The grasscutter: an untapped resource of Africa’s grasslands

E K Adu, A Asafu-Adjaye, B A Hagan and J K Nyameasem

CSIR-Animal Research Institute, P O Box AH 20, Achimota, Ghana
nhyirapapa@gmail.com

Abstract

The grasscutter, Thryonomys swinderianus, is one of the large rodents of Africa’s grasslands being domesticated. Naturally, the distribution of the grasscutter is influenced by occurrence of dense and thick cane-like grasses growing in damp places. The economic potential of the grasscutter is the reason many development agencies and non-governmental organizations interested in reducing poverty are promoting its production, particularly in countries in the West African sub-region where the meat of the animal is a delicacy. The meat of grasscutter enjoys a higher premium price per kilogram weight than chicken, beef, pork, mutton or chevon among many West Africans and elsewhere. Compared to other meats such as rabbit meat, grasscutter meat is very low in cholesterol and high in protein. It has a very high mineral (e.g. iron, calcium and phosphorous) content compared to beef, mutton, and chevon. The economic return on rearing is comparable to that of a cow, much higher than most livestock species, and only lower than that of the pig. However, there is very little information on basic production parameters for efficient economic exploitation under captive breeding, which has translated into poor production performance under captivity compared to the rabbit. Further research is required with regards to the nutrition of the grasscutter as well as the constraints associated with growth rates and reproductive efficiency in captivity. Genetic improvement of the grasscutter is another area that needs extensive research in order to improve upon its docility.

Key words: animal protein, economic exploitation, fibre digestibility, micro-livestock, poverty alleviation, Thryonomys swinderianus

Introduction

Grasslands, which are biomes found on every continent of the world except Antarctica, make up one fifth of the earth's land surface. Major grasslands of the world are found in the African savannas, the Australian grasslands, the cerrado and campo of South America, the prairies of North America and the Central Asian steppes. Among the economic value of grasslands are the fact that they provide natural grazing lands to ruminants including sheep and cattle producing wool, meat and milk for human consumption. Grasslands are a source of goods and services such as food, forage and energy and they also provide a natural habitat for a variety of wildlife that provide a bulk of the food resources for 800 million of the world’s population (FAOSTAT 2008).

Grasslands are characterized by multiple functions and values. Stypinski (2011) reported that grasslands provide forage for grazing and browsing animals, both domestic and wild, and support rural economies, functioning as the major source of livelihood for local communities (Table 1). Among the wildlife of the African grasslands is the giant rodent, known as the grasscutter by most Western Africans or the greater can rat in Eastern and Southern Africa.

Plate 1. The grasscutter, Thryonomys swinderianus Photo: E. K. Adu

The grasscutter, Thryonomys swinderianus, Temminck 1827 (Plate 1), is one of the large rodents of Africa (Rosevear 1969; National Research Council 1991; Van der Merwe and Van Zyl 2001). As a rodent of Africa’s grasslands, the grasscutter is surpassed in size only by the porcupine, Hystrix africaeautralis and Hystrix cristata (Van der Merwe and Van Zyl 2001).

The biology of the grasscutter

Taxonomy

The grasscutter belongs to the mammalian order Rodentia on account of the presence of one pair of chisel-like incisor teeth in each jaw. Based on the arrangement of the masseter muscles (i.e. the jaw muscles), it is further classified as belonging to the suborder Hystricomorpha. It belongs to the super family Thryonomyoidea and family Thryonomyidae. The family Thryonomyidae has only one known genus, Thryonomys, which is subdivided into two species: Thryonomys swinderianus (Temminck), commonly called the grasscutter or the greater cane rat, and Thryonomys gregorianus (Thomas), the lesser cane rat (Rosevear 1969; Wier 1974; Schrage and Yewadan 1999).

Morphology

The animal is heavy and compact with an adult female weighing between 3-4 kg (Schrage and Yewadan 1999; Adu and Yeboah 2000) whilst the male is slightly heavier, weighing between 3-6 kg (Adu and Yeboah 2003; Adu et al 2005). The grasscutter has a pale and very tender skin, especially the areas around the hind and forelimbs, that is easily damaged with the slightest rough handling. The fur is sub-spinous and ranges in shades of ginger-gray to grayish-black. It has a stocky body, thick and short neck, with a head that exhibits a slight sexual dimorphism with regards to its shape and size. The adult female has a slightly elongated head compared to the male. Though this anatomical feature is sometimes used for sexing the animal (Adu et al 1999), its efficiency can be low (Adu et al 2003).

