Livestock Research for Rural Development 35 (8) 2023 LRRD Search LRRD Misssion Guide for preparation of papers LRRD Newsletter

Citation of this paper

Effect of mixing ratios on quality of sweet potato (Ipomoea batatas) by-product silage

Ho Thanh Tham1 and Mai Truong Hong Hanh2

1 Faculty of Animal Sciences, College of Agriculture, Can Tho University, Vietnam. Campus II, 3/2 Street, Ninh Kieu District, Can Tho City, Vietnam
httham@ctu.edu.vn
2 Sub-Department of Animal Husbandry and Veterinary Medicine of Soc Trang Province, Vietnam

Abstract

The experiment was conducted on sweet potato vines (SPV) and sweet potato tubers (SPT) (type 3), arranged in a completely randomized design with 7 treatments and 3 replicates. The treatments (T) were the different ratios of SPV and SPT (on a dry basis) as follows: 0, 70, 60, 50, 40, 30 and 100% of SPT with 100, 30, 40, 50, 60, 70 and 0% of SPV on a dry matter basis, respectively, giving treatments TI, TII, TIII, TIV, TV, TVI and TVII. The treatments were evaluated for sensory and chemical composition at 0, 14, 28, 42, 56, 70 and 84 days. The results showed that after 84 days of ensiling, all of the treatments met the requirements of the silage. The sweet potato silage had a pale yellow colour from the SPV and a dark purple colour from the SPT. The colour in all treatments did not change after 14 days of ensiling up to 84 days. The silage on all treatments had a good smell, which did not change up to 84 days. With increasing ensiling time, DM contents decreased in all treatments (p<0.05). The CP content of silages did not change significantly. The NH3-N content in all treatments fluctuated in the range of 0.1 to 0.5% of total nitrogen. Ash content had no statistical significance (p>0.05) over the ensiling times, except for treatments I, II and VI. ADF content of the same treatment over the ensiling times was not statistically significant (p>0.05) except for T II, III, IV and VI, while NDF content has a large fluctuation. The pH value ranges from 3.3 to 3.84. Organic acids content tended to be low when the mixtures had a high SPT ratio. Levels of acetic acid ranged from 0.48 to 2.21% after ensiling 84 days. Propionic acid content increased slightly over the silage times, except for treatments II, III and V. Butyric acid content after ensiling for 84 days was not detected at the treatment I when only SPV was present in the mixture. All of the treatments resulted in good quality products.

Keywords: by-product, Ipomoea batatas, sweet potato, sensory evaluation, silage


Introduction

The ruminant production in Vietnam is showing a typical development trend as beef cattle increase from 5,496.6 thousand heads (2016) to 6,339 thousand heads (2022). Farmers are facing more difficulties due to the increasing price of animal feed and the outbreak of diseases. Facing that situation, it becomes more difficult to proactively feed sources for livestock. With an area of 109.3 thousand hectares of sweet potato planting (General Statistics Office 2021), the better use of sweet potato by-products will save feed costs and improve economic efficiency for farmers.

In the Mekong Delta of Vietnam, sweet potato has been grown for tuber production, mainly for human consumption (Vo Lam and Ledin Inger 2004). SPV and SPT by-products are used as feed for pigs and ruminants (Le Van An 2004, Etela et al 2008 and Etela et al 2009). For sweet potato vines, the results showed that the yield was quite high, ranging from 2.04 to 3.03 tons/ha. Harvested sweet potato tubers had a yield to be 26.97 tons/ha and the yield of their by-products (tubers did not meet commercial standards concerning the tuber size and other agents) was 4.76 tons/ha (accounting for 17.6%) (Ho Thanh Tham and Mai Truong Hong Hanh 2020). However, this type of feed if used for direct feeding will have many difficulties such as: large amounts produced in a short time, rapid spoilage, mold infection and high dependence on output and season. While the selling price of SPT is always very low, local farmers often preserve it by silage, combining SPV and SPT with salt as an additive to improve storage time and prevent mold growth. Salt is commonly used to inhibit the growth of butyric acid bacteria and increase fermentation (Ergin and Gumus 2020). Moreover, green forages have low levels of sodium and therefore adding salt in silage fulfills the needs of the animals. Therefore, ensiling by-products is a simple and low-cost solution, which can preserve feed for long periods. The objective of the present study was to evaluate sensory criteria and the chemical composition of different mixtures of SPV and SPT.


