Growth performance of grasscutter (Thryonomys swinderianus) eating leaf and stem fractions of Guinea grass (Panicum maximum)

S Y Annor, J K Kagya-Agyemang, J E Y Abbam, S K Oppong and I M Agoe

Department of Animal Science Education, University of Education, Winneba, P.O. Box 40, Mampong-Ashanti, Ghana
sayannor@yahoo.com

Abstract

This study was carried out to determine the growth performance of grasscutters (Thryonomys swinderianus) when fed on leaf and stem fractions of guinea grass.  Eighteen (18) three month old male grasscutters were randomly assigned in equal numbers to three treatment diets consisting of Guinea Grass Leave fraction (GGL), Guinea Grass Stem fraction (GGS) and Whole Plant of Guinea Grass (GGW). 

The results of the chemical analysis of the treatment diets indicated that the nutritive value of the GGL was better than that of GGS and GGW.  There were no significant differences between treatments for mean daily feed intake (P>0.05). However, animals fed GGL performed better than those fed GGS and GGW in terms of total weight gain, mean daily weight gain and feed efficiency (P<0.05). Animals fed GGS produced better lean meat compared to those on GGL and GGW (P<0.05).  The mean dressing percentage of animals fed GGL was significantly higher than those on GGW and GGS (P<0.05). 

The results of this work suggest that the leaf portion of elephant grass is more nutritious than the stem portion.  This implies that the current practice in Ghana whereby farmers cut off the leaves of the grass and feed only the stem fraction to the animals is a waste of feed resources, and should be discouraged.

Keywords: carcass characteristics, feed efficiency, feed intake, nutritive value, weight gain

Introduction

Within the last eight years the government of Ghana and some Non Governmental Organizations (NGOs) have been implementing projects to promote grasscutter (cane rats) production in poor communities.  The main objectives of these projects are to provide alternative source of income for farmers and increase farmers’ access to and utilization of animal protein for dietary needs (Heul-Rolf 2002; Wontewe 2002). 

One of the basic challenges confronting grasscutter farmers in Ghana is the provision of balanced diet to the animals.  The animals are basically fed on forages and eat a wide range of fresh and dry forages.  Some farmers supplement forages with crop residues (e.g. rice straw), agro-industrial by-product (e.g. pito mash) and kitchen leftovers.  Some of the fodder species that are routinely fed to the animals are Pennisetum purpureum (elephant grass) and Panicum maximum (guinea grass) (Adu et al 1999). 

Grasscutters are strictly herbivores, and prefer mainly tick-stemmed grass species (Schrage and Yewadan 1999). The feeding habits of grasscutter and other rodents (e.g. rabbits) are directly opposite.  Whereas the grasscutter prefers to eat stalks to leaves, the rabbit, for example, on the contrary chooses the leaves and wastes stem (Vietmeyer 1991; Schrage and Yewadan 1999).

The feeding behaviour of grasscutters has led most farmers in Ghana to adopt a practice whereby they (farmers) harvest grass by cutting off and throwing away the leaf fraction and feed only the stem fraction to the animals (Annor and Djang-Fordjour 2006).  This practice leads to waste of feed resources, especially during the dry season when there is scarcity of grass (Adu 2005). Moreover, there is enough scientific evidence to proof that the leaf portion of grass is more nutritious than the stem portion.  Leaves have higher dry matter digestibility and intake than the stem because the former contains more of the readily digestible mesophyll and less of the less digestible epidermis, vascular and sclerenchyma tissues (Wilson and Minson 1980).  Stem is highly lignified (containing high fibre) compared to leaves due to the high proportion of cell wall material in it (Wilson and Minson 1980).  Leafiness in pasture plants is commonly associated with forage quality, because there is usually a positive correlation between leaf percentage in a given plant species and protein, mineral composition and dry matter digestibility (Norton 1981).

There is limited data on the performance of grasscutters fed different fractions of grasses.  However, studies on the influence of fibre in the diet on the growth rates and digestibility of nutrients in grasscutters have indicated that animals fed high fibre diet had reduced digestibility of dry matter, protein and fat, and exhibited significantly lower growth rates than animals fed low fibre diet (Zyl van et al 1999).  Studies of the voluntary intake of sheep and cattle fed separated leaf and stem fractions of a range of tropical grasses have also indicated that leaf is eaten in much larger quantities than stem due to a shorter time the leaf is retained in the recticulo-rumen (Minson 1980; Poppi et al 1981).  The level of fibre and protein in the plant will also limit pasture intake.  When the level of crude protein in the feed falls below 6-8 % the appetite of the animal will be depressed by deficiency of protein, and voluntary food intake by the animal will be less than expected by a consideration of the fibre level (Minson 1980).

