Effect of stage of growth and processing methods on the nutritional content of Acacia angustissima leaf meal harvested for broiler feeding

Sharai Ncube, Petronella Tapiwa Saidi and Tinyiko Edward Halimani

University of Zimbabwe, Faculty of Agriculture, Department of Animal Science, Box MP167, Mt Pleasant, Harare, Zimbabwe
sharaincube7@gmail.com

Abstract

The objective of the study was to determine the effect of stage of harvest and drying technique on the chemical composition of Acacia angustissima leaves. Acacia angustissima leaves were harvested from the Southern African Development Community- International Centre for Research in Agroforestry (SADC-ICRAF) at three stages of maturity, 10 weeks after re-growth for the early growth stage, 15 weeks after re-growth for the mid growth stage and 20 weeks after re-growth for the late growth stage. Each harvest was divided into three portions and exposed to either shade drying, sun drying or oven drying. The AOAC (1990) procedure was used for determination of dry matter, crude protein, ash, crude fiber, calcium, phosphorus and condensed tannin analysis.

Stage of harvest and processing technique had an effect on the chemical composition of the leaf meal (P < 0.05). Crude protein and condensed tannins in early harvested leaves were higher than in the mid and late growth stage (P<0.05). A significant increase in dry matter, ash and crude fiber was noted with advancing leaf maturity (P<0.05). Shade drying retained significantly higher crude protein levels than both sun and oven drying (P<0.05). The crude fiber content in shade dried and sun dried samples was lower than the oven dried (P<0.05). Condensed tannins in oven dried samples were lower than both shade and sun dried (P<0.05). A significant interaction between stage of harvest and method of drying was noted on ash, crude fiber and condensed tannins. It was concluded that as a crude protein source in broiler diets, Acacia angustissima should be harvested at mid maturity stage and dried in a shade.

Keywords: drying, non-essesntial amino acids, poultry diets

Introduction

Acacia aungustissima has been recently widely propagated in agroforestry because of its potential as an animal feed ingredient. A member of the Mimosaceae family, Acacia angustissima is thought to have originated in Belize, Central America (Dzowela 1994). The tropical legume is recommended for use as a fodder tree due to its high potential growth rate and nitrogen fixing capabilities (Preece and Brook 1999). It tolerates occasional freezing conditions, withstands periods of drought, grows rapidily and responds well to regular cutting (Dzowela 1994). Acacia angustissima grows as a thornless shrub or small tree of 2-7 m high with a single short trunk (Ncube et al 2012). Biomass production can be as up to 12.4 t DM ha^-1 depending on tree spacing and cutting back height after each harvest (Preece and Brook 1999). It has mainly been used to feed ruminants, but in recent years there has been interest in using it in poultry diets. In a study by Ncube et al (2012), 10% Acacia angustissima leaf meal was successfully used to replace soyabean meal in broiler starter and finisher diets, indicating the possibility of using it as a protein ingredient.

Current research shows a variation on stages at which Acacia angustissima leaf material was harvested. Ncube et al (2012) harvested at mid maturity stage for use in broiler diets, Mukandiwa et al (2010) harvested mature leaves to feed goats while Rubanza et al (2007) and Gusha et al (2013) make no mention of stage of harvest in their nutritional quantification and recommendation on the shrub legume. The stage of harvest should be determined in consideration with the nutrient requirements of the animal to be fed, thus the need to assess the appropriate stage of harvesting Acacia angustissima leaves for broiler feeding.

The appropriate stage of harvesting and consumption of plant material should be based on nutritional information (Modi 2007) since harvesting stage affects nutritive value of plant material (Kökten et al 2012, Yu et al 2004). When plant material is harvested at the appropriate stage of maturity, optimum nutritional benefits can be obtained from the ingredient (Kökten et al 2012). Inclusion of Acacia angustissima in broiler diets must therefore be informed by the nutritional potential of the different stages of growth in relation to the nutritional requirements of broilers.

To feed poultry species, plant leaves are harvested and processed into leaf meals (Limcangco-Lopez 1989). The process involves drying leaves before grinding into a meal. A variety of processing techniques including sun, oven and shade drying are available for use. While most commercially available leaf meals are sun cured (Limcangco-Lopez 1989), different authors have also used different drying methods for different leaf meals and in some cases for the same leaf material. For example to dry cassava leaves, Ihekwumere et al (2008) and Onibi et al (2008) used the sun while Adeyemi et al (2013) dried it in a shade. To dry Moringa oleifera, Gadzirayi et al (2012), Donkor et al (2013) and Okafor et al (2014) used the shade drying technique, while Teteh et al (2013) used a shade conditioning system and Ayssiwede et al (2011) dried in the sun. Most reports indicate sun drying of Leucaena leucocephala (Onibi et al 2008, Ayssiwede et al 2010 and Franzel et al 2007). The limited information available for Acacia angustissima indicates that when used in broiler diets, the leaves were shade dried (Ncube et al 2012) although no justification was given for that drying approach. It would seem as if the basis of choosing a technique by different authors is not clear as there is lack of consistency in an approach used for drying the leaf meals.

