5. Discussion

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The main objective of the experiment conducted in the years 2002 and 2003 in location Berge with silage maize was to study the effect of leaf area development on dry matter yield and forage quality. During the vegetation period leaf area and leaf area index were measured by manual and LAI 2000 plant canopy analyser methods (harvest time). LAI 2000 was also used to measure light interception by plant canopy. Leaf senescence and stay green character of varieties and their effects on yield were studied. Plants were harvested to compare plant biomass production. At final harvest dry matter yield and forage quality were analysed using near infrared reflectoscopy method.

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The following results were obtained from the experiments in the years 2002 and 2003.

5.1. Temperature sum (GDD)

According to figure 2 in which temperature sums (GDD) during three successive years 2002, 2003 and 2004 growing seasons of forage maize at location Berge were compared, year 2003 attained temperature sums throughout the vegetation period earliest, this was indicated at silking phase, which was earliest of all the years indicated. At GDD 800, which corresponds to silking phase, was a 4-day difference between year 2002 and 2003. Its effect was also reflected in other physiological processes like leaf area (LAI) development (figures 3 and 4). Earlier expansion in leaf area and LAI in year 2003 in both maturity groups than in years 2002 and 2004 were a result of differences in accumulated temperature. Figures 3 and 4 also indicate that highest leaf area (LAI) was attained at the time of complete leaf area expansion compared to years 2002 and 2004. The above facts seem to suggest that year 2003 had more favourable conditions for leaf area development and growth, before the onset of water deficit than in 2002 and 2004. Temperature sum played an important role in overall development process. Flowering of the maize varieties started five days earlier in 2003 than in 2002. Earlier accumulated temperature sum increased leaf development rate and earlier attainment of maximum leaf area, which led to a higher light interception rate and consequently earlier dry matter accumulation in 2003.

5.2. Leaf area and leaf number

Initial leaf development was similar in that it was slow in all varieties in both years. At about 30 days after emergence, a sharp increase in leaf area development commenced with differences in rates of leaf development in early and mid-early maturity groups becoming defined (table 12 and 13). Early maturity group attained maximum leaf area at earlier dates than mid-early group. Mid-early group had slower leaf area development, but attained higher leaf area than the early group. Increase in leaf area in all varieties was a result of increase in leaf numbers and leaf sizes within the leaf generation of individual plants. Results of measurements of leaf area in leaf generation of individual plants indicated that the largest leaf sizes lied within the middle portion of overall plant leaf generations. In most varieties, both early and mid-early group, cob leaf had the largest leaf area, or at least a leaf above or below it. Cob leaf and 2-3 leaves above and below it accounted for nearly 70 % of total plant leaf area in both maturity groups. Loss of leaves due to senescence was low in 2002 with all varieties maintaining at least 4-5 green leaves below cob leaf at harvest time. In 2003 due to water deficit in July and August most leaves below cob leaf and often times, including cob leaves were lost to drought-imposed senescence. Senescence rate was also increased from top down the leaf generations in 2003 than in 2002 because of drought. As a result in 2003 leaf area (size) and total leaf numbers of individual plants (varieties) were greatly reduced due to drought-imposed senescence. Reduction in green leaf area meant reduction in ability to intercept light which was necessary for photosynthesis and hence affecting yield and quality parameters.

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Leaf area development was faster in 2003 than in both years 2002 and 2004. Leaf area of individual plants (varieties) varied from year to year as well as the leaf number. As figures 15 and 16 indicate, leaf areas and numbers in nearly all check varieties in both maturity groups were larger in year 2003 than in years 2002 and 2004, before severe drought in August 2003 set in. This seemed to indicate that the conditions for growth in year 2003 were more favourable during most of the vegetation period, especially from germination upto silking. Water deficit set in after all the leaves were already fully expanded. Under environmental conditions of Berge with limited water supply during plant growth, high leaf areas and leaf numbers are disadvantageous to maize production for silage. The optimum range of leaf numbers (generation) under Berge conditions for both maturity groups falls between 14 and 16.

