Effect on nutritive value of cassava (Manihot esculenta Crantz) stems of ensiling them with urea

Le Thi Thuy Hang and T R Preston¹

Department of Animal Sciences and Veterinary Medicine, Agricultural and Natural Resources Faculty, An Giang University, Vietnam
thuyhang.agu@gmail.com
¹ Centro para la Investigación en Sistemas Sostenibles de Producción Agropecuaria (CIPAV), Carrera 25 No 6-62 Cali, Colombia

Abstract

Cassava stems are used partly as plant material for the next crop, but the greater part is discarded after root harvest. The ready availability of this waste product has led to experiments in our laboratory to utilize them as the basal diet for goats. The stems contain about 33% DM but only 5.5% crude protein (CP) in the DM. It was therefore hypothesized that there could be a double benefit from ensiling the cassava stems with urea: (i) to provide the ammonia needed by rumen organisms; and (ii) to improve the digestibility of the stem DM as has been widely proven in the urea-ensiling of low-protein, fibrous feeds such as rice straw. The treatments in a random block 5*5 factorial design were: (a) five levels of urea (0, 1, 2, 3 and 4%, DM basis) added to freshly chopped cassava stems; and (b) five storage times (0, 2, 4, 6 and 8 weeks). Each treatment combination was replicated 4 times.

The positive effects of storing (ensiling) the cassava stem with addition of urea were the reduction in HCN levels and the possible synthesis of protein from the ammonia derived from the urea and the fermentation of part of the carbohydrate in the cassava stems. On the negative side was the considerable loss of biomass (about 24%) resulting from the fermentation of part of the cassava stem carbohydrate stimulated by the availability of ammonia from the added urea.

Key words: ammonia, fermentation, HCN, protein, tannins

Introduction

Cassava (Manihot esculenta Crantz) is a perennial woody shrub of the family Euphorbiaceae. It originated in the Caribbean and South America and is extensively cultivated as an annual crop in the tropics and sub-tropics for the dual purpose of tuberous roots for human consumption and roots and foliage as a feed for animals. Cassava foliage is recognized as a source of bypass protein with a high content of digestible nutrients for both non-ruminants and ruminants (Wanapat 1997). The foliage can be used as a supplement for animals in either fresh or wilted form or as hay (Phengvichith and Ledin,2007; Wanapat et al 1997). At root harvest, 9 to 10 months after planting, the foliage production can be about 5 tonnes dry matter (DM)/ha (Mui 1994) . It is estimated that more than 2.5 milion tonnes of cassava foliage are produced in Vietnam, of which about 15,000 tonnes in An Giang, Cassava foliage is usually thrown away after harvesting the root, because of its content of cyanogenic glucoside, mainly linamarin and lotaustralin (Alan and John 1993). Hydrolysis of these cyanogenic glucosides liberates hydrogen cyanide (HCN) (Poulton 1988) and causes toxicity symptoms in animals when the tolerated dose is exceeded.

Cassava foliage consists of the leaves, petioles and small branches which attach to the highly lignified stem. Observations at the Rabbit and Goat Center in Bavi, North Vietnam indicated that the stem was well appreciated by goats and this led to the experiment reported by Thanh et al (2013) in which chopped cassava stems supplemented with fresh cassava foliage supported live weight gains in growing goats of 57 g/day, 100% higher than when Guinea grass was used to supplement the cassava stems.

According to Thanh et al (2013), cassava stems contain 33% DM but only 5.5% crude protein (CP) in the DM. It was therefore hypothesized that there could be a double benefit from ensiling the cassava stems with urea: (i) to provide the ammonia needed by rumen organisms; and (ii) to improve the digestibility of the stem DM as has been widely proven in the urea-ensiling of low-protein, fibrous feeds such as rice straw (Trach et al 1998).

The specific objectives were to determine if the addition of urea to cassava stems would facilitate the storage of this feed resource and at the same time improve its digestibility.

Material and methods

The experiment was carried out at An Giang University in An Giang Province in the South of Vietnam from March to June 2015.

Treatments and experimental design

The treatments in a random block 5*5 factorial design were: (a) five levels of urea (0, 1, 2, 3 and 4%, DM basis) added to freshly chopped cassava stems; and (b) five storage times (0, 2, 4, 6 and 8 weeks). Each treatment combination was replicated 4 times. Cassava stems were collected from farmers’ fields directly after root harvesting and chopped by hand. Representative amounts were analyzed for DM by infra-red radiation (Undersander et al 1993) prior to hand mixing 20 kg quantities with the indicated amounts of crystalline urea followed by storage in polyethylene bags which were then sealed.

