6. Conclusions

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With the aim of studying the influence of leaf area development on dry matter yield and forage quality of early and mid-early maturity varieties of maize, two year experiments were conducted in 2002 and 2004 at Berge research station, belonging to the Institute of Crop Science, Faculty of Agriculture and horticulture, Humboldt-University Berlin.

Temperature sum (GDD) was accumulated from the day after sowing to the day of harvest to determine the various phases of development.

Maintaining green leaf area at harvest especially during adverse environmental conditions, like water deficit and high temperatures, was seen as an indicator of stay green trait. Comparisons were made between varieties within the groups and between the years for high green leaf areas at harvest and dry matter yield.

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Since the objective of the experiment was to find out to what extent changes in leaf development during the course of plant growth and development would influence dry matter yield and forage quality, varieties were grown to full physiological maturity before harvest. Timing for optimum harvest that would make for best forage quality results was an integral part of the experiments. From the results obtained in the two years research work, including additional information on results in 2004, the following conclusions are made:

Between the varieties and years significant LAI differences existed. The maximum leaf area was reached at the phase of silking in both maturity groups. Maximum LAI lied between 2.8 and 4.6. LAI increased with increase in leaf number and leaf size during the vegetation period. LAI started to decline with onset of senescence due to aging of leaves from the lower leaf generations upwards. LAI in both maturity groups remained high in 2002 at harvest time, which also corresponded to the high green leaf area at harvest because of available water and cooler growing conditions. LAI fell sharply after silking in 2003 with a sharp reduction in leaf area and leaf number due to water deficit and heat.

Leaf area development during the vegetation periods was governed by temperature sums (GDD) for each year. The required temperature sum for each phase of development was reached earliest in 2003, which also corresponded to early attainment of higher leaf area and leaf area index at a given time in 2003 than in 2002 and 2004. Silking dates differed between the years in association with differences in temperature sums, with earliest silking in 2003, with a 4-day difference in beginning of silking dates for year 2002 and 2003. Although rainfall distribution throughout the vegetation period was lower in 2003 (average mm) than in 2002 (average mm), also with higher temperature means than 2002, higher leaf area and leaf area index were produced in both maturity groups in 2003 than in 2002 before drought. This also explains the effect of increased temperatures in accelerating rates of physiological processes in plants, including leaf area development, sometimes shortening the duration of the processes involved.

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Although less water was received in 2003 than 2002 during the vegetative periods, the plants seemed to have used it more effectively during the vegetative growth, reflected in higher attainment of leaf area and leaf area index in 2003 than in 2002. This also illustrates that higher water requirement is needed during the early generative period of development than in the vegetative phase. Achieving high leaf area development alone during the vegetative period in 2003 was not enough to guarantee higher dry matter yields and good forage quality under acute water deficit during silking and post silking phases.

At the onset of water deficit in mid July of 2003, all varieties in both maturity groups had attained maximum leaf area (approximately 4300 cm² for early and 4800 cm² for mid-early core varieties) and leaf area indices of 3.3 and 3.8, respectively. Water deficit hastened senescence rates and effectively reduced green leaf area and leaf number. Using cob leaf as reference position in relation to leaf generation of the plants, in 2002, all varieties in both maturity groups maintained a minimum of 4 leaves below the cob leaf green and active at harvest. In 2003, drought-imposed senescence affected cob leaves, reduced leaf areas of those leaves above the cob leaf, with the result that green leaf area and leaf area index of varieties in both maturity groups were greatly reduced in comparison to the year 2002.

Overall vegetation period in 2003 was reduced by drought and high temperature, especially the post silking phases. Harvest dates were influenced by environmental conditions, therefore, in addition to estimation of harvest dates based on temperature sum, a combination of methods and approaches are required to combat the uncertainty of time, intensity and duration of unfavourable conditions. To determine optimum harvest time, whole plant dry matter content is normally used, which must fall within the required values for each maturity group. However, under unfavourable growing conditions, like in year 2003, of water deficit and high temperature, plant leaf generation could also be used as a check for timing harvest. It was adviseable to harvest when at least two leaf generations below the cob leaf were still green. By the time the cob leaf was also dry, dry matter content had already risen to 39-40 %. Under drought-imposed leaf senescence in 2003, dry matter content increased rapidly with increased loss of leaves to senescence. However, in locations susceptible to water deficit, selection of varieties for high leaf numbers is not adviseable, because of greater demand for water. In location Berge, varieties with leaf numbers between 14 and 16 and maximum leaf area index of 3 are recommendable. Variety FAO 750, for instance had total leaf number of 19 (Appendix 16) but in both favourable and unfavourable growing conditions of 2002 and 2003 was caught up requiring longer vegetation period (2002) and by drought (2003). Afterall varieties with high leaf numbers lost more leaves to senescence than those with fewer leaves.

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Specific leaf area (SLA), an indicator of leaf thickness, was an important parameter in determining leaf generation in a plant. Leaf area is a product of leaf dry weight (g) and specific leaf area. Specific leaf area was higher in 2003 in both maturity groups than in 2002. Cob leaf positions indicated 15 m² kg-1 in early and mid-early maturity groups in 2002 and 18 m² kg-1 in both maturity groups in 2003. Similarily, leaf generations above and below the cob leaf showed in both maturity groups in 2002 lower values than in 2003. Leaf thickness seemed to increase more from the cob leaf downwards than upwards. Both maturity groups had similar patterns of specific leaf area in both years.

Plant biomass composition (cob, cob sheath, stem and leaf) expressed in percent of each variety varied between 2002 and 2003. Cob dry weight (%) varied inversely to stem and leaf dry weights. Higher cob dry weight (%) in 2002, which lied between 52.8 and 63.5 % in early and 52 and 59.4 % in mid-early maturity check varieties corresponded to lower stem/leaf dry weight (%). However, in 2003, higher stem/leaf dry weights resulted in lower cob dry weight of between 45.0 and 50.9 % for early and between 40.6 and 53.2 % for mid-early maturity check varieties.

Dry matter yield was positively influenced by leaf area development under favourable environmental conditions, giving higher yields in 2002, but under unfavourable environmental conditions of water limitation and high temperatures in 2003, dry matter yield was greatly reduced in all maturity groups, irrespective of how well leaves had developed during the vegetative phase. The differences in dry matter yield and dry matter content between 2002 and 2003 can be seen as the differences in the green leaf area and numbers maintained by the varieties at harvest, which resulted from availability of water and temperature levels.

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Varieties within the maturity groups (check varieties) differed significantly between the years in green leaf area at harvest and dry matter yield. Varieties in both maturity groups reacted differently in relation to drought. Early check varieties had high green leaf area but lower yield or vise versa (Figure 23 and 24). This probably indicated genotypic variations among the varieties and yield potentials.

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