Varieties with more leaf numbers had also higher leaf areas and consequently higher leaf area indices. This, however, was not a general rule because some varieties had small individual leaf areas, which accounted for lower average leaf areas of such varieties, although they had high leaf numbers. Variety PR39H32 of early maturity group for instance had an average of 14 leaves per plant, lower than most varieties within this group, but had relatively larger leaf areas of individual leaves compared to other varieties, which accounted for high leaf area index of 4.1. On the other hand, variety Eurostar (mid-early) had 16 numbers of leaves, with leaf area index of 4.5. However other factors like leaf angle, plant height might have also determined the varying values of LAI among varieties. Comparing experimental years 2002 and 2003, leaf numbers, leaf areas and leaf sizes were greatly reduced in 2003 due to water deficit which occured in August 2003. The differences were expressed as differences in maximum total plant leaf area and total green leaf area at harvest (figures 15 and 16).

Maximum sum of leaf area per plant in early maturity check varieties ranged between 4000 and 5000 cm², while total leaf number was between 14 and 18 (figure 15). At harvest time, the maximum sum of leaf area was reduced to between 1500 and 2500 cm², total leaf number was between 5 and 8. Leaf senescence left between 9 and 12 dried leaves per plant. Check varieties with higher number of leaves lost more leaves through senescence than those with fewer leaf numbers. Check variety Pernel for instance, had total leaf number 18, lost 12 to senescence and remained with 6 green leaves at harvest, while check variety Talman with total leaf number of 14, lost 9 and had 5 green leaves at harvest.

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Maximum leaf area per plant in mid-early maturity check varieties ranged between 4000 and 5000 cm², while the total leaf number was between 16 and 18 (figure 16). At harvest, the maximum sum of leaf area was reduced to between 1000 and 2500 cm² and the total leaf number to between 6 and 8. Leaf senescence left 9 – 11 dried leaves per plant. The number of senesced leaves was higher in varieties with higher total leaf numbers than in varieties with lower total leaf numbers. For instance, check varieties Sandrina and Acapulco both had total leaf numbers of 17, lost 11 to senescence and had 6 vital and green leaves each at harvest, while check variety Rivaldo and Flavi, both had total leaf numbers of 16 each, lost 9 and had 7 green and vital leaves at harvest.

In 2002, all varieties in all maturity groups maintained up to 3 and 4 green leaves above and below cob leaf at time of harvest, with cob leaf or those adjescent to it (top or below), having the largest leaf area in most varieties. Senescence was predominantly from below. However, in 2003, due to drought at harvest nearly up to the 11th leaf was dry, including cob-leaf (a majority of varieties had cob-leaves on 9th-12th leaf). As a result of drought-imposed leaf senescence, greater leaf areas and sizes were reduced in 2003 than in 2002.

5.3. Leaf senescence

In the experimental years 2002 and 2003, leaf senescence markedly differed, mainly as a result of water availability to the plants. In 2003, senescence was hastened by water deficit in mid August, which affected not only the rate at which leaves dried out, but also leaf sizes, numbers and duration of senescence. Before the onset of water deficit, all varieties in both early and mid-early maturity groups had lost averagely up to the 5th lower leaf due to senescence. Results on leaf area development showed that all varieties in both maturity groups had already attained maximum leaf area (expansion) before water deficit set in (table 3 and 4). The effect of water deficit was mainly on the grain-filling phase of corn. However, the remaining leaves began to dry out rapidly from both ends of the plant as drought intensified in mid August 2003. There was a drastic reduction in leaf number and area (also size) of individual leaves as a result of drought as compared to year 2002. A comparison between maximum leaf area per plant at harvest and total number of remaining green and vital leaves were made for both years and maturity groups. High total plant leaf area was not necessarily a result of high leaf number of a variety, but in some cases a result of larger leaf sizes (area) of individual leaves. At harvest time, total green leaf area (sum) also did not always correspond to leaf number of green leaves, but to the sizes (areas) of individual green leaves. Leaf senescence seemed to favour varieties (early and mid-early check varieties) with fewer total leaf number. The more leaves (total) a variety had, the more leaves it lost to senescence, especially during the period of extreme water deficit. Water deficit did not only hasten senescence from both ends of the plant, but also caused a shift in leaves with the largest leaf areas (sizes) from middle (cob zone), upwards, above cob zone. The last leaves to dry under extreme drought condition were 2-3 leaves above cob-leaf. The next question to ask is whether these leaves were still photosynthetically active or cosmetically green.

