Citation of this paper

Effects of wilting and rice bran supplementation on the quality of taro (Colocasia esculenta) leaf and petiole silage

Lam Thai Hung, Ly Thi Thu Lan, Nguyen Thi Anh Thu, Nguyen Thiet¹, Trinh Thi Hong Mo², Nguyen Thi Hong Nhan³ and Nguyen Trong Ngu³

School of Agriculture and Aquaculture, Tra Vinh University, 126 Nguyen Thien Thanh, Tra Vinh City, Vietnam
ntngu@ctu.edu.vn
¹ Collegeof Rural Development, Can Tho University, 3/2 Street, Can Tho City, Vietnam
² Applied Biology Faculty, Tay Do University, Can Tho City, Vietnam
³ College of Agriculture, Can Tho University, 3/2 Street, Can Tho City, Vietnam

Abstract

This study was carried out to determine how wilting and supplementation of rice bran (10% and 20%) would affect the ensiling characteristics of taro petioles and leaves (TPL).

Samples were collected and measured for pH, dry matter, ammonia, oxalate, and organic acids on days 1, 7, 14, 21, 30, 45, and 60 of ensiling. It was shown that wilted TPL silage was high in dry matter (DM) and crude protein (CP) but low in oxalate content compared to fresh TPL silage. The addition of 20% rice bran in fresh TPL resulted in high fermentation quality with higher DM content and consistently low oxalate compared to TPL added with 10% or without supplementation. This result implies that TPL should be wilted before ensiling or should be ensiled with 20% of rice bran for better silage quality.

Keywords: ensiling, fermentation, oxalate, lactic acid

Introduction

In the Mekong Delta of Vietnam, Dong Thap province has the biggest area for growing taro (Colocasia esculenta) as tubers for human consumption. This province has 1,338 hectares for taro production, annually producing 50,000 tonnes of leaves and petioles after harvesting tubers (Tri 2019). Taro petioles and leaves (TPL) have a high nutritional content with crude protein (CP) ranging from 19.5 to 26% for leaves and 6.2 to 7.3% for petioles (Hang and Preston 2010). However, TPL also contain high levels of calcium oxalate which causes irritation (Nhan et al 2011); and to overcome this problem some processing methods have been applied including drying, cooking, and ensiling. A previous study found that ensiling can decrease calcium oxalate content up to 50% and reduce negative effects from oral irritation (Hang and Preston 2010). The objective of this study was to evaluate the effect of wilting TPL and rice bran levels on quality and chemical composition of TPL silage.

Materials and methods

The experiment was conducted in a completely randomized design with 4 treatments and 4 replicates. Before ensiling, the ratio of materials was calculated as a dry matter ratio. The treatments were as follows:

Treatment 1 (F): 100% fresh TPL

Treatment 2 (W): 100% wilted TPL (30-40% DM)

Treatment 3 (Rb₁₀F): 89.5% fresh TPL + 10% Rb + 0.5% salt

Treatment 4 (Rb₂₀F): 79.5% fresh TPL + 20% Rb + 0.5% salt

Taro petioles and leaves, after being harvested, were chopped into small pieces (2 to 4 cm long). After that, taro was ensiled either fresh or wilted under sunshine (to 30-40% DM). The other treatments were addition of rice bran (10% and 20%, DM basis) with 0.5% salt. The mixtures were packed into polyethylene bags of 5 kg capacity with air removed from the bags. During storage, the bags were checked regularly and, if found to be loose, would be re-opened, made compact and re-tied.

Silage samples were collected on days 1, 7, 14, 21, 30, 45, and 60 during the ensiling process. All samples were divided into two parts; the first part was immediately measured (fresh) for pH, ammonia, lactic acid (LA), and acetic acid (AA) according to AOAC (2005) and oxalate content according to the method of Savage et al (2000); the second part was dried in an oven at 65^oC until the weight was stabilised and then analysed for CP and ash according to AOAC (2005).

Statistical analysis

The data were subjected to analysis of variance using the General Linear Model procedure of Minitab software version 16.2.1.

Results and discussion

Initially the CP and oxalate of TPL were 13.7% and 5.68%, respectively (Table 1).

Table 1. Chemical composition of silage ingredients
Ingredients	% DM
Ingredients	DM, %	CP	Ash	Oxalate

Taro petiole and leaf	9.32	13.7	16.4	5.68
Rice bran	91.2	16.5	7.80	-

The pH and oxalate of taro silage declined, but ammonia, LA, and AA dramatically increased over the 60-day period (Table 2). On day 14, the pH of all treatments dropped dramatically below 4.2 and stabilised until the 60^th day of ensiling. Decreased pH values due to increasing LA indicated good quality of silage. For the conversion of pyruvate to lactate, the concentration of LA and AA increased with a decreased pH that inhibited the aerobic respiration of plant cells and microbial activity, which otherwise lead to negative effects on silage (Rooke and Ronald 2003). According to Muck (2010), a successful fermentation is recognised by the number of LA-producing bacteria that dominate under anaerobic conditions, which is in agreement with the present study. The increased LA content over time had led to a decreased pH, which, in addition to the activity of lactic bacteria, may also involve other microorganisms such as probiotics and fungi that activated changes in soluble carbohydrates to LA, AA, and other organic acids inhibiting harmful bacteria and enzymes (Russell and Diez-Gonzalez 1997).

