Digestibility, nitrogen balance and methane emissions in goats fed cassava foliage and restricted levels of brewers’ grains

Le Thi Thuy Hang, T R Preston¹, Nguyen Xuan Ba² and Dinh Van Dung²

Faculty 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
² Hue University of Agriculture and Forestry, Hue University, Hue City, Vietnam

Abstract

Four “Bach Thao” goats (14 ± 2 kg) were fed fresh cassava foliage (sweet variety) ad libitum and 4 levels (0, 2, 4 and 6%, DM basis) of brewers’ grains in a 4*4 Latin square changeover design with periods of 14 days.

Adding 4% of brewers’ grains to the diet of cassava foliage increased the DM intake, the apparent DM digestibility, the N retention and the biological value of the absorbed nitrogenous compounds. The methane levels in eructed gas increased with a positive curvilinear trend as the proportion of brewers’ grains in the diet was increased. The benefits of small quantities of brewers’ grains in the diet are believed to be related to their “prebiotic” qualities in enhancing the action of beneficial microbial communities along the digestive tract of the animal.

Key words: Bach Thao, biofilms, biological value, microbial communities, prebiotics

Introduction

Cassava (Manihot esculenta) is a major crop in Vietnam, grown on 570,000 ha producing annually some 1 million tonnes of roots (GSO 2016). The roots are used mainly for manufacture of starch and as an ingredient in livestock feed. Growing the crop as a semi-perennial forage with repeated harvesting at 2 to 3month intervals is a recent development (Wanapat 1997; Preston et al 2000; San Thy and Preston 2001). Several reports have shown the benefits of the fresh foliage as a source of bypass protein in ruminant diets based on molasses-urea (Ffoulkes and Preston 1978), rice straw (Do et al 2002; fresh cassava stems (Thanh et al 2013) and ensiled cassava pulp-urea (Toum et al 2017; Binh et al 2017).

The use of fresh cassava foliage as the sole diet of goats was pioneered by Vor Sina et al (2017). Growth rates on a diet of fresh cassava foliage were 65 g/day and were doubled to 160 g/day when a small supplement (5%) of ensiled brewers’ grains was included in the diet, It was proposed that this “synergistic” effect of the brewers’ grains was due to its role a s a source of beta-glucan, a component of the cell walls of cereal grains and fungi such as yeasts, that has been shown to have prebiotic properties (Novak and Vetvicka 2008).

The present experiment was designed to provide further evidence for the prebiotic effect of brewers’ grains in a basal diet of cassava foliage fed to growing goats. Proportions of ensiled brewers’ grains above (6%) and below (2%) the 4% level were compared to identify the optimum level.

Materials and methods

Experimental design

The experiment was conducted from July to November 2016 at An Giang University farm, An Giang province, Vietnam. Four “Bach Thao” goats (14 ± 2 kg) were fed the 4 levels if ensiled brewers’ grains (0, 2, 4 and 6% DM basis) as the only supplement to a diet of ad libitum fresh cassava foliage (sweet variety). The design was a Latin square (Table 1) with four treatments and four periods, each lasting 15 days (ten days for adaptation and 5 days for collection of feces and urine).

Animals and management

The goats were housed in metabolism cages made from bamboo, esigned to collect separately feces and urine. They were vaccinated against Pasteurellosis and Foot and Mouth disease and treated with Ivermectin (1ml/10 kg live weight) to control internal and external parasites. They were weighed between 06:30 and 07:30h before feeding at the start and end of the experimental periods, and prior to the start of each 5-day collection period.

Feeds and feeding management

The cassava (sweet cassava variety) was planted in sandy soil in the An Giang University farm. from April to October 2016. It was fertilized with 8 tonnes/ha of cattle manure, 175 kg  Urea, 200 kg Super phosphate and  130kg Potassium chloride. The first application was between 25 and 30 days after planting and the second application from 50 to 60 days after planting.

The foliage was harvested 50-60cm above soil level at intervals of 120 days when it had attained a height of 100 - 120 cm. Harvesting of the cassava was done 2hprior to each feed, morning and afternoon. On rainy days the cassava foliage was harvested the day before feeding so as to avoid excessive levels of moisture in the foliage. The forage was chopped by hand prior to being put into the feed troughs.  The brewers’ grains were brought from Kien Giang Province every 5 days. They were stored in closed plastic bags. The chosen amounts were offered twice daily in troughs separate from the cassava foliage. Feed refusals were weighed every morning prior to giving the new feed. Samples of each diet component were collected daily, stored at -18C, and bulked at the end of each period for analysis. 

Analytical procedures

The sub-samples of feeds offered and refused, and the feces, were analysed for dry matter, ash and nitrogen by AOAC (1990) methods. Neutral detergent fiber (NDF) and acid detergent fiber (ADF) were analyzed according to the procedure of Van Soest and Robertson (1985).). Nitrogen in urine and ammonia in rumen fluid were determined by the Kjeldahl method (AOAC 1990). The pH of the rumen fluid was determined by using an electronic meter (Eco Testr pH2). The concentration of ammonia nitrogen in the rumen fluid was determined by diluting 15 ml of ruminal fluid with 5 drops of concentrated H2SO4 and distilling and titrating the released ammonia by the standard Kjeldahl procedure (AOAC 1990). The protozoan population in the rumen fluid was estimated by diluting 8 ml of ruminal fluid with 16 ml of formaldehyde-saline solution (37 % formaldehyde with saline solution 1:9) and counting the protozoa under light-microscopy (100x magnification) using a 0.2 mm deep Dollfus counting chamber. Four fields in the counting chamber were filled and protozoa counted, according to the method described by Jouany and Senaud (1979) and Dehority (1993).

