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 cutting	Photo 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)
Ratio (% DM)	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
Parameter	Treatment	1	14	28	42	56	70	84	SEM	p

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	^b^c24.0 ^B	^a^b25.3 ^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	^c^d22.7 ^B	^c22.9 ^B	^c^d24.2 ^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
Treatment	1	14	28	42	56	70	84	SEM	p

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
Parameter	Treatment	1	14	28	42	56	70	84	SEM	p

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
Treatment	0	14	28	42	56	70	84	SEM	p

I	^e5.21^a	^a3.83^b	^a3.84^b	^a3.66^c	^a3.77^bc	^a3.71^bc	^a3.76^bc	0.03	0.001
II	^b5.59^a	^d3.43^b	^bc3.51^b	^a3.55^b	^d3.42^b	^d3.46^b	^b3.52^b	0.04	0.001
III	^c5.42^a	^cd3.46^c	^bc3.56^b	^ab3.47^c	^c3.51^bc	^cd3.51^bc	^b3.51^bc	0.02	0.001
IV	^d5.32^a	^bc3.51^d	^bc3.61^b	^ab3.51^d	^b3.56^bc	^bc3.53^cd	^b3.54^cd	0.01	0.001
V	^d5.29^a	^b3.56^c	^b3.63^b	^a3.55^c	^b3.57^c	^b3.58^bc	^b3.59^bc	0.01	0.001
VI	^e5.23^a	^b3.56^b	^d3.35^c	^a3.56^b	^b3.58^b	^bc3.57^b	^b3.59^b	0.01	0.001
VII	^a5.71^a	^e3.33^c	^cd3.47^b	^b3.30^c	^e3.31^c	^e3.28^c	^c3.34^b	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
Parameter	Treatment	14	28	42	56	70	84	SEM	p

Acetic acid	I	^b0.005 ^C	0.20^BC	^a0.64^AB	^a^b0.70 ^A	^a^b0.99 ^A	^a^b0.91 ^A	0.1	0.001
	II	^a0.046 ^C	0.21 ^BC	^a^b0.49^a^b ^c	^a^b0.73 ^ABC	^a^b1.16 ^AB	^a^b1.26 ^A	0.21	0.006
	III	^b0 ^B	0.18 ^B	^a^b0.47 ^B	^a1.34 ^AB	^a1.99 ^A	^a^b1.11 ^AB	0.32	0.006
	IV	^b0^B	0.09^B	^b0.32^B	^a^b0.71^B	^b0.62^B	^a2.21^A	0.22	0.001
	V	^b0^C	0.03 ^C	^b^c0.27^C	^a^b0.70^BC	^a^b1.05^AB	^a^b1.58^A	0.16	0.001
	VI	^b0^C	0.007^C	^c0.077^C	^b0.11 ^C	^a^b1.67^A	^a^b1.06^B	0.06	0.001
	VII	^a0.063 ^D	0.11 ^D	^b^c0.30 ^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	^a0.48^B	^a0.48^B	^a0.65^A	^c0.09 ^C	^bc0.08^C	^b0.03^C	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

It can be concluded from this present study that all of the 7 mixtures of SPV and SPT were successfully ensiled with 0.5% salt, resulting in good quality products that could be stored at least 3 months.
The actual ratios of SPV and SPT in the silage under practical conditions will depend on the ratios of SPV and SPT produced at harvest.

Acknowledgments

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

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