These findings were similar to those reported by Tu et al (2014), who found that pH values in the first week of the ensiling process remarkedly decreased as the speed of fermentation happened immediately after ensiling. This is an important period because with neutral pH, harmful bacteria can develop, thereby destroying the ensiling materials over the following two to three weeks. Furthermore, this also eliminates the development of mold and prevents the decrease of CP content in silage (Muck 1988). Changes in pH in the present study were similar to the conclusion of Moran (2005), that the silage process succeeds when the pH of silage is between 3.5 and 4.5.

Table 2. Effect of wilting and rice bran levels on ensiling characteristics of Taro petiole and leaf
Parameters	Days of ensiling							SEM	p
Parameters	1	7	14	21	30	45	60	SEM	p

pH
F	5.42^a	4.36^b	3.68^c	3.69^c	3.57^c	3.41^c	3.52^c	0.09	0.001
W	5.59^a	4.23^b	3.77^b	3.75^b	3.55^b	3.50^b	3.58^b	0.16	0.001
Rb₁₀F	5.09^a	4.12^b	3.50^bc	3.29^bc	3.36^bc	3.20^c	3.14^c	0.20	0.001
Rb₂₀F	5.17^a	4.67^b	3.60^c	3.48^cd	3.44^cd	3.33^d	3.36^d	0.04	0.001
Dry matter (%)
F	10.5^a	9.5^ab	9.4^ab	8.7^b	8.8^b	8.6^b	7.2^c	0.27	0.001
W	50.4^a	47.5^b	43.1^c	40.0^d	39.9^d	38.6^de	36.6^e	0.47	0.001
Rb₁₀F	17.8^a	16.9^ab	15.8^abc	15.1^bc	14.8^c	14.0^c	13.9^c	0.48	0.001
Rb₂₀F	26.2^a	24.2^ab	23.3^bc	22.1^bc	21.7^c	18.9^d	18.6^d	0.47	0.001
Crude protein (% of DM)
F	12.9	12.0	12.2	12.1	12.2	12.3	12.3	0.45	0.902
W	13.1	13.5	13.4	13.7	13.3	13.0	13.3	0.69	0.991
Rb₁₀F	13.7	13.3	13.7	13.6	13.8	13.5	13.1	0.26	0.380
Rb₂₀F	13.2	12.7	13.3	13.7	13.3	13.1	13.3	0.24	0.185
Ammonia (% of DM)
F	0.62^d	1.25^c	1.54^ab	1.58^a	1.58^ab	1.54^b	1.57^ab	0.01	0.001
W	0.64^c	0.93^b	1.13^ab	1.23^a	1.28^a	1.29^a	1.34^a	0.05	0.001
Rb₁₀F	0.63^c	0.89^bc	1.17^ab	1.25^a	1.33^a	1.31^a	1.42^a	0.07	0.001
Rb₂₀F	0.60^c	0.92^bc	1.16^ab	1.25^ab	1.22^ab	1.31^ab	1.40^a	0.09	0.001
Oxalate (% of DM)
F	6.14^a	4.95^b	3.97^c	3.62^cd	3.24^d	3.36^d	3.22^d	0.09	0.001
W	6.01^a	4.77^b	3.35^c	3.21^c	2.56^c	2.57^c	2.67^c	0.19	0.001
Rb₁₀F	5.26^a	4.26^b	3.17^c	2.89^cd	2.78^cd	2.28^d	2.29^d	0.17	0.001
Rb₂₀F	4.19^a	3.69^a	2.83^b	2.22^b	2.29^b	2.35^b	2.34^b	0.18	0.001
Lactic acid (% of DM)
F	1.53^c	1.53^c	2.04^b	2.19^b	3.43^a	3.43^a	3.39^a	0.04	0.001
W	1.64^c	2.14^c	3.23^b	3.90^ab	4.30^a	4.40^a	4.38^a	0.17	0.001
Rb₁₀F	1.42^d	2.69^c	3.50^b	4.41^a	4.24^ab	4.76^a	4.78^a	0.18	0.001
Rb₂₀F	1.57^d	2.57^c	3.34^bc	3.98^ab	3.90^ab	4.40^a	4.71^a	0.21	0.001
Acetic acid (% of DM)
F	0.84^c	0.84^c	0.84^c	0.95^b	0.95^b	1.02^a	1.07^a	0.02	0.001
W	0.81^c	0.90^c	0.91^b	1.41^a	1.45^a	1.55^a	1.58^a	0.20	0.030
Rb₁₀F	0.97^b	1.09^ab	1.30^ab	1.49^ab	1.54^ab	1.61^ab	1.77^a	0.16	0.015
Rb₂₀F	1.05^b	1.11^ab	1.37^ab	1.44^ab	1.52^ab	1.59^ab	1.66^a	0.13	0.012
F: 100% fresh taro leaf and petiole; W: 100% wilting leaf and petiole (30-40% of DM); Rb₁₀F: 89.5% of fresh taro leaf and petiole + 10% rice bran + 0.5% salt; Rb₂₀F: 79.5% of fresh taro leaf and petiole + 20% rice bran + 0.5% salt