This study was carried out to determine the growth performance of Thryonomys swinderianus when fed on leaf and stem fractions of Panicum maximum (guinea grass).

Materials and methods 

The experiment was conducted at the Animal Science Section of the University of Education, Winneba, Mampong-Ashanti, Ghana.  Mampong-Ashanti lies within latitude 07⁰ 04’N and longitude 01⁰ 24’W and is generally described as being a hot humid forest-savannah transitional zone.  The mean daily minimum and maximum temperatures during the experimental period were 21.2⁰C and 30.6⁰C, respectively while the mean monthly rainfall was 51.5 mm (MSD 2004).  The experiment was carried out for a period of 12 weeks i.e. from 15^th March to 7^th June 2004. 

Eighteen (18) male grasscutters each of about 3 months of age were used in a Completely Randomized Design (CRD) for the study.  The animals were initially weighed and then randomly allotted in equal numbers (6 animals per treatment) over three (3) treatments consisting of Guinea Grass Leave fraction (GGL), Guinea Grass Stem fraction (GGS) and Whole Plant of Guinea Grass (GGW). 

Guinea grass was normally harvested at a stubble height of about 5 cm. The leave fraction (GGL) of the plant consisted of the foliage portion, beginning from the lowest basal leaf to the tip of the apical leaf.  When GGL was cut off, the rest of the plant constituted the stem fraction (GGS).  The whole plant (GGW) consisted of GGL and GGS.    Each treatment was replicated six times and each replicate used one animal.  Animals were fed two times a day in the morning (7:00 hours GMT) and afternoon (16:00 hours GMT), respectively.  Feed was weighed every time before being offered.  Feed leftover was collected every morning at 6:00 hours GMT and weighed after sorting out faeces from leftovers.  Water was offered ad libitum throughout the experimental period. The animals were housed singly in wire floor wooden cages with each cage measuring 60 cm x 50 cm x 40 cm.

Samples of the treatment diets were analyzed for crude protein, crude fibre, ether extract, ash, nitrogen free extractives (NFE) and dry matter (DM) according to the procedure outlined by A.O.A.C. (1990).  The results of the analysis are presented in Table 1.

The parameters studied were: feed intake, liveweight gain and feed conversion efficiency.  Body weights of experimental animals were recorded every fortnight at 6:30 hours GMT.  The average weights of animals on each treatment were similar at the start of the experiment (Table 2).  On the last day of the trial, all animals from each treatment were slaughtered for dressing percentage and carcass quality determination. The weight of fat extracted from the animals after evisceration in the three different dietary treatments was used in assessing the carcass quality. 

The treatment effects on the various parameters were analyzed by the General Linear Model (GLM) procedure of Statistical Analysis System (SAS 1988). 

Results and discussion 

The results of the chemical analysis of the treatment diets are presented in Table 1.  The crude protein, ether extract, ash and dry matter content of GGL were higher than GGS.  However, the crude fibre and NFE content of GGS were higher than that of GGL.  The crude protein, crude fibre, ether extract, ash, NFE and total dry matter content of GGW were intermediate to those of GGL and GGS.

The results of the chemical analysis follow the well known pattern of grasses which shows that as the percentage of protein diminishes that of fibre increases (e.g. Bredon et al 1963; Landsbury et al 1965).  It was noted in the foregoing studies that the changes that occur in the ash and ether extract content of grasses were similar to that of protein, but to a much lesser extent, with the nitrogen-free extractives being the least affected.  In the current study higher crude protein content of grass was associated with high ether extract and ash, but low fibre and nitrogen-free extractives contents (Table 1).  The high crude protein content of GGL compared to the other treatments indicates that the former is more nutritious than the latter because leafiness in pasture plants is commonly associated with forage quality that results from high crude protein, mineral composition and dry matter digestibility (Norton 1981).