After the appropriate storage times, samples of the treated stems were taken for measurement of pH (ORION model 420 A) and proximate composition. The DM, ash and HCN content were determined according to the standard methods of AOAC (2016). Nitrogen was determined by the Kjeldahl procedure. NDF and ADF were analysed according to the procedure of Van Soest et al(1991). Total tannin content was determined according to the method (955.35) of AOAC (2016).

Statistical analysis

The data were subjected to an analysis of variance (ANOVA) using the General Linear Model (GLM) procedure of Minitab 16. Sources of variation were levels of urea, storage time, the interaction urea levels*storage time and random error.

Results and discussion

There were major effects of urea level and storage time on chemical attributes of the urea-ensiled cassava stems (Tables 1, 2 and 3; Figures 1 – 8).

The content of tannin was reduced after 4 weeks of storage and the effect tended to be greater the higher the level of urea (Figure 1).

Figure 1. Effect of urea level and storage time on tannins in cassava stems

The content of HCN in the stems was reduced gradually over the first two weeks and then more rapidly after 4 weeks with none being detected after 6 weeks of storage (Figure 2).

Figure 2. Effect of urea level and storage time on HCN in cassava stems

Ammonia level increased massively in the second week of storage, then fell by half at 4 weeks the levels being proportional to the amounts of urea added (Figure 3).

Figure 3. Effect of urea level and storage time on ammonia in cassava stems

There were consistent effects of urea level on the pH in the stored stems with curvilinear increases to maximum values after 4 weeks of storage declining subsequently (Figure 4). Within storage times the pH was positively related to the level of urea added at the beginning of storage.

Figure 4. Effect of urea level and storage time on pH in cassava stems

After the second week of storage, NDF and ADF levels were reduced linearly by increasing levels of urea and by length of storage time; however, the changes were of relatively small order (Figures 5 and 6).

Figure 5. Effect of urea level and storage time on NDF in cassava stems

Figure 6. Effect of urea level and storage time on ADF in cassava stems

As expected, the crude protein level in the stems was related linearly to the proportion of urea added at the beginning (Figure 7). There were only slight reductions in overall CP levels with length of storage

Figure 7. Effect of urea level and storage time on crude protein in cassava stems

Urea level had no effect on the DM content of the cassava stems during the first two weeks of storage, when the DM content of the cassava stems did not change (Table 3); but from 4 to 8 weeks of storage, the DM content declined linearly, and the decline was increased linearly with the level of added urea (Figure 8).

Figure 8. Effect of urea level (0 to 4% in DM) and storage time on the DM content in the cassava stems

Discussion

The increase in ammonia and in pH in the stored cassava stems is similar to what has been reported for urea-treatment of other fibrous byproducts such as rice straw (Thuy Hang et al 2005; Trach et al 1998).

The decrease in tannin with urea treatment is likely to be a result of the high pH caused by evolution of ammonia from urea (Price et al 1979; Makkar 2003a,b). Tannins are easily oxidized at alkaline pH values to quinines, which may promote covalent bonds to other compounds (Rawel et al 2000).

The decrease in HCN with storage time may similarly be the result of the high pH (>7.00) following 2 weeks of storage with urea and would appear to be related to chemical reactions resulting in neutralization of the hydrocyanic acid by the ammonia. A decrease in HCN toxicity has been reported as a result of increasing the pH of the medium (Huertas et al 2010).

The data for crude protein (N*6.25) is misleading as they do not differentiate between true protein and the products of multiplying the N content by 6.25. The result of major concern for the farmer is the loss of DM from the combined effect of storage time and level of added urea, which resulted in the DM content of the stored stems declining from initial values of 23.6% to 17.6% after 8 weeks of storage with 4% added urea (a loss of about 24%; Figure 8). The slight decline in the percentages of NDF (about 10%) and ADF (4%) account for only part of the losses; the remainder supposedly being in the form of soluble carbohydrates. There may have been some gain in true protein during storage, but this could not be ascertained in the absence of analytical data for true protein.

Conclusions

• The positive effects of storing (ensiling) the cassava stem with addition of urea are the reduction in HCN levels and the possible synthesis of protein from the ammonia derived from the urea and the fermentation of part of the carbohydrate in the cassava stems.
  
  
• On the negative side is the considerable loss of biomass (about 24%) resulting from the fermentation of part of the cassava stems stimulated by the availability of ammonia rom the added urea.

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Received 20 May 2019; Accepted 20 May 2019; Published 4 June 2019

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