Citation of this paper |
A laboratory experiment on ensiling sweet potato (SP) included 5 different ratios of sweet potato roots (SPR) and vines (SPV): 70, 60, 50, 40 and 30% of SPR with 30, 40, 50, 60 and 70% of SPV on a dry matter basis, respectively, giving treatments SP7:3, SP6:4, SP5:5, SP4:6 and SP3:7. Samples of SP silage were analysed at 0, 7, 14, 21, 28, 42, 70 and 84 days after ensiling to determine chemical composition and fermentation and physical characteristics.
When SPR levels decreased from treatment SP7:3 to treatment SP3:7 the colour changed from yellow to a deeper green because of the dark green colour of SPV. However, the colour did not change from 14 to 84 days of ensiling. The silage on all treatments had a good smell at all times up to 84 days. With increasing ensiling time, dry matter content increased and crude protein decreased in all treatments, but the changes were not significant. Other chemical components such as NDF, calcium, and phosphorus did not change during the 84 days of ensiling in all treatments. The pH value in all treatments decreased rapidly in the first week (from around 6.4 to around 3.8) and continued to decrease up to day 14 (to around 3.6), then remained low until 84 days. Acetic acid and lactic acid increased quickly in the first 2 weeks (P<0.01) and then remained constant to 84 days. As the SPR level of the SP silage increased, pH values decreased (during the 84 days of ensiling) and lactic acid increased during the 2 first weeks of ensiling. The NH3-N content in all treatments fluctuated at around 2-3% of total nitrogen and was not affected by ensiling duration or ratio of root to vine.
All of the treatments resulted in good quality products.
Key words: Composition, fermentation, silage, sweet potato roots, vines
In Vietnam around 269,000 ha of land are used for growing sweet potato, producing 1,745,300 tons of sweet potato roots in 1998 (Statistical Year Book 1999). The root is a good energy source (15.6 MJ of metabolizable energy/kg dry matter) and the vine is a source of protein (17.7% CP in dry matter) (NIAH 2001), and both are considered as good livestock feeds. One of the harvests of sweet potato coincides with the wet season, which means that large amounts of sweet potato need to be stored for use in the off-season. Sweet potato roots (SPR) have a delicate skin that is very easily broken, so the flesh also is easily bruised, broken or cut, which makes for good conditions for bacteria to attack, resulting in the sweet potato decaying. In addition, the stored roots often come under attack from weevils and rats, and farmers may lose as much as half of their stored feed. Sweet potato vines (SPV), especially after harvesting, are considered as waste, because animals cannot consume all of the huge amounts produced in the short time available before the vine decays, which occurs within 2 or 3 days. However, the vine is very expensive to purchase during the off-season.
Ensiling by-products is a simple and low-cost option, which can preserve feed for long periods (Lien et al 1994). Ensiling can also render some previously unpalatable products useful to livestock by changing the chemical nature of the feed (Kayouli and Lee 1998). Tinh et al (2000) concluded that SPV ensiled with chicken manure resulted in the highest quality feed, increasing diet crude protein content and dry matter conversion rates. Tinh et al (2001) reported that rice bran, cassava leaf meal and chicken manure are good additives for fermenting SPR, in combination with salt, and the fermented SPR can be stored at least for 90 days in the laboratory without any significant reduction of the quality, and for 4.5 months on farm.
This research then focuses on the effects of ensiling SPR and SPV together in different ratios, without any additives, and the aim was to preserve sweet potato for several months and evaluate it for later use as fattening pig feed in the off-season.
After harvesting the sweet potato vines were chopped into very small pieces (1-2 cm) and the root was washed to remove soil, then ground (f = 1-2 mm) by machine.
The prepared SPR and SPV were mixed together in 5 different ratios as 5 experimental treatments, namely 70, 60, 50, 40, 30 % of SPR with 30, 40, 50, 60, 70% of SPV, respectively, on a dry matter basis, without any additives, and designated SP7:3, SP6:4, SP5:5, SP4:6, SP3:7, respectively. Each SPR and SPV mixture had a total weight of 24 kg (fresh basis) after mixing, and was then divided into 24 equal parts (each 1 kg) and placed in 24 plastic bags that were sealed to avoid air contamination. Three bags from a treatment were put into a 10L covered jar to prevent external mechanical damage and each jar thus represented 3 replications of one sample analysed for chemical composition for every ensiling period (there were 8 different ensiling periods). All jars were stored at room temperature (25-30o C).