The DM content of taro silage in all treatments decreased with the time of ensiling (Figure 1b). These results concur with the report of Malavanh et al (2008) that DM of taro leaves declined when ensiling with sugar cane molasses. On the contrary, CP concentration of silage remained unchanged. Previously, the findings of An and Lindberg (2004) showed that CP concentration tended to slow down by 0.7 to 1.3% after 56 days of ensiling while Lin et al (1988) stated that the nutritional values of silage including ME, fatty acids, and amino acids did not change during ensiling. Ammonia of the silage dramatically increased after 30 days because TPL protein was broken down into amino acids, amines and ammonia. This result was in agreement with the conclusions of Kinh et al (1997), who reported that the concentration of ammonia went up from 8.54% to 9.73% after 30 days of ensiling cashew apples and poultry litter.





Figure 1. Effect of wilting on changes of pH (a), DM (b), CP (c), ammonia (d), oxalate (e), lactic acid (f) and acetic acid (g) of taro silage during the ensiling period. Fresh TPL: 100% fresh taro leaf and petiole; Wilting TPL: 100% wilting leaf and petiole (30-40% of DM)

There was a dramatic decrease in the concentration of oxalate after 30 days and it was stable thereafter because the anaerobic ensiled environment at first resulted in production inhibition of glycolate oxidase enzymes that stopped the synthesis of oxalate. This, coupled with the decomposition of oxalate, led to a reduction of oxalate (Rooke and Ronald 2003). The result was similar to the report of Hang and Preston (2010), who found that oxalate decreased in ensiled TPL. A reduction of oxalate in ensiled taro leaves was also reported previously (Malavanh et al 2008; Hang et al 2016; Hang et al 2018).

pH of both fresh and wilted TPL silage registered a fall of 2 units after 60 days. Meanwhile, the concentration of DM, CP, LA, and AA of wilted taro were higher than those of taro silage made from fresh taro foliage. This could be attributed to the fall in moisture percentage as a result of wilting. Also, organic matter of wilted TPL increased, giving rise to an increase in the concentration of acetic and lactic acid.

The concentration of ammonia and oxalate in wilted TPL silage was reduced remarkedly compared to that of fresh TPL silage. The reduction of oxalate concentration from wilting TPL silage was impacted by both sunshine and the anaerobic ensiling environment that inhibited the synthesis of oxalate and decomposition of oxalate. The result of Hang et al (2011) indicated that taro petioles wilted at 37-38^oC for 18 hours reduced by 14.2% the oxalate while taro petioles ensiled for 14 days reduced 36.8% of the oxalate. According to the report of Hang et al (2013), TPL (purple stem and light green stem varieties) processed by wilting for 18 hours caused an overall 5.9% reduction of soluble oxalates. Therefore, TPL wilted before ensiling led to a major decline in the concentration of oxalate of TPL silage.

The results in Figure 2 indicate that DM of TPL silage with added 20% of rice bran was higher than that of TPL silage supplemented with rice bran at 10%. However, the concentration of ammonia, oxalate, LA, and AA of TPL silage in Rb₁₀F and Rb₂₀F on day 60 of ensiling were similar. Hang et al (2016) suggested that the silage was optimal for using when oxalate no longer changed. Based on the decreased level of DM and stability of CP, the treatment Rb₂₀F ensiled for 30 days was considered to produce excellent silage for animals.





Figure 2. Effect of rice bran addition on changes of pH (a), DM (b), CP (c), ammonia (d), oxalate (e), lactic acid (f) and acetic acid (g) of taro silage during the ensiling period. Rb₁₀F : 10% rice bran + 89.5% of fresh taro leaf and petiole + 0.5% salt; Rb₂₀F : 20% rice bran + 79.5% of fresh taro leaf and petiole + 0.5% sal

Conclusions

Wilting TPL before ensiling resulted in better quality TPL silage compared with not wilting before ensiling.
Inclusion of 20% of rice bran in silage made from fresh TPL proved to give optimal quality because of the increase in lactic acid the decrease in oxalate.

Acknowledgments

This research was funded by the scientific research grant of Tra Vinh University.

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Received 23 March 2020; Accepted 14 April 2020; Published 1 May 2020

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