There is no documented information about the stage of harvesting and drying technique suitable for Acacia angustissima for use in poultry diets. For the full potential of Acacia angustissima leaves to be realized, research must determine the stage of growth and processing technique that would result in the best leaf meal quality with reference to broiler nutrient requirement. The study hypothesized that the chemical composition of Acacia angustissima leaf meal will vary with the stage at which it is harvested and the drying technique used. To test the hypothesis, the study determined the effect of stage of growth and method of processing on the nutritive value of Acacia angustissima leaf meals.

Materials and methods

Acacia angustissima leaves were harvested from the Southern African Development Community- International Centre for Research in Agroforestry (SADC-ICRAF) research plots at Domboshava Training Centre, 25 km northeast of Harare. Domboshava is 31°13´E and 17° 30´S at an altitude of 1530 m. Soils the site a sandy-loam in texture and largely of granitic origin. Summer temperatures rise up to a maximum mean monthly of 27.9°C in October and winter temperatures fall to a minimum monthly mean of 5.5°C in July. An area 15 x 15 m was marked out at the Southern African Development Community- International Centre for Research in Agroforestry (SADC-ICRAF) research plots at Domboshava Training Centre. The area was cleared by cutting branches of Acacia angustissima shrubs 1 meter from the ground to allow for re-growth. The area was subdivided into 15 small plots of 1 x 1m. The plots were randomly allocated to three treatments of early stage, mid stage and late stage of re-growth. Each treatment was replicated five times. Leaves from different plots were harvested manually by stem pruning at different stages of growth, 10 weeks after re-growth for the early growth stage, 15 weeks after re-growth for the mid growth stage and 20 weeks after re-growth for the late growth stage. For each replicate, leaves were divided into three portions and subjected to the following processing techniques.

1. Sun drying by placing the leaves on cotton sheets and then putting the sheets in direct sunlight on a concrete floor platform. Leaves were turned occasionally to ensure even drying and brought back indoors at night.
2. Shade drying by placing the leaves on cotton sheets in a well ventilated shade with constant turning.
3. Heat drying by loading the leaves in trays. The trays were put in a 45°C preheated oven for 24 hrs.

The dried samples were ground through a 1mm sieve and tightly sealed in bags, stored in a cool dry place for dry matter, crude protein, crude fibre, calcium, phosphorus and condensed tannins determination (AOAC 1990). Data on chemical analysis was analyzed using an analysis of variance (General Linear Model (GLM) procedure) of SAS (2000) to determine the effect of stage of growth and drying technique on chemical composition of the leaf meals.

Results

Effect of stage of growth on nutritional composition of the leaf meal

Stage of growth had an effect on the dry matter, ash, crude fibre, calcium, phosphorus condensed tannins and crude protein content of the leaf meal. Dry matter content of the leaf meals increased from 90.2% to 90.9% with stage of leaf maturity (Table 1). The early growth stage had the lowest dry matter but this was not different from the mid stage of growth (P=0.1011) while the late stage had the highest dry matter content which was also not different from the mid stage of growth (P= 0.4174). An increase in ash content from 4.34% to 5.12% with plant maturity was noted (Table 1). No difference was noted with regard to ash content between the mid and late stages of growth (P= 0.9656).

Crude protein ranged from 21.7% - 24% and decreased with leaf maturity (Table 1). Although numerically different from each other, the crude protein from the mid and late maturity leaves were not different (P= 0.0697). The crude fiber content increased from 11.5% to 16.6% with age of the plant material (Table 1). However for the mid and late stages of growth, no differences were noted (P= 0.1072). Calcium content increased from 0.924% to 1.006% with age of plant material (Table 1). Levels in the early harvested leaves were not different from leaves harvested at mid-maturity (P=0.505). Phosphorus and condensed tannins content declined with stage of maturity from 0.193 % to 0.143% and 1.087% to 0.973% respectively (Table 1). However no differences were noted in condensed tannin levels between the mid and late growth stages (P= 0.5179).