5.4. Stay-green characteristics and yield

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Loss of green leaves to senescence reduces a plant’s capability to intercept light energy that is necessary for photosynthesis. As the assimilation surface is reduced through leaf senescence, dry matter production decreases. One of the most devastating effects on silage maize production is caused by drought-imposed leaf senescence. Therefore, among the major challenges for crop improvement programmes is to develop plants that have an advantage in water-limited environments. Stay-green or delayed foliar senescence, is one of such traits in test for any advantage in yield over non stay-green. During postanthesis drought, genotypes (varieties) possessing the stay-green trait are said to maintain more photosynthetically active leaves than genotypes not possessing this trait (Rosenow et al.1983). Expression of stay-green has been reported in cereals including Zea mays L. (Crafts-Brandner et al. 1984, Rajcan & Tollenaar 1999 a).

Stay-green trait of varieties tried in 2002 and 2003 was expressed as green leaf area at physiological maturity. This could also be deduced from the maximum (total) plant leaf area minus duration and rate of leaf senescence. Stay-green character was harder to distinguish from non stay-green varieties under favourable growing conditions with adequate precipitation in year 2002, than in 2003. Under growing conditions in 2002, senesced leaves of the varieties in both maturity groups tested went through normal aging and death. A narrow margin of percent green leaf area at harvest existed in 2002 within varieties of the same maturity group as well as between varieties of different maturity groups (figures 13 and 14). In 2002, all check varieties in both maturity groups maintained between 90 and 96 % of green leaf area at harvest. In year 2003 however, stay-green trait, due to water deficit were expressed in mid-early and probably in early maturity group. There were greater fluctuations in percent green leaf area at harvest within and between the maturity groups. Percent green leaf area at harvest was between 20 and 55 % in both maturity groups, far less than those of 2002 figures. In both early and mid-early maturity check varieties, those varieties which had high yield under favourable growing conditions in 2002 also indicated better yield under adverse water deficit conditions in 2003. Nearly similar patterns in decrease in yield and reduction of green leaf area at harvest were followed in both maturity groups, but in varying degrees (figures 23 and 24). High green leaf area at harvest was not necessarily accompanied with higher yield. Varieties with higher maturity numbers S 240 and S 250 (LG 3226, Rivaldo Sandrina and Flavi) maintained higher green leaf area at harvest during adverse weather conditions of water deficit (2003) than S 230 varieties (Acapulco and Topper). Within the maturity groups, there were variations in yield and green leaf area at harvest. Variety Acapulco within mid-early group (S 230), with the lowest green leaf area of the check varieties, maintained high yield, while variety Tassilo within early maturity group (S 200), indicated greater tendency to stay-green in water deficit conditions with improved yield. Variety Talman (S 210), with much reduced green leaf area at harvest maintained better yield during water deficit than other check varieties within that group, indicating yield stability under adverse weather conditions. Mid-early maturity group was preferably harvested at a much later date as compared to early maturity group, taking the advantage of wider harvest window in the former maturity group than in the latter (Schmidt 2002). The difference in harvest date between early and mid-early maturity groups was 8 days, 15.08 03 for early and 23.08.03 for mid-early. Earlier harvest date (3-4 days) for mid-early maturity varieties would have shown a better picture in percent green leaf area retained at harvest within and between the maturity groups (Mid-early maturity group was harvested on a much later date due to some technical faults with the harvestor). Dry matter content was higher in mid-early maturity group (mean 43.61 %) than in early maturity group (mean 39.29 %). The vegetation period was shortened in 2003 due to water limitation and high temperature, resulting in earlier harvest dates than expected.