Each jar of 3 bags of sweet potato silage (SPS) on each treatment included 3 replications, with samples taken at: 0, 7, 14, 21, 28, 42, 70 and 84 days after ensiling for analysis of chemical composition, including dry matter (DM), crude protein (CP), NDF, calcium (Ca), and phosphorus (P) and fermentation characteristics such as organic acids, pH and NH3. Physical characteristics of SPS such as colour and smell were also observed and recorded.
The analyses were done in the Department of Feed Analysis of NIAH. DM, CP, NDF, Ca and P were determined by using standard AOAC procedures (AOAC 1990). pH was measured in the liquid extracted from SPS samples.
The data were analysed using the General Linear Model procedure of ANOVA in MINITAB 12.21 program (1998). Tukey pair-wise comparisons were used to determine the differences between treatments with confidence level 95.0%.
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Table 1. Analyzed chemical composition of sweet potato vines (SPV) and sweet potato roots (SPR) |
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|
Parameter |
SPV |
SPR |
|
Dry matter, % |
15 |
19.1 |
| As % of dry matter | ||
|
Crude protein |
16.2 |
4 |
|
NDF |
29.8 |
13.9 |
|
Calcium |
1.16 |
0.4 |
|
Phosphorus |
0.42 |
0.23 |
With increasing ensiling time, DM contents increased and CP decreased in all treatments, but the changes were not significant (P>0.05). However, DM and CP contents were different between treatments at all sampling times, due to different SPR and SPV ratios (Table 2).
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Table 2. Effect of sweet potato root and vine ratio on dry matter (DM, %) and crude protein (CP, % of DM) content in sweet potato silage |
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|
Parameter |
Treatment* |
Time of ensiling, days |
SE |
P |
|||||||
|
0 |
7 |
14 |
21 |
28 |
42 |
70 |
84 |
||||
DM |
SP7:3 |
18.0 |
18.3 |
18.3 |
18.7 |
18.3 |
18.5 |
18.8 |
19.0 |
0.329 |
0.426 |
|
SP6:4 |
17.2 |
17.3 |
17.4 |
17.7 |
17.5 |
17.6 |
17.9 |
17.9 |
0.293 |
0.712 |
|
|
SP5:5 |
16.8 |
16.9 |
17.0 |
16.7 |
17.0 |
17.2 |
17.3 |
17.6 |
0.248 |
0.258 |
|
|
SP4:6 |
15.9 |
16.1 |
16.2 |
16.1 |
16.3 |
16.3 |
16.0 |
16.3 |
0.218 |
0.838 |
|
|
SP3:7 |
15.1 |
15.2 |
15.3 |
15.3 |
15.3 |
15.4 |
15.2 |
15.2 |
0.138 |
0.831 |
|
CP |
SP7:3 |
8.1 |
8.1 |
8.0 |
8.0 |
7.9 |
7.8 |
7.9 |
7.9 |
0.134 |
0.762 |
|
SP6:4 |
9.4 |
9.2 |
9.3 |
9.1 |
9.2 |
9.1 |
8.9 |
9.0 |
0.199 |
0.784 |
|
|
SP5:5 |
10.5 |
10.3 |
10.3 |
10.3 |
10.2 |
10.2 |
10.3 |
10.1 |
0.134 |
0.745 |
|
|
SP4:6 |
11.7 |
11.4 |
11.5 |
11.2 |
11.3 |
11.2 |
11.3 |
11.2 |
0.285 |
0.877 |
|
|
SP3:7 |
13.0 |
12.9 |
12.9 |
12.9 |
13.0 |
12.7 |
12.6 |
12.5 |
0.208 |
0.565 |
|
|
*SP7:3 consists of 70%SPR and 30%SPV, SP6:4 consists of 60%SPR and 40%SPV,
SP5:5 consists of 50%SPR and 50%SPV, SP4:6 consists of 40%SPR and 60%SPV,
SP3:7 consists of 30%SPR and 70%SPV |
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The NH3-N content in all treatments fluctuated in the range of 2 to 3 % of total nitrogen and was not affected by ensiling time or ratio of root to vine (Table 3).