Effect of drying method on nutritional composition of leaf meal

Method of drying had an effect on dry matter, ash, crude protein, crude fiber, calcium, phosphorus, condensed tannin content retained in the leaf meal (Table 2). The highest dry matter content of 91.4% was obtained in the oven dried leaf meal and lowest, 89.8%, in the shade dried leaf meal. The ash content in sun dried samples was 5.13% and higher (Table 2) than both oven (4.65%) and shade dried (4.77%) ash content. The shade and oven dried samples were however not different from each other with regard to ash content (P=0.6450). However an interaction on ash content was observed between stage of growth and drying technique (Table 3). While the early and mid harvested leaves retained the same amount of ash regardless of the drying method, ash in the late stage harvested leaf was highest in the shade and sun dried samples with the lowest value in the oven dried sample (Table 3). The crude protein after drying ranged from 22.3% to 23.4%. Crude protein in the shade dried samples was higher than that for sun and oven dried samples (Table 2). Crude protein content from sun dried leaves was not different from that dried in the oven. Shade drying retained highest levels of calcium and shade drying retained more phosphorus, than oven drying (Table 2).

Condensed tannins varied from 0.917% to 1.06% depending on the drying technique used. The lowest level of tannins was found in oven dried samples and it was lower than both shade and sun dried samples. The amount of condensed tannins in shade dried samples was not different from the sun dried sample (P=0.5824). While retained condensed tannins in the mid and late maturity leaves did not depend on the method of drying, a lower concentration of condensed tannin was obtained in the early harvested leaves after oven drying (Table 3).

Oven dried samples had crude fiber content of 16.83%, higher than both the sun (13.52%) and shade dried samples (13.01%) (Table 2) but there were no noted differences in crude fiber levels between sun and shade dried samples (P=0.6848). An interaction was noted between stage of growth and harvesting stage on crude fiber. For the early and late harvested Acacia angustissima, the crude fiber content was not affected by method of drying but for the mid stage of growth harvested samples sun and shade dried leaves retained the lowest crude fibre content (Table 3).

Discussion

Effect of stage of growth on nutritional composition of the leaf meal

The characteristic increase in dry matter content with advancing age of maturity is also reported by Kökten et al (2012) and Oduntan and Olaleye (2012). Dry matter content is related to the amount of moisture in feed samples. The lower the dry matter content the higher the water levels in an ingredient. The more water content in a feed ingredient, the likely it is that the ingredient will have a shorter shelf life since microorganism that can cause spoilage thrive in foods having high moisture content (Oduntan and Olaleye 2012, Adeyeye and Ayejuyo 1994). High moisture content also indicates low total solids and likewise a higher dry matter content would also translate to high total solids in a feed ingredient. As such the more mature the Acacia angustissima leaves are, the less moisture they contained, the more total solids and also making it easier to dry the leaves. The younger the leaf material, the more moisture it contains and the lower the total solids. The reduction in moisture content with leaf age indicates a possible reduction in chances of spoilage, thus better keeping quality. The results therefore indicates that the mid and late stages of growth will give best dry matter content.

The observed increase in ash content with leaf maturity was also observed by Oduntan and Olaleye (2012), Kökten et al (2012), and Bamishaiye et al (2011) from different shrubs. Since ash is a rough estimate of the mineral content in a sample (Oduntan and Olaleye 2012, Aye 2012, Aliero and Abdullahi 2009) more ash contents in an ingredient can imply high levels of minerals. However at levels ranging from 5.75% to 9.25% for Moringa oleifera Bamishaiye et al (2011) concluded that some minerals may not have fully developed and are likely to be unavailable for use. In this study ash content was even lower than values reported for other legume tree leaves by Aye and Adegun (2013), implying that among other legume trees, Acacia angustissima leaf meal may not be used as the major source of minerals for poultry. As a source of minerals however, the mid and late maturity leaves of Acacia angustissima are better than the early harvested leaves.