5.5. Percentage green leaf area at harvest

Comparison between years 2002 and 2003 showed that greater green leaf area was retained in year 2002 than 2003 in both maturity groups (figures 13 and 14). Above 90 % of green leaf was retained in 2002, while only 20-55 % green leaf was retained in 2003 in both maturity groups. The great difference in green leaf area retention at harvest in both years is explained by the diverse differences in weather conditions in both experimental years, precipitation (water availability) being the greatest single factor. More precipitation was received during the 2002 vegetation period than 2003 (figure 1). Drought in August 2003 accelerated rate of leaf senescence, thereby quickly reducing green leaf area, size and leaf number. According to figures 3 and 4, the same leaf areas were attained earliest in 2003 compared to 2002 and 2004, by both early and mid-early maturity groups (only core varieties represented in the figures). The highest leaf areas were also attained in 2003 in comparison to 2002 and 2004, before intensive drought in August 2003 set in. In the absence of drought, a longer plateau of maximum and highest leaf area in years 2002 and 2004 might have resulted.

5.6. Leaf area duration

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The general pattern of individual leaf development in both maturity groups was similar, varying mainly in maximum leaf area and leaf area duration. The lower leaves of the plants which emerged during the early stage of plant growth expanded to maximum leaf area between GDD 200 and 400 (figure 25), dried up at a much earlier stage than the rest of the leaves. These were mainly the first 5-6 lower leaves of the plants. Between GDD 600 and 800, all leaves had attained maximum leaf area, which also corresponded to the period of continuous stem elongation, beginning and end of tassel emergence, pollination and flowering. At the phase of silking (maximum leaf area), cob leaf had attained one of the highest leaf areas within the leaf genaration. This fell mostly between 9th and 12th leaf generation. Under normal growing conditions with sufficient precipitation, leaf generation within the cob zone (at least 2 leaves below cob leaf), cob leaf inclusive, are the last to senesce, but figure 25 below depicts leaf senescence as accelerated by water deficit. Senescence affected cobleaves, leaving only 13th, 14th and 15th leaves above cob leaf green, which had comparatively lower leaf areas than those within the cob zone. The difference between maximum leaf area (fully expanded leaves) and leaf area at harvest (green) indicate the rate at which leaf senesced, which also defined the duration or longevity of each individual leaf in GDD (not calculated). The more intense the drought, the steeper was the slope, (the faster was the rate, the shorter was the duration of leaf senescence). Figure 25 is characteristic of individual varieties in both maturity groups in year 2003 under drought conditions. Between GDD 1000°C and 1200°C was seen a great reduction in leaf area in leaves below cob leaf. Rapid reduction in leaf area above cob leaf through senescence was between GDD 1200°C and 1400°C. Grain development and kernel set were affected by rapid leaf senescence under water deficit, dry matter yield was low, dry matter content high as a result. Under favourable growing conditions of 2002, a longer plateau for maximum leaf area and a more gentle slope resulted in increased leaf area duration, higher green leaf area at harvest (functional photosynthetic apparatus), which contributed to improved dry matter yield and dry matter content.

Figure 25:Leaf area development and senescense in two check varieties of early and mid-early group of forage maize having the same number of leaves in 2003 (Tassilo and LG3226)

Figure 25:Leaf area development and senescense in two check varieties of early and mid-early group of forage maize having the same number of leaves in 2003 (Tassilo and LG3226)

