Quality and preference of zebu heifers for legume or elephant grass-silages with cassava peel

O A Olorunnisomo and O H Fayomi

Department of Animal Science, University of Ibadan, Ibadan, Nigeria
sholanisomo@yahoo.com

Abstract

Three legumes (Leucaena leucocephala, Gliricidia sepium and Enterolobium cyclocarpum) and elephant grass (Pennisetum purpureum) were ensiled with different proportions of cassava peel (0, 25 and 50% on a wet basis). Physical characteristics of silage, pH and chemical composition were assessed at 21 days of ensiling. Twelve zebu heifers were used to assess preference for the silages.  

All silages were firm in texture while smell was greatly influenced by the original fresh forage. The color varied from light green with elephant grass to brown with leucaena. The pH ranged from 3.6 to 5.4, the silage with cassava peel had the lower values. Crude protein level varied from 6.7 to 22.9 and crude fibre from 12.0 to 24.0%. The addition of cassava peel to the legumes greatly reduced silage pH, while protein levels were maintained at acceptable levels for ruminants. The order of preference was: elephant grass > leucaena > gliricidia > enterolobium.  Legume forages with their high protein ensiled with cassava peel can contribute  to solving the problem of poor nutrition and low milk production in zebu cattle in Nigeria.

Key words: cattle, enterolobium, gliricidia, leuceana, Pennisetum purpureum

Introduction

A major constraint to milk production in Nigeria is inadequate nutrition and inability of zebu cattle to meet their energy and protein needs from low quality native pastures.  This problem is further compounded by the scarcity and extremely low quality of forages during the dry season (Pamo and Pieper 2000).  It has been shown that the crude protein (CP) content of native grasses during the dry season is between 1.5 and 3.0% (Adamu et al 1993). This is far below the minimum level of 6 – 7% CP required to enhance voluntary intake, digestibility and utilization by ruminants (Milford and Haydock 1965).  It is common in Nigeria to find ruminants that are well-fed and robust during the rainy season which lose appreciable weight in the following dry season (Adegbola 1998). The problem of low quality feeds and forage scarcity during the dry season can be reduced drastically by utilizing tree legumes and agro-industrial by-products, like cassava peel. Leucaena leucocephala and Gliricidia sepium are common tree legumes found in the humid and sub humid parts of Nigeria. Enterolobium cyclocarpum is another tree legume that is gradually gaining acceptance (Babayemi 2006, Ezenwa 1998). These tree legumes have high protein leaves with potential to improve feed quality and milk production in zebu cattle. 

Legume forages usually make poor silage due to their low content of carbohydrates and their high buffering capacity (Wilkins 2001) hence they require addition of readily fermentable carbohydrates (RFC) in order to obtain good silage. Cassava peel, an abundant by-product of cassava root processing in Nigeria, is a source of RFC (Onua and Okeke 1999). Its addition to legume forage can enhance fermentation and improve energy concentration. 

The objectives of this study was to assess the  characteristics, chemical composition  and preference by zebu heifers of three legumes (Leucaena leucocephala, Gliricidia sepium, Enterolobium cyclocarpum)  and elephant grass ensiled with different proportions of cassava peel. 

Materials and methods

The experiment was conducted at the Dairy Unit of the Teaching and Research Farm, University of Ibadan, Ibadan, Nigeria (3º45´E, 7º27´N; at 220m above sea level). Annual rainfall ranges from 1150 - 1500mm.

Preparation of silage
 

Three legumes, cut at 16 weeks of age, and elephant grass, cut at 10 weeks, were wilted for 24 h and chopped to 3cm in length using an automated chopping machine. The forages were mixed with chopped cassava peel at 0, 25 and 50% levels, on a wet basis, for laboratory studies, and at 50% for acceptability study. The mixtures were ensiled in 4-litre plastic buckets (mini silos) in triplicates for laboratory analysis, and 120 litre plastic drums for acceptability study.

Silage examination and laboratory analysis
 

 Mini silos were opened at 21 days to determine pH, physical characteristics (colour, smell and texture) and proximate composition using the general procedures of AOAC (2005). Dry matter (DM) was determined using a forced draught oven at 65^oC, correcting values for the loss of volatile compounds by multiplying with the factor of 1.056 (Fox and Fenderson, 1978). Detergent fibre components were determined according to methods of Van Soest and Robertson (1985).