Materials and methods

Location

The experiment was conducted in the laboratory of the Faculty of Animal Sciences, College of Agriculture, Can Tho University from November 2021 to February 2022.

Silage preparation

Sweet potato vines (SPV) and sweet potato tubers (SPT) (type 3) of the Japanese purple sweet potato variety were collected from the field in Binh Tan district, Vinh Long province, with a growing time of about 135 days. SPT (type 3) is characterized by a weight less than 50g, broken or scratched (Photo 1).

After harvesting SPV were chopped into small pieces (2-3 cm) and dried for 4-6 hours under sunlight conditions. The tubers were washed to remove soil, then ground by a chopper with a 0.3-0.5 cm thickness.

The prepared SPV and SPT were mixed in 7 different ratios as 7 experimental treatments, namely 100, 30, 40, 50, 60, 70 and 0% of SPV with 0, 70, 60, 50, 40, 30, 100% of SPT, respectively, on a dry matter basis, with 0.5% salt as additives. They were designated TI, TII, TIII, TIV, TV, TVI and TVII, respectively. Each SPV and SPT mixture weighed 2 kg (fresh basis) after mixing and placed in plastic bags that were sealed by a vacuum machine to avoid air contamination. All bags were stored at room temperature.

Photo 1. Sweet potato tubers by-product (type 3) (a) Weight less than 50g, (b) Broken or scratched,
(c) Damage caused by fungal diseases and (d) Damaged by rodents


Photo 2. Sweet potato tubers after cuttingPhoto 3. Collecting sweet potato vines in the field
Experimental design

The treatments were levels of SPV and SPT in the mixtures, arranged in a completely randomized design with 7 replications and 3 replications (Table 1). The silages were surveyed at 7 time points: 1, 14, 28, 42, 56, 70 and 84 days after ensiling.

Table 1. Experimental layout

Ratio (% DM)

Treatment (T)

TI

TII

TIII

TIV

TV

TVI

TVII

Sweet potato vines (SPV)

100

30

40

50

60

70

0

Sweet potato tubers (SPT)

0

70

60

50

40

30

100

Sensory assessment and chemical composition

Sensory characteristics of sweet potato silages such as colour and smell were observed and recorded. Samples were taken at 0, 14, 28, 42, 56, 70 and 84 days after ensiling for analysis of the chemical composition, including dry matter (DM), ash, crude protein (CP), crude fiber (CF), acid detergent fiber (ADF), neutral detergent fiber (NDF) and ether extract (EE) and fermentation characteristics such as organic acids, pH and NH3-N. Dry matter, ash, CP, CF, ADF, NDF and EE were determined by using standard AOAC procedures (AOAC 2002) and Van Soest et al (1991). pH was measured in the liquid extracted from sweet potato silage samples.

Statistical analysis

The data were analysed by ANOVA using the General Linear Model (GLM) procedure in the Minitab 16.2.0 software (2010). Tukey pair-wise comparisons were used to determine the differences between treatments with a confidence level of 95.0%.


Results and discussion

Sensory evaluation

The sweet potato silage had a pale yellow colour from the SPV and a dark purple colour from the SPT. In the three treatments of TI, TIV, TV and TVI, the yellow colour was dominant, while the dark purple colour of SPV was dominant in treatments TII, TIII and TVII (Photo 4). The colour in all treatments did not change after 14 days of ensiling up to 84 days. The silage on all treatments had a good smell, that did not change up to 84 days. There was an exudate liquid in silage in treatment TVII after 84 days of ensiling.

Photo 4. Silages after 84 days of ensiling Labels of (a), (b), (c), (d), (e), (f), (g)
designated TI, TII, TIII, TIV, TV, TVI and TVII, respectively
Chemical composition of sweet potato by-products

The results showed that the chemical composition of SPV was relatively high compared with that of sweet potato tuber, especially the CP content of leaves was significantly higher than that of stem (An et al 2003). The CP content of SPV (11.5%) was lower than the result of Hoang Huong Giang et al (2004) (16.2%). In this study, SPV was harvested at 135 days and SPR was used as a by-product (type 3). Moreover, in the rainy season, SPV has better nutritional value than in the dry season (Katongole et al 2008).