The levels of crude protein obtained in this study confirm a general statement by Mokoboki et al (2005) that crude protein content in leaves of Acacia species is above 10%. The values were also within the range obtained by previous authors ranging from 19.9% (Mukandiwa et al 2010), 22.9% (Rubanza et al 2007) to 26.5% (Gusha et al 2013) although some of the authors make no mention of the stage of harvest. The characteristic decrease in crude protein levels as the plant matured has also been noted by Baloyi et al (2013), Modi (2007), Omoyola et al (2012) and Kökten et al (2012). Crude protein percentage of forage is primarily a function of physiological age of the plant tissue (Heitschmidt et al 1982). As such the observed trend in crude protein content emphasizes the importance of harvesting the leaves at early stages of growth. This will safeguard availability of crude protein for the synthesis of non-essential amino acids by the birds. Contrary to the observed trend in this study Oduntan and Olaleye (2012) reported an increase in crude protein content from early to mid stage of growth and a decline in the late stages of growth with sesanum radiatum leaves. A complete opposite of the observed trend is reported by Bamishaiye et al (2011) in which Moringa oleifera showed an increase in protein content as the plant leaves matured. The different trends could be explained by species difference, thus the need to identify an appropriate nutrient dense growth stage for each plant resource. Given that the crude protein requirement for synthesis of non-essential amino acids is around 14-18% for layers and 18-23% for broilers (NRC 1994), the three stages of growth can be potential sources of crude protein but the early harvested leaves have the greatest potential with regard to the supply of crude protein compared to the mid and late harvested leaves.

The increase in crude fiber levels shows that as the plant matures, it becomes more fibrous, hence the increase in the fibre content (Ahmed et al 2013, Oduntan and Olaleye 2012). Oduntan and Olaleye, (2012) also observed a general increase in crude fibre content with sesanum radiatum leaves as they matured. The trend however seems to be different in other species, implying species differences in factors influencing the rate of lignifications in plant leaves. Thus a study by Bamishaiye et al (2011) with Moringa oleifera there was no difference in crude fiber content with leaf maturity. While crude fiber has always been considered as an antinutritional factor for the monogastric animal, reasonable levels of fibre in the diet of such animals may be good (Oduntan and Olaleye 2012). As such, ingredients with high crude fibre levels like sunflower meal (14-32%) (Raza et al 2009) are included in poultry diets at low inclusion levels. In general the sources of crude fiber used in formulating poultry feeds contain 10-35% crude fiber (Dohms 2006). With 11.53% - 16.57% crude fiber, Acacia angustissima falls within the range, and thus inclusion levels can be manipulated to take advantage of the crude protein whilst minimizing the negative effects of crude fiber. It is important however to ensure that the final feed composition contains total crude fiber of not more that 7% (Varastegani and Dahlan 2014). With regard to crude fiber levels, the early harvested leaf meal would be most suitable for broiler feeding that both the mid and late harvested leaves.

A general increase in calcium content as the plant matures was also reported by Modi (2007) with Amarathus. Although in this study there is clear decline in phosphorus levels with the age of the plant, Modi (2007) with Amarathus leaves, concluded that phosphorus content was not affected by stage of plant development. The differences in trend could be specie related. As a source of calcium and phosphorus in poultry diets the younger and mature leaves can be used respectively.

The observed decline in condensed tannins with advances in the age of the leaves was also observed by Zhang et al (2009). The authors reported higher levels of condensed tannins in young branchlets than mature branchlets of Casuarinas equisetifolia. Contrary to this trend and using a number of Mediterranean shrubs Kökten et al (2012) found a general increase in condensed tannins as the plants matured. Although the values in this study are within the 0.1-8% range reported by Mokoboki et al (2005) for some Acacia species, they are on the lower range than most species. This could mean that among the Acacia family, Acacia angustissima could be the best for broiler feeding as it has lower levels of condensed tannins. The trend in this current study could be explained by the fact that the primary role of tannins in plant material is defending against herbivory. There is need therefore for higher levels in younger plants as damage through herbivory at this stage of plant growth could be detrimental to the future development of plants (Zhang et al 2009). This is because with younger leaves, the plants has less photosynthetic units, and thus defoliation at that stage of growth is likely to be more destructive than at later stages of growth. It could therefore be nature’s way of ensuring continuation of species. The implication is that the younger leaves are likely to produce more toxic effects when fed to broilers than older ones.

Although it has also been noted that the early harvested leaves contained more protein and less crude fiber thus ideally would be a better stage of harvest for inclusion in broiler diets, the results should be interpreted together with the variation in other nutrients of important consideration in broiler feeding. One greatest limitation in using legume leaf meals in broiler diets is the presence of such antinutritional factors as tannins. As such the higher the levels of antinutritional factors in any stage of growth the more unsuitable the ingredients becomes for broiler feeding. Given the variations of all the nutrients, the mid maturity leaf meal is considered the best stage of harvesting Acacia angustissima leaves for broiler feeding as it contains reasonably higher levels of crude protein and lower crude fiber content that the late growth stage as well as much lower condensed tannins than the early growth stage.