Digestibility and N retention

During the data collection periods, the feces and urine were recorded twice daily at 7:00 and 16:00 and added to jars containing 100 ml of 10% sulphuric acid. The pH was measured and, if necessary, more acid added to keep the pH below 4.0. After each collection period : (i) a sample of 10% of the urine was stored at ­-4^o C for analysis of nitrogen (AOAC 1990); (ii) the feces were mixed and a sample (10%) stored frozen at –20^oC.C.

Gas emission measurement

At the end of each period the goats were confined individually in a gas-proof chamber (a bamboo frame covered with plastic) for sampling of eructed gases and residual air in the chamber. Measurements of the concentrations of methane and carbon dioxide were taken continuously over a 10-minute period, using a Gasmet infra-red meter (GASMET 4030; Gasmet Technologies Oy, Pulttitie 8A, FI-00880 Helsinki, Finland).

Statistical analysis

Data were analyzed with the General Linear Model option of the ANOVA program in the MINITAB software (Minitab 2000). Sources of variation were treatments, animals, periods and error.

Results and discussion

Composition of diet ingredients

The crude protein (CP) of the cassava foliage (leaf and petiole combined) was considerably lower than the value of 21% CP in DM reported by Vor Sina et al (2016) where the leaf alone had 29% CP in DM and the petiole 9.6% in DM).

Feed intake and digestibility

DM intake followed a curvilinear trend with the peak intake occurring when the BG content of the diet DM reached 4%, declining when the BG was raised to 6% (Table 3 and Figure 1). The same trend was seen for change in live weight (Figure 2) and DM feed conversion (Figure 3).

Figure 1. Effect of level of ensiled brewers’ grains on DM intake	Figure 2. Effect of level of ensiled brewers’ grains on live weight gain

Figure 3. Effect of level of ensiled brewers’ grains on DM feed conversion

Coefficients of apparent digestibility of crude protein and DM showed the same curvilinear trends as were recorded for DM intake, LW gain and feed conversion, with maximum values when the ensiled brewers’ grains were approximately 4% of the diet DM (Table 4 and Figure 4).

Figure 4. Effect of level of ensiled brewers’ grains on apparent digestibility of DM and crude protein

Rumen parameters

All criteria of rumen fermentation showed linear decreasing trends as the level of ensiled brewers’ grains in the diet was increased (Table 5; Figures 5 and 6). The probable explanation of this trend is the stimulus to eating, and therefore to rumen fermentation, following the offering of fresh feed in the morning. Reduction in ammonia levels and protozoal numbers are the logical result of the decrease in pH due to the increased rate of fermentation.

Figure 5. Effect of level of ensiled brewers’ grains on rumen pH before and after offering new morning feed	Figure 6. Effect of level of ensiled brewers’ grains on rumen ammonia before and after offering new morning feed

Nitrogen retention

Retention of nitrogen, per day and as a percentage of the nitrogen digested, showed curvilinear trends with the optimum coinciding with the 4% level of ensiled brewers’ grains in the diet (Table 6; Figures 7 and 8). The effect of adding 4% brewers’ grains to the diet was a 65% increase in N retention and a 14% increase in N retained per unit of N digested.

Figure 7. Effect of dietary level of ensiled brewers’ grains on N retention	Figure 8. Effect of dietary level of ensiled brewers’ grains on N retention as a percentage of N digested

Methane emissions

The ratio of methane to carbon dioxide in the mixture of eructed gas and air in the plastic-enclosed chambers increased with a curvilinear trend as the level of brewers’ grains in the diet was increased (Table 7; Figure 9). The trend was similar to that reported when cassava foliage was replaced by brewers’ grains in a fattening diet fed to cattle (Binh et al 2017: Toum et al 2017) ; however, the replacement rate in both these cases was over a much wider range of brewers’ grains (eg: Figure 10), the proportion of cassava foliage was lower and the basis of the diet was ensiled cassava pulp-urea.

Figure 9. Effect of increasing proportions of brewers’ grains replacing cassava foliage on methane: carbon dioxide ratio in mixed air-expired breath of the goats	Figure 10. Methane: carbon dioxide ratio in mixed air-expired breath of cattle fed increasing proportions of brewers’ grains replacing cassava foliage in a fattening diet based on cassava pulp:urea (from Toum et al 2017)

Discussion

The 65% increase in N retention, and corresponding increase in live weight gain, with addition of 4% brewers’ grains to an exclusive diet of fresh cassava foliage, followed by the decline in N retention when the proportion of brewers’ grains was increased to 6%, shows that the benefit of the brewers’ grains was not by enhancing the supply of bypass protein. On the other hand, the 14% increase in N retention as percentage of digested nitrogen indicates that the biological value of the absorbed amino-acids was improved by supplementation with brewers’ grains, the implication being that the brewers’ grains had facilitated the activity of rumen microbes in the synthesis of microbial protein. We suggest that these results strengthen the original proposal of Binh et al (2017) “that the brewers’ grains act as a site (substratum) for biofilm attachment of detoxifying microbes and as a source of nutrients for their detoxifying activity”. In this respect, the benefits of the small quantity of brewers grains in the animals’ diet suggest that on this context their role is as a “prebiotic” enhancing the activities and effectiveness of beneficial microbial communities.

Conclusions

• Adding 4% of brewers’ grains to a diet of cassava foliage increased the DM intake, the apparent DM digestibility, the N retention and the biological value of the absorbed nitrogenous compounds.
  
  
• The benefits of such small quantities of brewers’ grains are believed to be related to their “prebiotic” qualities in enhancing the action of beneficial microbial communities along the digestive tract of the animal.

References

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Received 29 November 2017; Accepted 25 February 2018; Published 1 April 2018

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