Table 2. Chemical composition of sweet potato by-products

Parameter

DM

Ash

CP

CF

ADF

NDF

EE

SPV

23.7

11.8

11.5

29.6

32.5

42.0

4.19

SPT (type 3)

26.5

3.57

3.69

3.81

7.18

10.2

0.6

DM: Dry matter, CP: Crude protein, CF: Crude fiber, ADF: Acid detergent fiber, NDF: Neutral detergent fiber, EE: Ether extract

With increasing ensiling time, DM contents decreased in all treatments (p<0.05). However, DM and CP contents were different between treatments at all sampling times, due to different SPV and SPT ratios (Table 3). As reported by McDonald et al (1991), the accepted DM change during ensiling is less than 5%. The change of CP in the silage is one of the most important criteria in the ensiling process. The CP content decreased much after ensiling, the quality of the feed was poor (Truong La, 2012). In the present study, after 84 days of ensiling, the CP content of silages did not change significantly, which facilitated the use of feed for a long time while the quality remained stable.

The NH3-N content in all treatments fluctuated in the range of 0.1 to 0.5% of total nitrogen and was not affected by ensiling time. According to the AOAC standard (2002), a quality silage sample requires an NH3-N content < 5% of total N. In the present study, all of the 7 different ratios of SPV and SPT were successfully ensiled with 0.5% salt.

Table 3. Effect of SPV and SPT ratio on dry matter (DM, %), crude protein (CP, % of DM) contents and ammonia nitrogen (NH3-N, % of total N) in sweet potato silage

Parameter

Treatment

Time of ensiling (days)