Effect of drying method on nutritional composition of leaf meal

The variation in dry matter content could with processing method is also reported by other authors with higher dry matter content representing a more effective method of removing moisture. In a study by Özcan et al (2005), compared to sun drying, oven drying removed more moisture. However Oluwulana et al (2011), concluded that for drying water leaf, sun drying and oven drying would give the same result. Results from this study show that oven drying removed more moisture from the leaves than both shade and sun drying. The higher temperatures in the oven can help explain this scenario.

The high ash content in sun dried leaves was also reported by Oluwulana et al (2011) and Özcan et al (2005) but Satwase et al (2013) reported higher ash content in shade dried samples in comparison with sun and oven dried leaves. The high ash content in sun dried samples in this study could be a result of possible contamination from uncontrolled dust during the drying process as noted by Limcangco-Lopez (1989) and Aliero and Abdullahi (2009).

The trend observed on the impact of drying procedure on crude protein content agrees with that obtained by Satwase et al (2013) after using the same drying techniques. The authors reported a higher crude protein content in shade dried drumstick leaf samples compared to the sun and oven dried samples. In a study by Mbah et al (2012) with Moringa oleifera, crude protein content was also found to be higher in the shade dried samples compared to oven and sun dried samples. Such trends as in previous and current study can be explained by the levels of heat treatment (Aye 2012) implied in the three drying techniques. Higher drying temperatures are associated with a reduction in crude protein content. According to Reddy and Elanchezhian (2008) oven drying is associated with an increase in fiber bound nitrogen even at temperatures below 600c, thus low crude protein content in oven dried samples. Contrary to this trend however, Oluwulana et al (2011) reported a higher crude protein content for oven dried than sun dried samples.

The trend in amount of calcium and phosphorus retained is not in line with the observation by Oluwalana et al (2011). The authors noted a general increase in the calcium and phosphorus contents with increasing drying temperature. In this study there is no clear relationship between drying temperature and retained minerals. The implication of these results is that if ever Acacia angustissima leaf meal is to be used as a calcium and phosphorus, shade drying would retain more of those nutrients than oven and sun drying.

Oven drying was most effective in reducing the levels of condensed tannins. This concurs with a study by Fasuyi (2005) and Yakubu and Wafar (2014), in which oven dried samples retained the least amount of tannins. In another study with a number of selected vegetables Adegunwa et al (2011) noted the inability of sun drying to reduce tannin content.

The effect of oven drying on crude fiber was also noted by Satwase et al (2013). The authors reported high crude fiber in drumstick leaves after oven drying in comparison with shade and sun dried samples. The increase in crude fiber levels in oven dried samples could be an exaggeration from complexes between fiber and protein molecules as well as some polyphenolics substances. Such reactions with crude fiber could also be related to low crude protein and condensed tannin levels in oven dried samples. When fiber bound, the substances are then measured as crude fiber. In this regard and for purposes of feeding broilers, it would be better to shade dry the Acacia angustissima leaves.

Observed interactions between processing method and stage of growth imply that for different stages of growth, a particular method of processing would be appropriate when targeting optimum levels of ash, crude fiber, and condensed tannins for broiler feeding. With regard to ash content the early and mid stages can be dried by any of the three proposed methods without any significant effect on content. However if late stage of growth is harvested sun and shade drying would give the highest ash content, implying that to maximize on ash content from mature leaves, sun and shade drying would give the best results. To optimize on the levels of crude fiber, the early and late stages of growth can be dried by any of the three techniques. With medium maturity leaves shade drying should be used as it results in lower levels of crude fiber compared to sun and oven drying. At early stages of growth, the levels of tannins after processing depended on method of processing, with oven drying giving the lowest values compared to sun and shade drying. This could be because of high levels of condensed tannins in the younger fresh leaf. However removal of condensed tannin at the mid and late stage is not affected by method of drying and as such any dying technique can be used.

Conclusion

• In general the younger leaves contained more crude protein, more condensed tannins and less dry matter and less crude fiber. As a crude protein source in broiler diets Acacia angustissima should be harvested at mid maturity growth stage to maximize on the dry matter, crude protein and condensed tannins levels. The interaction between stage of harvesting and drying technique also showed the best drying technique for this stage of growth to be shade drying. Since the removal of condensed tannins at mid maturity stage did not vary with drying technique, shade drying can also be used effectively.

Acknowledgements

The authors want to thank DAAD for funding this work through a scholarship to Sharai Ncube: Award number A1296185. We also thank the Department of Animal Science technical staff for the support given throughout the project.

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Received 15 March 2015; Accepted 26 March 2015; Published 1 May 2015

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