4 RESULTS
4.2. Giessen 2005
4.2.3 Anaerobic digestion results
harvest time. The highest sugar content (17,7 %) was produced at the second har-vest by Mikado and the lowest (6,1 %) by Gavott at the third harhar-vest. A significant cultivar harvest time interactions observed might have been due to the sharp increase in sugar content shown by Gavott in the transition period from first to second harvest.
Maize starch content
Table 4.18 shows delaying harvest to affect maize starch content in a pattern which is an exact mirror image of the sugar content pattern. Except for Mikado, delaying harvest increased the starch content of the rest of the cultivars to the highest level at the third harvest from where it dropped to the fourth. Increasing starch contents by delaying maize harvest have also been reported by Hunt et al. (1989) but the values of the sugar contents of all the cultivars in this experiment are far above those observed by Ballard et al. (2001) and Thomas et al. (2001).
Table 4.18: Maize starch content (%) according to cultivar and harvest time, Giessen 2005
Cultivar(CV) Harvest time (HT)
1 2 3 4
Gavott 5,3 21,9 28,1 23,3
KXA5226 4,0 15,0 26,3 22,1
KXA5233 3,3 15,1 26,4 21,8
KXA5243 5,1 6,1 17,8 15,2
Mikado 4,7 1,4 11,8 15,2
CV HT CV*HT
p -value 0,000 0,000 0,000
LSD (5%) 2,5 2,2 5,0
In Mikado starch content decreased from first (4,7 %) to the second (1,4 %) harvest and increased continuously from second through the third to the fourth harvest time.
A significant cultivar harvest time interaction (LSD = 5,0) was observed may be due to the sharp drop (1,4% ) in sugar content shown by Mikado at the second harvest.
Maize organic matter content
By comparing the dry matter yields and the volatile solids (VS) yields of all the maize cultivars presented in table 4.19, one realises that their differences are significantly small. The significance of whole plant silage maize rich in organic matter and the impacts of its lignification to digestibility been highlighted in the literature.
Table 4.19: Maize dry matter yield, dry matter content, volatile solids, biogas yield, methane yield and percentage methane concentrations according to Cultivar and harvest time, Giessen 2005
CV HT DMY
dt /ha
VS dt/ha
DMC
%
Gas nL /kg VS
CH4 nL /kg VS
CH4
%
Gavott 1 109,0 103,1 19,0 656 384 58,5
Gavott 3 152,8 145,5 33,4 701 392 55,9
Gavott 4 147,9 139,4 40,6 643 375 58,3
KXA5233 1 120,3 112,6 17,1 735 417 56,7
KXA5233 3 176,8 168,4 29,6 764 421 55,1
KXA5233 4 184,0 173,6 36,7 687 384 55,9
DMY-dry matter yield, DMC-dry matter content, VS-volatile solids, nL-norm litre, HT- harvest time, CV-cultivar
Each sample from each selected harvest time was digested over a retention time of twenty one days. The daily specific biogas and methane productivity within these twenty one days depended on the composition of the organic matter content of each sample. Hence the ratio of cell wall (NDF) contents to cell contents. The higher gas productivity observed during the first few days mainly came from the cell contents. As time went on more and more of the organic matter left constituted mainly the cell wall material. Due to the slow digestibility of the cell wall contents the gas curves slowly plateaued. as can be inferred in figures 4.4 to 4.9 below for Gavott and KXA5233.The values shown for each measurement day is a sum of the actual values measured for that day and the values of the preceding measurements. Hence the total biogas and methane productivity of each harvest time is the final value at the end of the curve.