SEM

p

1

14

28

42

56

70

84

DM

I

f23.7 B

b22.2 C

d21.9 C

a26.2 a

e22.2 c

c22.5 c

b22.0 c

0.20

0.001

II

b26.9

a26.3

ab25.3

a25.2

ab26.1

ab25.3

a25.8

0.44

0.001

III

c26.4 A

ab23.9 B

bc24.0 B

ab25.3 AB

bc24.7 AB

bc23.9 B

b23.7 B

0.44

0.001

IV

d25.3 A

ab24.4 AB

c22.9 B

bc23.6 AB

bc24.8 AB

bc23.8 AB

b23.5 AB

0.48

0.001

V

e24.9 A

ab24.8 A

cd22.7 B

c22.9 B

cd24.2 AB

bc24.1 AB

b23.5 AB

0.36

0.001

VI

d25.2 A

ab24.9 A

c23.7 AB

bc23.7 AB

de22.8 b

bc23.6 AB

b23.8 AB

0.42

0.001

VII

a27.6 A

a26.6 AB

a25.6 B

a26.7 AB

a26.8 AB

a26.9 AB

a25.9 B

0.31

0.001

SEM

0.605

0.641

0.314

0.353

0.338

0.454

0.391

p

0.001

0.003

0.001

0.001

0.001

0.001

0.001

CP

I

a11.45

a9.85

7.98

a9.90

a10.95

a10.06

a9.64

1.01

0.372

II

b5.23 AB

b5.71 AB

4.84 B

ab6.88 A

c6.02 AB

bc5.15 AB

bc4.96 AB

0.42

0.043

III

b5.67

b6.01

5.32

ab6.58

c7.19

b6.17

b7.03

0.68

0.438

IV

b5.71

ab7.13

6.41

bc8.13

c6.87

b7.01

bc6.15

0.63

0.233

V

b5.79 AB

bc4.92 b

8.68 A

c7.91 AB

b9.16 a

b6.97 AB

bc6.74 AB

0.75

0.013

VI

b6.21 C

ab8.19 ABC

9.05 AB

bc8.26 ABC

ab10.00 A

b7.26 BC

bc6.60 bc

0.52

0.020

VII

b4.11 A

c2.01 B

3.85 A

a3.35 A

d4.45 A

c3.36 A

c4.18 A

0.26

0.001

SEM

0.444

0.729

1.111

0.639

0.255

0.477

0.530

p

0.001

0.001

0.030

0.001

0.001

0.001

0.001

NH3-N

I

0.15

0.45

0.27

ab0.30

a0.33

0.18

0.29

0.09

0.165

II

0.19

0.10

0.25

ab0.19

ab0.14

0.28

0.24

0.06

0.449

III

0.15

0.16

0.50

ab0.21

b0.09

0.19

0.17

0.12

0.105

IV

0.14

0.20

0.40

ab0.29

ab0.20

0.17

0.19

0.07

0.196

V

0.37

0.15

0.25

ab0.16

ab0.17

0.29

0.32

0.11

0.724

VI

0.82

0.10

0.29

b0.11

ab0.24

0.33

0.37

0.14

0.40

VII

0.28

0.38

0.36

a0.44

a0.35

0.31

0.16

0.05

0.062

SEM

0.150

0.095

0.124

0.061

0.048

0.082

0.0789

p

0.062

0.115

0.753

0.029

0.015

0.657

0.436

a,b,c within columns, values with different superscript letters are different (p<0.05) A,B,C within rows, values with different superscript letters are different (p<0.05)
TI: 100% SPV: 0% SPT; TII: 30% SPV: 70% SPT; TIII: 40% SPV: 60% SPT; TIV: 50% SPV: 50% SPT; TV 60% SPV: 40% SPT; TVI: 70%SPV: 30%SPT; TVII: 0%SPV: 100% SPT.

Ash content

In the same treatment over the ensiling times, ash content had no statistical significance (P>0.05), except for treatments I, II, VI.

Table 4. Effect of SPV and SPT ratio on ash (%) in sweet potato silage

Treatment

Time of ensiling (days)

SEM

p

1

14

28

42

56

70

84

I

a11,79 B

a13,13 AB

a12,57 AB

a13,84 A

a12,64 AB

a12,96 AB

a13,31 A

0,290

0,01

II

c7,93 A

c6,58 B

d7,17 AB

c7,20 AB

e6,88 B

d6,78 B

d6,91 AB

0,214

0,01

III

c7,88

c7,56

cd8,40

bc8,88

de7,75

c8,45

cd7,97

0,582

0,70

IV

c8,47

b9,25

bc9,17

bc9,63

cd8,90

c9,33

c9,00

0,323

0,32

V

b9,36

b9,40

b10,28

ab10,90

ab11,10

c9,83

bc9,55

0,500

0,12

VI

b9,51 B

b9,86 AB

b10,10 ab

b10,74 AB

bc10,38 AB

b11,38 A

b10,95 AB

0,345

0,02

VII

d3,96

d4,08

e3,64

d4,00

f4,43

e3,88

e4,52

0,300

0,43

SEM

0,143

0,347

0,340

0,626

0,452

0,318

0,357

p

0,001

0,001

0,001

0,001

0,001

0,001

0,001

a,b,c within columns, values with A,B,C within rows, values with different superscript letters are different (P<0.05)

ADF and NDF content

ADF content of the same treatment over the ensiling times was not statistically significant (p>0.05) except for T II, III, IV and VI. According to Nguyen Xuan Trach et al (2006), the difference was not significant between the ensiling times because in the green forage sample, the ADF content was difficult to be degraded because it contained lignin-hemicellulose linkage, so microorganisms were not able to degrade. NDF content over the ensiling times of each treatment was significantly different (P<0.05), except for TM IV. It can be seen that the NDF content has a large fluctuation.

Table 5. Effect of SPV and SPT ratio on ADF and NDF content (%) in sweet potato silage

Parameter

Treatment

Time of ensiling (days)