Acceptability study
 

Twelve female zebu cattle aged about 12 months and weighing between 120 and 140 kg were used for the preference trial. Animals were treated against internal and external parasites using ivermectin and housed in one group in a pen. The silages (20 kg each on a wet basis) were offered in various troughs in the pen for a period of 14 days. Location of silages within the pen was interchanged daily to prevent association of diets with a particular location. Fresh water was offered free choice on a daily basis. Silage intake was measured 4 hours after it was offered by deducting remnants, but  animals were allowed eat the remnant silage for the rest of the day. The coefficient of preference (COP) was calculated as the ratio of individual silage intake to the average intake of all silages.  Preference was calculated as the ratio of individual intake to total intake multiplied by 100. Silage was regarded as acceptable when the COP was greater than unity while ranking was based on percentage of preference.

Experimental design/statistical analysis

The experimental design adopted for the mini silos study was the completely randomized design. Data obtained were subjected to analysis of variance and significant means were separated by Duncan’s multiple range tests using the procedures of SAS (1995). 

Results

Table 1 shows the physical characteristics of the silages.

Colour of silages varied from pale green to brown. Gliricidia and elephant grass silage largely maintained a green colour but leucaena and enterolobium silage changed from green to brown. Silages containing cassava peel had a brown and white speckled appearance which is attributed to the brown skin colour and white flesh of cassava tuber. The smell of the silages was generally influenced by the fresh forage and appeared to improve with increasing proportion of cassava peel. Gliricidia silages had a strong smell which is characteristic of coumarin while enterolobium forages had its characteristic sharp prickly smell. All the silages had a firm texture and there were no apparent differences among them. The pH decreased significantly (P < 0.05) as the proportion of cassava peel increased while there were no significant differences (P < 0.05) in pH values between legume silages at similar levels of cassava peel addition. The pH value of grass silage without cassava peel was significantly lower than legumes silages without cassava peel, however, when cassava peel was added, there was little or no difference in pH values between legume and grass silages.

Table 2 shows the chemical composition of silages. Dry matter content increased with the inclusion of cassava peel while crude protein content reduced. Legume silages had higher protein content than the grass silage. The fibre components of the diets (CF, NDF, ADF, ADL) reduced as cassava peel in the mixture increased. The fibre level in elephant grass silage was generally higher than legume silages although NDF values were similar between elephant grass and leucaena silage. In Figure 1, the CP content and fibre components were compared. Legume silages showed a clear advantage over elephant grass silage in terms of CP while there were only slight differences  among the legume silages (leucaena > gliricidia > enterolobium).

Intake and coefficient of preference (COP) for elephant grass and legume silages among zebu cattle are presented in Table 3. There were significant differences (P < 0.05) in intake and coefficient of preference for the different silages with cattle showing a higher preference for elephant grass silage than legume silages. After 4 hours the highest intake (fresh basis) was for elephant grass and the least for enterolobium  (elephant grass > leucaena > gliricdia > enterolobium). Figure 2 shows the preference of zebu cattle for elephant grass and legume silages as percentage of total forage intake. There were significant differences (P < 0.05) in preference between the silages. (elephant grass > leucaena > gliricidia > enterolobium).

Discussion

A light colour in silage generally indicates good fermentation while a dark colour indicates spoilage. The brownish colour of leucaena and enterolobium silage in this study (Table 1) may not be indicative of spoilage, since pH of these silages shows that there was adequate fermentation. Unidentified chemical reactions in these silages with the addition of cassava peels may be responsible for the colour change; hence further research is required to explain this observation.  The brown and white speckled appearance observed in forages ensiled with cassava peel was due to the characteristic brown colour of cassava root skin and the white colour of the flesh which was peeled together with the skin. The smell of the silage was largely influenced by the smell of the fresh forage and appears to improve with increasing proportion of cassava peel in the mixture.  All the silages had firm texture. This is partly due to the fibre components which showed significant levels of lignin in all the forages used.  