347 412
508
580 620
233
290 335 360
656
257
384
194 143 0
100 200 300 400 500 600 700
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Re te ntion time (Days) Specific Gas /methane productivity (nL /kg VS)
Biogas sum curve nl/kg VS Methane sum curve nL/kg VS
Fig. 4.4: Cumulative curves of biogas and methane yields of Gavott grown in Giessen and harvested at the and first harvest time in 2005
0 100 200 300 400 500 600 700 800
0 2 4 6 8 10 12 14 16 18 20 22
Tage Gassummenkurve [l n/kg oTS]
Gavott ET-1_GI-05 Gavott ET-3_GI-05 Gavott ET-4_GI-05
Fig. 4.5: Cumulative curves of biogas yields of Gavott (S 250) according to retention time and harvested at the first, third and fourth harvest times. ET-1-First harvest, ET-3-third harvest time, ET-4-Fourth harvest time ;oTS-Volatile solids; GI-Giessen 2005
0 50 100 150 200 250 300 350 400 450
0 2 4 6 8 10 12 14 16 18 20 22
Tage Methansummenkurve [l n CH4/kg oTS]
Gavott ET-1_GI-05 Gavott ET-3_GI-05 Gavott ET-4_GI-05
Fig. 4.6: Cumulative curves of methane yields of Gavott (S 250) according to retention time and harvested at the first, third and fourth harvest times. ET-1-First harvest, ET-3-third harvest time, ET-4-Fourth harvest time;oTS-Volatile solids; GI-Giessen 2005
374 448
562
638
697
244
310 356 392
272
735
417
202 144 0
100 200 300 400 500 600 700 800
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Retention time (Days) Specific Gas/methane productivity (nL/kg VS)
Biogas sum curve nl/kg VS Methane sum curve nL/kg VS
Fig. 4.7: Cumulative curves of biogas and methane yields of KXA5233 grown in Giessen and harvested at the and first harvest time in 2005
0 100 200 300 400 500 600 700 800
0 2 4 6 8 10 12 14 16 18 20 22
Tage Gassummenkurve [l n/kg oTS]
KXA5233 ET-1_GI-05 KXA5233 ET-3_GI-05 KXA5233 ET-4_GI-05
Fig. 4.8: Cumulative curves of biogas yields of KXA5233 (S 270) according to retention time and harvested at the first, third and fourth harvest times. ET-1-First harvest, ET-3-third harvest time, ET-4-Fourth harvest time ;oTS-Volatile solids; GI-Giessen2005
0 50 100 150 200 250 300 350 400 450
0 2 4 6 8 10 12 14 16 18 20 22
Tage Methansummenkurve [l n CH4/kg oTS]
KXA5233 ET-1_GI-05 KXA5233 ET-3_GI-05 KXA5233 ET-4_GI-05
Fig 4.9: Cumulative Curves of methane yields of KXA5233 (S 270) according to retention time and harvested at the first, third and fourth harvest times. ET-1-First harvest, ET-3-third harvest time, ET-4-Fourth harvest
time;oTS-Maize dry matter content
The importance of optimum dry matter content in deciding optimum yield and yield quality for any products envisaged has also been mentioned in the preceding literature too. Table 4.19 shows DMC of Gavott and KXA5233 to have increased with delay in harvest and that both cultivars produced optimum DMCs only at the second harvest. How this affects biogas and methane productivity might become clear in the results below.
Maize biogas and methane productivity
Table 4.19 and fig 4.10 below shows maize biogas productivity of both Gavott and KXA5233 increasing from first harvest to the highest value at the third harvest from where a further delay in harvest to the fourth harvest decreased the biogases of both cultivars. The biogas values considered are those at the end of each cumulative curves for each cultivar and harvest time after a retention time of twenty one days.
KXA5233 (S 270) can be observed to have produced higher specific biogas volumes than Gavott (S 250) at each of the three harvest times. The rates of increase and decrease can be seen to be faster in Gavott than in KXA5233.
Methane productivity of both cultivars also increased from first harvest to the highest values at the third harvest from where it dropped with further delay to the fourth harvest.
656 701
643
735 764
687
384 392 375 417 421 384
0 111,2 222,4 333,6 444,8 556 667,2 778,4 889,6
HT1 HT3 HT4 HT1 HT3 HT4
Gavott KXA5233
Cultiv ar (Gav ott and Doge ) and harv e st time (HT) Specific Gas / methane productivity (nL /kg VS)
Gas nL /kg VS CH4 nL /kg VS
Fig. 4.10: Biogas and methane productivity of Gavott according to harvest time HT1 to HT4 = harvest time 1to harvest time 4, Giessen 2005
Similar to biogas productivity, KXA5233 (S 270) produced the highest specific methane volumes at all the three harvest times than Gavott (S 250) and the rates of increase and decrease are also faster in Gavott than in KXA5233.