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Table 3. Effect of sweet potato root and vine ratio on ammonia nitrogen (NH3-N, % of total N) and NDF content in sweet potato silage |
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|
Parameter |
Treatment* |
Time of ensiling, days |
SE |
P |
|||||||
|
0 |
7 |
14 |
21 |
28 |
42 |
70 |
84 |
||||
|
NH3-N |
SP7:3 |
2.08 |
2.20 |
2.15 |
2.45 |
2.35 |
2.19 |
2.49 |
2.56 |
0.155 |
0.310 |
|
SP6:4 |
2.28 |
2.52 |
2.45 |
2.51 |
2.61 |
2.26 |
2.40 |
2.58 |
0.286 |
0.977 |
|
|
SP5:5 |
2.19 |
2.43 |
2.76 |
2.76 |
2.58 |
2.67 |
2.56 |
2.64 |
0.227 |
0.676 |
|
|
SP4:6 |
2.24 |
2.39 |
2.53 |
2.85 |
2.69 |
2.50 |
2.65 |
2.37 |
0.217 |
0.585 |
|
|
SP3:7 |
2.21 |
2.67 |
2.75 |
2.86 |
2.75 |
2.31 |
2.88 |
3.09 |
0.251 |
0.286 |
|
|
SE |
0.321 |
0.493 |
0.158 |
0.108 |
0.114 |
0.144 |
0.212 |
0.174 |
|
|
|
|
P |
0.993 |
0.969 |
0.126 |
0.075 |
0.225 |
0.227 |
0.585 |
0.142 |
|
|
|
NDF |
SP7:3 |
19.5 |
18.4 |
18.2 |
18.2 |
18.6 |
19.2 |
18.3 |
18.5 |
0.332 |
0.126 |
|
SP6:4 |
22.7 |
22.5 |
21.3 |
20.7 |
21.6 |
20.2 |
20.8 |
21.6 |
0.792 |
0.379 |
|
|
SP5:5 |
24.0 |
24.0 |
22.8 |
22.6 |
23.3 |
22.2 |
23.4 |
22.1 |
0.665 |
0.332 |
|
|
SP4:6 |
26.5 |
25.3 |
25.3 |
25.6 |
26.3 |
27.1 |
25.2 |
25.1 |
0.562 |
0.174 |
|
|
SP3:7 |
28.1 |
27.8 |
27.7 |
27.9 |
27.6 |
28.1 |
27.8 |
27.6 |
0.275 |
0.710 |
|
|
*See footnote in table 2 |
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Ca and P in all treatments were unchanged during the 84 days of ensiling (P>0.05) (Table 4).
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Table 4. Effect of sweet potato root and vine ratio on calcium (Ca, % of DM) and phosphorus (P, % of DM) content in sweet potato silage |
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|
Parameter |
Treatment* |
Time of ensiling, days |
SE |
P |
|||||||
|
0 |
7 |
14 |
21 |
28 |
42 |
70 |
84 |
||||
Ca |
SP7:3 |
0.58 |
0.55 |
0.56 |
0.62 |
0.61 |
0.59 |
0.61 |
0.59 |
0.020 |
0.172 |
|
SP6:4 |
0.67 |
0.63 |
0.67 |
0.63 |
0.65 |
0.70 |
0.67 |
0.66 |
0.018 |
0.345 |
|
|
SP5:5 |
0.80 |
0.79 |
0.77 |
0.79 |
0.83 |
0.81 |
0.82 |
0.81 |
0.014 |
0.198 |
|
|
SP4:6 |
0.85 |
0.88 |
0.85 |
0.86 |
0.87 |
0.91 |
0.82 |
0.83 |
0.023 |
0.306 |
|
|
SP3:7 |
0.92 |
0.93 |
0.95 |
0.97 |
0.94 |
1.00 |
0.97 |
0.93 |
0.024 |
0.286 |
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