5.7. Specific leaf area

Specific leaf area (SLA projected leaf area per dry mass) has become an important variable in comparative plant ecology because it is associated with many critical aspects of plant growth and survival. For instance SLA is often positively correlated with seedling potential relative growth rate (Muller & Garnier 1990, Poorter & Remkes 1990) and leaf net photosynthetic rate (Field & Mooney 1986, Reich et al. 1997, Shipley & Lechowicz 2000), it is negatively correlated with leaf life span (Reich et al. 1992) and palatability to herbivores (Lucas & Pereira 1990). In the experimental years 2002 and 2003 maize varieties in both early and mid-early maturity groups showed similar trends in SLA. Individual plants had cob leaves with the largest leaf area and highest dry weight. While both leaf area and leaf dry weight decreased towards both ends of the cob leaf, SLA increased downwards below cob leaf. Leaves above cob leaf nearly maintained the same level as the cob leaf, except the last 2-3 uppermost leaves, which had higher SLA than the proceeding ones. The last leaf at apex had the lowest dry mass compared to the corresponding leaf area hence a much higher SLA than the proceeding leaves below it. Although there were fluctuating values of leaf area and leaf dry weight up and down the leaf generation, the general trend for the curve was the same in all varieties in both maturity groups, leaves below the cob leaf having higher SLA than those above cob leaf. Most cob leaves or at least 1-2 leaves above or below it registered the highest leaf dry weight. Similar results were also observed with cob leaf areas being highest or at least 2-3 leaves above and below cob leaf. The upper leaves (above cob) had higher leaf dry weight than those below the cob. Similarily, leaves above the cob had higher (larger) leaf areas than those below. SLA at plant level, except for the 2-3 upper most leaves which were relatively small in size (area) and weight that resulted in higher SLA than those below, had a similar trend. The general trend for nearly all varieties was that of increasing SLA from top downwards. Leaves above cob had lower SLA than those below. If SLA indicates ‘leaf thickness’, then ‘leaf thickness’ increases from top to bottom if the last two top-most leaves were exempted, due to their relatively small sizes (and dry mass). According to figs. 17, 18, 19 and 20, SLA was lower in 2002 in both maturity groups than in 2003 (referring only to cob leaf position), at 15 and 18 kg m-2 in 2002 and 2003 respectively. This result possibly agrees with Reich et al. 1992, which stated that SLA is negatively correlated with leaf life span. In 2002, leaf life span of the varieties in both maturity groups were longer, hence lower SLA than in 2003. According to table 17 the leaf area was a product of leaf dry weight and SLA, therefore SLA is an important parameter that can be used to calculate either of the parameters, when the other is known. The table also indicates that these three parameters are useful in roughly determining leaf generation of a plant in relation to cob position. When cob leaf position and leaf generation are unknown, with the three parameters in place, then cob leaf is positioned where SLA is lowest, leaf dry weight highest and leaf area largest. This result agrees with the assessment of Birch et al. (1999), who stated, that SLA is the likely consequence of leaf area expansion and dry matter accumulation in leaves.

5.8. Dry matter yield and dry matter content

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Dry matter yield is an important trait because most production costs are incurred on a unit area basis. Improved dry matter yield often results in more efficient use of plant nutrients.