SEM

p

1

14

28

42

56

70

84

ADF

I

a32.5 B

a37.0 A

a35.0 AB

a35.6 AB

a33.6 AB

a34.5 AB

a33.6 AB

0.79

0.022

II

e12.4

e13.8

e13.1

e13.6

d12.0

cd10.9

bc13.5

0.67

0.086

III

d16.1

d17.5

d17.2

de15.5

c16.6

bc16.4

b18.7

0.92

0.314

IV

c18.1

cd19.5

c21.1

cd19.6

c18.0

bc19.3

ab26.2

2.65

0.398

V

b21.7 B

bc21.9 B

b25.0 A

bc21.7 B

b21.8 B

b21.3 B

ab20.9 B

0.58

0.005

VI

b22.3 B

b23.9 AB

b26.8 A

b24.4 AB

b25.2 AB

bc23.7 AB

ab24.8 AB

0.83

0.049

VII

f4.14

f4.40

f4.21

f4.46

e3.14

d2.83

c3.01

0.44

0.067

SEM

0.325

0.658

0.662

0.946

0.741

0.563

2.725

p

0.001

0.001

0.001

0.001

0.001

0.001

0.001

NDF

I

a42.0 a

a41.1 a

a39.5 a

a40.3 a

a39.2 a

a30.6 b

a40.5 a

0.97

0.001

II

e17.7 a

cd15.4 a

e13.5 ab

ef16.6 a

e15.6 a

cd10.5 b

e16.1 a

0.94

0.002

III

de21.0 ab

bc21.9 ab

d21.7 ab

de20.0 ab

de19.0 ab

bc17.0 b

d22.9 a

1.15

0.041

IV

cd24.4

bc25.7

cd24.3

cd23.9

bc27.2

b20.5

bc27.7

2.24

0.395

V

c25.8 ab

ab31.2 a

bc28.3 a

bc27.4 ab

cd24.8 ab

b19.5 b

cd25.3 ab

1.72

0.011

VI

b29.9 a

bc27.2 a

b30.2 a

b31.4 a

ab32.2 a

bc17.5 b

b29.8 a

1.87

0.001

VII

f2.26 bcd

d4.28 cd

e11.2 a

f11.2 a

f7.50 b

d3.32 d

f6.52 bc

0.63

0.001

SEM

0.737

2.563

0.899

1.494

1.483

1.529

0.556

p

0.001

0.001

0.001

0.001

0.001

0.001

0.001

a,b,cwithin columns, values with different superscript letters are different (p<0.05) A,B,C within rows, values with different superscript letters are different (p<0.05)

pH and organic acid content

The decrease in pH was very rapid in the first two weeks (from around 5.59 to around 3.33) and remained low up to day 84 (Table 6). pH increased slightly as the ratio of vines increased in the mixtures. The pH value of the treatments ranged from 3.28 to 3.84. It was shown that treatments were of good quality, consistent with the results of silage of SPV and SPT of Nguyen Thi Thu Huyen (2011) and Hoang Huong Giang et al (2004) with pH in the range of 3.59-3.94. These results agree with McDonald et al (1991), who concluded that good silages are characterized by having low pH values, usually between pH 3.7 and 4.2.

Table 6. Effect of SPV and SPT ratio on pH in sweet potato silage

Treatment

Time of ensiling (days)

SEM

p

0

14

28

42

56

70

84

I

e5.21a

a3.83b

a3.84b

a3.66c

a3.77bc

a3.71bc

a3.76bc

0.03

0.001

II

b5.59a

d3.43b

bc3.51b

a3.55b

d3.42b

d3.46b

b3.52b

0.04

0.001

III

c5.42a

cd3.46c

bc3.56b

ab3.47c

c3.51bc

cd3.51bc

b3.51bc

0.02

0.001

IV

d5.32a

bc3.51d

bc3.61b

ab3.51d

b3.56bc

bc3.53cd

b3.54cd

0.01

0.001

V

d5.29a

b3.56c

b3.63b

a3.55c

b3.57c

b3.58bc

b3.59bc

0.01

0.001

VI

e5.23a

b3.56b

d3.35c

a3.56b

b3.58b

bc3.57b

b3.59b

0.01

0.001

VII

a5.71a

e3.33c

cd3.47b

b3.30c

e3.31c

e3.28c

c3.34b

0.03

0.001

SEM

0.006

0.013

0.029

0.048

0.011

0.012

0.025

p

0.001

0.001

0.001

0.004

0.001

0.001

0.001

a,b,c within columns, values with different superscript letters are different (p<0.05) A,B,Cwithin rows, values with different superscript letters are different (p<0.05)

Organic acid content tended to be low when the mixtures had a high SPT ratio. There was a significant difference (p<0.05) in acetic acid content between treatments at the same time, except for the 28-day period. Levels of acetic acid ranged from 0.48 to 2.21% after ensiling 84 days. According to McDonald et al (1991) a quality silage product requires that the product contain less than 2.5% acetic acid. Propionic acid content increased slightly over the silage times, except for treatments II, III and V (p<0.05). According to the silage time, this acid ratio increased slightly in all treatments. According to Kung et al (2018) it is common to add propionic acid-containing additives to improve silage stability and increase its concentration in the silage by 0.15-0.30%. Butyric acid content after ensiling for 84 days was not detected at the treatment when only SPV was present in the mixture. The presence of butyric acid indicates metabolic activity from Clostridial organisms, leading to DM loss and reduced energy (Pahlow et al 2003). Butyric acid does not reduce the quality of the silage, but they are an indicator that the silage is not very good.