There was no significant difference between mean daily feed intakes of the various treatment means (Table 2). This result was unexpected because studies have confirmed that leaf is eaten in much larger quantities than stem due to the shorter time the leaf is retained in the stomach (Minson 1980; Poppi et al 1981).  The fibre and crude protein levels of diets also limit feed intake. In the current study the crude protein content of the diet was GGL> GGW>GGS whilst the fibre level GGL<GGW<GGS (Table 1).  It has been observed that when the level of crude protein in the feed falls below 6-8 % the appetite of the animal will be depressed by deficiency of protein, and voluntary food intake by the animal will be less than expected by a consideration of the fibre level (Minson 1980).  In this study the crude protein content of GGS was below 8 % whilst that of GGW was within the range of 6-8 % and that of GGL was above 8 %.

The mean total weight gain of animals fed GGL was higher than those on GGW and GGS (P<0.05) (Table 2).   However, animals fed GGW and GGS significantly gained the same weight (P>0.05).  Similarly, the mean daily live weight gain of animals fed GGL was significantly higher than those fed GGW and GGS (P<0.05). Animals fed GGW and GGS had the same performance in terms of mean daily weight gain (P>0.05) (Table 2).

The trend of performance in mean total weight gain and daily weight gain follows the reverse of the trend of crude fibre content of the treatment diets but similar to that of the crude protein content of the treatments.  The high fibre levels and low crude protein content of GGW and GGS compared to that of GGL might have caused the reduction in performance.  An increase in fibre levels in the diet of grasscutters has been found to be associated with a decrease in the digestibility of dry matter, protein and fat in these animals leading to reduction in growth rate (Zyl van et al 1999).  Studies in cattle have also shown that there is a closer positive relationship between the amount of protein in grass and the dry matter digestibility (Bredon et al 1963).

Animals fed GGL were more efficient to convert grass into muscle than those fed GGW and GGS, and those fed GGW were also more efficient than those on GGS (Table 2) (P<0.05).  Comparing the feed intake values, the results of the feed conversion ratio could be explained by the differences in weight gain resulting from feed intake.  The order of performance in weight gain was GL>GGW>GGS.  The mean feed conversion ratio followed the same order.

The mean fat content was the same for animals fed GGL and GGW (P>0.05), but the means for GGL and GGW were significantly (P<0.05) different from that of GGS (Table 2).  Animals on GGS recorded the best lean meat compared to the other treatments, probably because of the high fibre content of the former compared to the latter.  This observation has been confirmed by Zyl et al (1999) who investigated the influence of fibre in the diet on the growth rates and digestibility of nutrients in grasscutters.  They observed that animals fed high fibre diet had reduced digestibility of fat, and exhibited significantly lower growth rates than animals fed low fibre diet.  It has also been established in farm animals that the consumption of high fibre diet is associated with low fat deposition (e.g. Adeniji 2004; Skiba et al 2006). 

The dressing percentage of animals fed GGL was significantly (P<0.05) higher than those fed GGW and GGS (Table 2).  The difference may be attributed to the fibre content of the diet.  Many studies in farm animals have confirmed that animals fed high fibre diet have reduced dressing percentage compared to those fed low fibre diets (e.g. Adeniji 2004).

Conclusions 

• Growth performance of grasscutter fed on the leaf portion of guinea grass was better than those fed on the stem portion. Animals fed on the leaf fraction had higher growth rates, were more efficient to convert grass into muscle and had higher dressing percentage compared to those fed on the stem fraction.  However, animals fed on the stem portion of the grass produced less fat (lean meat) than their counterparts fed on the leaf portion.  The performance of animals fed the whole plant of leaf and stem was intermediate to the other two treatments. 

  
  

• It can be deduced from the results that the current practice in Ghana whereby farmers cut off the leaves of the grass and feed only the stem fraction to the animals is a waste of feed resources, and should be discouraged.  Farmers should rather be encouraged to feed the whole plant consisting of both leaf and stem.  This is significant, especially, during the dry season, when there is general scarcity of grass, and farmers have to trek long distance in search of grass.  

Acknowledgement 

The authors are grateful to the African Women Leaders in Agriculture and Environment (AWALE) in the United States of America for providing the funds for this study.  We are also grateful to the Animal Science Department, Kwame Nkrumah University of Science and Technology, Ghana for the chemical analysis of the experimental diets.

References 

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Received 3 June 2008; Accepted 22 July 2008; Published 5 August 2008

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