Dry matter accumulation is closely associated with leaf area development. The development of leaf area is a function of both leaf numbers and leaf size, these factors may change differently, depending on the genetic material involved and the environment in which the plants are grown. Leaf area development differed in both experimental years 2002 and 2003 due to contrasting weather conditions during the vegetation periods. Although the rate of leaf development was faster in 2003 than in 2002 between BBCH 19-65, the normal trend of leaf development after silking was interfered with by water deficiency, which also interrupted the grain filling process, hence affecting both yield and quality. Even though the maximum leaf area and leaf area index per plant were relatively higher in 2003 than in 2002 (average of 4701 cm² and 3.76 for early 4829 cm² and 3.86 for mid-early (2003), compared to 4193 cm² and 3.35 for early and 4514 cm² and 3.61 (2002), leaf area and LAI of green leaves at harvest were comparatively small in 2003 (1609 cm² and 1.29 for early and 1522 cm² and 1.22 for mid-early), 2002 (3852 cm² and 3.08 for early and 3911 cm² and 3.13). In experimental year 2003, adverse weather conditions at the research station of Berge, namely drought stress during the vegetation period, accompanied with high temperature in mid July/August, resulting in earlier than expected harvest in mid August, accounted for relatively low dry matter yield and high dry matter content compared to 2002. The average dry matter yield was 105.0 dt ha-1 for early and 125.4 dt ha-1 for mid-early maturity groups in 2003 (App. 4 and 5) as compared to 176.5 dt ha-1 and 181.5 dt ha-1 respectively, in 2002 (App. 1 and 2). There were significant differences in dry matter yield within early maturity group tested in 2002 and 2003 (13 varieties). This might have been caused by differences in soil textures of the plots in both years. The plot where early maturity group was grown in 2003 was more sandy than that of the previous year. Water retention capability was low and under condition of drought, sandy soil lost water faster than the more sandy loam. This affected the amount of water taken in by the roots, which in turn depended on other factors like root depth of individual variety, total surface area of root hairs available for water absorption. Under such conditions, individual traits of a variety were much more expressed than under normal growth conditions. There were insignificant differences within mid-early group tested in 2002 and 2003 (14 varieties). Core varieties, in 3 years trial between 2002 and 2004 (3 early and 6 mid-early varieties), also showed insignificant differences in dry matter yield between varieties within each group. However, significant differences were found in Year * Variety interaction in both maturity groups. This indicated that yearly changes in environmental conditions, apart from genotypic differences among individual varieties, played a vital role in initiating and directing the course of growth and development among the varieties, which also determined yield and forage quality. Although weather conditions at the research station for both years sharply contrasted, namely one being more favourable than the other, yet the relation of average dry matter yield and dry matter content between the maturity groups did not alter. As it was the case in year 2002, the average dry matter yield and content in year 2003 for mid-early maturity varieties were higher than those of early maturity varieties. However the average dry matter yield for year 2003 of early maturity varieties (105.0 dt ha-1) and mid-early maturity varieties (125.4 dt ha-1) were much lower than for year 2002: early 176.5 dt ha-1 and mid-early 181.5 dt ha-1. The average dry matter content for year 2003 for both early (39.29 %) and mid-early (43.61 %) maturity varieties were higher than the values of year 2002 of 34.4 % and 38.2 % respectively. Significant differences were found in dry matter content in early maturity varieties tested in 2002 and 2003, but insignificant differences in mid-early varieties and core varieties. Dry matter content in both maturity groups exceeded the optimum level required at harvest of 30-32 % for early and 34-36 % for mid-early maturity group. This was a result of high temperatures during the grain filling period which hastened the process, thereby increasing the content above normal.

Dry matter yield was closely linked to leaf area index. However, LAI higher than 3.5 (the average maximum LAI for both maturity groups in 2002 by LAI 2000) was not a guarantee to improved dry matter yield or dry matter content. Year 2002 had one of the good weather conditions under which silage maize could be grown in Berge, under optimum LAI between 3-3.5. Average dry matter yield and dry matter content were higher in mid-early maturity group than in early in both years. Figs. 22 and 23 show higher green leaf area at harvest in 2002 than in 2003, which also corresponded to higher dry matter yield in 2002 than in 2003. However, within each maturity group within each year, high green leaf area did not necessarily indicate greater yield. This means that improvement in dry matter yield under water limited conditions could not be attributed to green leaf area at harvest only. Moreover, there was no confirmation as to whether the visually green leaves at harvest were actually photosynthetically active or just cosmetically green and therefore unable to photosynthezise. The intensity and duration of drought also determined the activity and duration of leaves and their effect on dry matter yield.

5.9. Forage quality

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Quality parameters of silage maize considered in the maturity groups tested were: Starch content, energy content, starch yield and energy yield. Other parameters that were analysed, beside those above included: crude fibre, crude protein, VIVO DOM, enzyme soluble organic substances.