Table 7. Effect of sweet potato root and vine ratio on organic acids (%) content in sweet potato silage

Parameter

Treatment

Time of ensiling (days)

SEM

p

14

28

42

56

70

84

Acetic acid

I

b0.005 C

0.20BC

a0.64AB

ab0.70 A

ab0.99 A

ab0.91 A

0.1

0.001

II

a0.046 C

0.21 BC

ab0.49ab c

ab0.73 ABC

ab1.16 AB

ab1.26 A

0.21

0.006

III

b0 B

0.18 B

ab0.47 B

a1.34 AB

a1.99 A

ab1.11 AB

0.32

0.006

IV

b0B

0.09B

b0.32B

ab0.71B

b0.62B

a2.21A

0.22

0.001

V

b0C

0.03 C

bc0.27C

ab0.70BC

ab1.05AB

ab1.58A

0.16

0.001

VI

b0C

0.007C

c0.077C

b0.11 C

ab1.67A

ab1.06B

0.06

0.001

VII

a0.063 D

0.11 D

bc0.30 C

b0.43 BC

b0.58 A

b0.48 AB

0.03

0.001

SEM

0.006

0.061

0.048

0.182

0.236

0.315

p

0.001

0.187

0.001

0.013

0.007

0.043

Propionic acid

I

a0.20 C

0.17 C

d0.22 C

bc0.33 BC

a0.71 A

a0.50 B

0.04

0.001

II

ab0.12

0.13

b0.26

bc0.37

ab0.44

ab0.22

0.07

0.050

III

ab0.15

0.17

bcd0.24

cd0.24

ab0.43

ab0.33

0.08

0.227

IV

ab0.17 C

0.14 C

b0.23 BC

ab0.42 B

a0.68 A

ab0.32 BC

0.04

0.001

V

ab0.11

0.17

b0.23

ab0.44

ab0.40

ab0.35

0.08

0.074

VI

ab0.17 CD

0.15 D

cd0.19 CD

a0.56 A

ab0.33 BC

a0.47 AB

0.04

0.001

VII

b0.04 C

0.05b C

a0.14 A

d0.09 ABC

b0.12 A

b0.097 AB

0.01

0.001

SEM

0.032

0.026

0.019

0.036

0.104

0.075

p

0.065

0.059

0.013

0.001

0.018

0.027

Butyric acid

I

d0.15 B

c0.19 AB

d0.26 a

c0.02 C

c0 C

b0 C

0.02

0.001

II

ab0.38

b0.30

bc0.40

ab0.63

a0.92

a0.78

0.16

0.093

III

ab0.37

b0.33

bcd0.37

a0.69

ab0.60

ab0.36

0.09

0.085

IV

bc0.29 AB

bc0.29 AB

bc0.40 A

abc0.35 AB

bc0.08 B

ab0.29 AB

0.07

0.072

V

cd0.21

b0.31

b0.43

bc0.23

bc0.27

ab0.56

0.1

0.166

VI

cd0.23 BC

bc0.24 BC

cd0.30 AB

c0.09 C

abc0.45 A

ab0.18 BC

0.03

0.001

VII

a0.48B

a0.48B

a0.65A

c0.09 C

bc0.08C

b0.03C

0.02

0.001

SEM

0.027

0.021

0.024

0.088

0.115

0.140

p

0.001

0.001

0.001

0.001

0.001

0.014

a,b,c within columns, values with different superscript letters are different (p<0.05) A,B,C within rows, values with different superscript letters are different (p<0.05)


Conclusions


Acknowledgments

This research is funded by the Ministry of Education and Training, Vietnam under grant number B2020-TCT-07.


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