Addition of cassava peel to elephant grass and legume forages significantly reduced pH of the silage (Table 1). This suggests that cassava peel content improves fermentation in silages. Cassava processed in the southwest of Nigeria leaves a significant proportion of the flesh in the peeled skin, hence high starch content (Onua and Okeke, 1999). Addition of cassava peel to legume and grass forage therefore provided sufficient substrate for lactic acid bacteria to act on and significantly lowered the pH of the wet forage, thus preserving it. Elephant grass alone was better preserved (lower pH) than legume forages ensiled without cassava peel due to the higher buffering capacity associated with legume forages (Wilkins, 2001). However, when cassava peel was mixed with the forages, there were no differences in pH values between legume and grass silages, showing that cassava peel significantly reduced the buffering capacity of legume silages. Since high buffering capacity in legumes usually limits their use as silage crops, addition of cassava peel to tropical legume forages may expand their utilization as silage materials for feeding cattle during the dry season.  

The higher dry matter and lower protein content (Table 2) observed in silages containing cassava peel compared to forage ensiled alone is largely a reflection of the higher dry matter and lower protein content of cassava peel compared to the legumes and elephant grass. Expectedly, legume silages had higher protein content than grass silage due to the higher protein content in the legume forage. Although addition of cassava peel to legume forages significantly reduced the crude protein content of the silage, the protein content was still within acceptable range for ruminants (Milford and Haydock, 1965). Since protein content in grass silages were low compared to legume silages, ruminants fed a basal diet of grass-cassava peel silage will require higher protein supplementation than their counterparts fed legume-cassava peel silage. 

The fibre fractions in the silages reduced with inclusion of cassava peel in the mixture. Chemical composition of silage ingredients shows that fibre levels in cassava peel is much lower than in legume or grass forage.  Although the elephant grass (10 weeks old) used in this study was younger than the legumes (16 weeks), the fibre level was generally higher in grass than legume silage. The higher protein content and lower fibre fractions in legume silages compared to elephant grass silage together with the higher digestibility associated with tropical legumes suggests that legume silages might provide better nutrition for ruminants than grass silage during the dry season. 

The COP is a measure of acceptability of forages by ruminants and is determined as the ratio of individual forage intake to the mean intake of all the forages. When values are equal or greater than 1, the forage is considered to be acceptable to the animals but when values are less than 1, the forage is considered to be unacceptable. Based on this, elephant grass and leucaena silages would be considered as acceptable to the cattle in this study while gliricidia and enterolobium silages were not acceptable.  This method however, appears to be a poor measure of acceptability of forages among ruminants since previous experience of the author (Olorunnisomo, 2011) shows that cattle which previously did not accept elephant grass ensiled with cassava peel (COP, 0.65 - 0.69) now readily accept it and prefers it above other silage mixtures. Since acceptability changes with time and dietary experience of the animals, expressing acceptability (or preference) as a percentage of total forage intake may be a better index of preference because it judges acceptability in proportion to overall intake.  

Preference indices show that zebu cattle preferred elephant grass silage to legume silages while leucaena was preferred to glircidia and enterolobium among the legume silages. The initial aversion of cattle and lower preference for gliricidia and enterolobium silages despite the addition of cassava peel may be attributed partly to the strong characteristic smell of these forages and the little previous exposure of cattle in this study to gliricidia and enterolobium forages. Ikhimioya (2008) and Provenza and Cincotta (1994) reported that the previous dietary experience of ruminants with particular forages can influence their choice among a variety of forages. 

Conclusions

• Physical properties and pH indicate that all silages containing cassava peel were better preserved than silage with grass or legume alone.

• Preference indices (COP and % preference) indicate that cattle preferred grass silage to legume silage.

• Crude protein level in the silage however suggests that legume silage will provide better nutrition for cattle and improve milk production than grass silage.

• In order to strike a balance between nutrition and preference of the animals, it is suggested that composite silage should be made from mixtures of elephant grass, cassava peel and available legume forage.

• Further research is recommended to determine the appropriate proportion of grass, cassava peel and legume forage required in the mixture.

References

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Received 18 July 2012; Accepted 14 August 2012; Published 3 September 2012

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