Generally, lower starch content was obtained in 2003 than in 2002 in both maturity groups. Average starch content for all early maturity group tested in 2002 was 34.9 % compared to 27.8 % in 2003, 37.6 % in 2002 and 31.3 % in 2003 for mid-early maturity group (tables 27 and 28). Analysis of variance of varieties tested in both years in both maturity groups (13 of early and 14 of mid-early) indicated insignificant differences in starch content between varieties within each group, but significant differences between the years. Early core (varieties in 3 year trial) varieties however showed insignificant differences within the group and also between the years. The mid-early core varieties showed significant differences between the varieties within the group as well as between the years. The results for early and mid-early varieties tested in 2002 and 2003 showed that year to year differences in starch content were a result of yearly changes in environmental conditions in Berge. In this case, the differences in starch content between 2002 and 2003 were a result of contrasting relatively favourable weather condition of sufficient precipitation (737 mm) and average temperature of 10°C in 2002, compared to (342 mm) and 10°C in 2003, accompanied by drought end July and August. Poor starch fill was a result of unfavourable conditions, like high temperature. Starch content affects energy content, which is one of the determinants of forage quality.

Higher values of energy content were obtained in both maturity groups in the year 2002 than in 2003. Average energy content of early maturity group was 6.52 MJ NEL kg-1 (2002) and 5.87 MJ NEL kg-1 (2003), 6.61 and 5.95 MJ NEL kg-1 for mid-early maturity group in year 2002 and 2003 respectively (table 29 and 30). Analysis of variance showed insignificant differences in energy content between varieties within mid-early maturity group and mid-early core varieties. Significant differences were seen between the years. In both years, the average energy content was higher in the mid-early than in the early maturity group.

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Starch yield was also affected by unfavourable weather conditions in the year 2003, as a result lower values of starch yield were obtained than in 2002 (tables 27 and 28). Average starch yield in 2002 was 61.5 dt ha-1 and 29.4 dt ha-1 in 2003, 68.3 dt ha-1 and 39.4 dt ha-1 for the mid-early maturity group in 2002 and 2003 respectively. However, insignificant differences were seen within the year among varieties of the same maturity group, while significant differences were noticed between the years. In both years, mid-early maturity varieties indicated higher starch yield than early.

Higher values of energy yield were obtained in 2002 than in 2003 in both maturity groups (tables 29 and 30). Analysis of variance showed significant differences between varieties within the early maturity group and between the years. Insignificant differences were found within varieties of the mid-early group and in core varieties of both maturity groups. In both years, the mid-early maturity group indicated a higher energy yield than the early group.

Similar results were obtained with crude fibre in both maturity groups. Crude fibre is one of the important indicators of forage structure, in addition to ADF and NDF, which affects digestibility of maize forage. Insignificant differences were found between varieties of the same group, but significant differences between the years. Core varieties of the early maturity group showed insignificant differences both within and between the years, which suggests that crude fibre was not affected by changes in environmental conditions.

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According to analysis of variance for both maturity groups and core varieties, crude protein was not affected by yearly changes in environmental conditions in early maturity group and early maturity core varieties. There were insignificant differences within and between the years. Mid-early maturity group however showed insignificant differences within the group, but significant differences between the years.

In conclusion, there were significant differences in forage quality between year 2002 and 2003. The differences were a result of interaction between environment and the varieties. Under favourable environmental conditions, like in 2002, dry matter yield were high with better forage quality, however under unfavourable conditions of water limitation and high temperature as in 2003, low dry matter yield, high dry matter content resulted, with low forage quality. The results showed that crude fibre and crude protein were insiginificantly affected between the years.

Very high, positive correlation existed between enzyme soluble organic substances and vivo digestible organic matter, starch content, enzyme-soluble organic substances, netto energy for lactation in both maturity groups and in both years. Very high, but negative correlation existed between crude fibre and enzyme soluble organic substances, starch content, vivo digestible organic matter and netto energy for lactation. Crude fibre was negatively correlated with all given parameters in both years and maturity groups. Crude protein indicated between low positive to low negative correlation with other parameters in both years and maturity groups. Dry matter content of early maturity group in 2003 expressed higher correlation with other parameters than in 2002. Generally, dry matter yield and dry matter content had low correlation with other parameters given. Higher correlation were seen in all parameters of early maturity group in 2003 than in 2002, suggesting that under unfavourable weather conditions, correlation between the parameters were strongly expressed than under favourable growing conditions of 2002.


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