the ideotype (preferred variety) should have a combination of the following traits: • functional leaf chlorophyll concentration until around physiological maturity; • high grain filling duration; • many viable kernels per ear maintained until physiological maturity; • many ears per plant (i.e. prolific); and • non-dwarf plant stature.
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