Geographical distribution

The grasscutter occurs naturally only in Africa, its distribution ranging from as far north as Senegal (latitude 15^o N) to as far south as South Africa (latitude 32^o S) (Fig. 1). The distribution of the grasscutter is influenced by the occurrence of dense grasses, such as thick cane-like grass such as elephant grass (Pennisetum purpureum) and guinea grass (Panicum maximum) growing in damp places (Rosevear 1969). It also lives on edges of wetlands and marshes (Feldhamer et al 1999), in areas of plant cover comprising savanna, open woodland and rocky ground (National Research Council 1991). Its habitat is characterized by annual rainfall of over 750 mm with seven or eight months of rainfall and an average annual temperature between 22˚C and 27˚C (Schrage and Yewadan 1999).

Figure 1.The geographical distribution of the grasscutter, Thryonomys swinderianus and Thryonomys gregorianus (■) (Source: National Research Council 1991)

Economic importance

The meat of the animal enjoys a higher premium price per kilogram weight than chicken, beef, pork, mutton or chevon among many West Africans on the continent and elsewhere in the Diaspora (National Research Council 1991; Yeboah and Adamu 1995). The sale of the meat is a major industry in Ghana (Yeboah and Adamu 1995) and elsewhere in Africa (Baptist and Mensah 1986; Kyle 1987).

Grasscutter meat is also most preferred both in urban and rural centers of South Africa (Blignaut and Van der Merwe 1996 cited by Van Zyl et al 1999a). Blignaut et al 1996, (cited in Van Zyl et al 1999a) reported that consumer acceptance trials indicate that grasscutter meat is accepted by an urbanized South African group in terms of appearance, taste and general acceptability. Consumer preference for grasscutter meat was found to be 49.7% compared to 28.9% and 21.5% for beef and goat respectively (Blignaut and Van der Merwe 1997). Apart from its high protein content (18.1%) the cholesterol content of grasscutter meat is low (48.5-53.4 mg/100 g fresh mass) compared to values for the rabbit (135 mg/100 g; Adu et al 2005) and beef (58.9-68.6 mg/100 g) and goat (61.5-76.1 mg/100 g; Table 2).

These positive attributes of grasscutter meat notwithstanding, when the grasscutter occurs in cultivated forest regions, in sugar cane plantations and fields where groundnut, maize, rice and tuber crops such as cassava, sweet potato etc. are grown, farmers regard them as pests (Asibey 1974).

Though the grasscutter has great potentials as a micro-livestock species for poverty alleviation (Adu et al 2005), there is very little information on basic production parameters for efficient economic exploitation of the animal under captive breeding. The paucity of information on the biology of the animal has translated into poor production performance under captivity compared to the rabbit (Adu et al 2005). Among the major constraints are poor growth rates and reproductive efficiency (Adu and Wallace 2003).

Meat quality and carcass characteristics

The nutritive value of meat is among the important factors that influence the meat quality and consumer acceptability. The main components of meat quality are the protein and lipid contents. Meat fat contains several types of lipids, including triglycerides as the main components, phospholipids and cholesterol, with the phospholipids component being relatively constant compared to the triglycerides and cholesterol (Hernández and Gondret 2006). Compared to other meats such as rabbit meat, grasscutter meat is very low in cholesterol (48.5-53.4 mg/100 g v. 135 mg/100 g fresh weight) and high in protein (18.1% v. 14-25%) (Van Zyl et al 1999b; Adu et al 2005). Like rabbit meat, it has a very high mineral (e.g. iron, calcium and phosphorous) content compared to beef, mutton, and chevon (Table 2).

Though it has a much smaller body weight compared to cattle, sheep, goats and pigs, the meat yield is higher, with a higher dressing percentage compared to the major livestock species such as sheep, goats and cows, with the exception of the pig which has a dressing percentage of 74% ( c. 57% compared with just about 49% for goats or a maximum of 55% for rabbits, Table 2). The meat-bone ratio is also higher for the grasscutter compared to both goats and cattle; and comparable to sheep (Table 3). Grasscutter meat suffers only a minimal chilling loss of just about 2% compared to a minimum of 11% for rabbit meat (Table 3).

Feeds and feeding

The idea of feeding an animal in captivity is to provide it with a balance combination of nutrients for optimum productivity, be it meat or milk production or some other products of interest. The grasscutter is selective in its feeding habit and prefers the more succulent portion of graminaceous plants (Schrage and Yewadan 1999). This feeding habit, though makes available a better food to the animal than the feed on offer, it also results in a lot of wastage. Table 4 shows a number of food items used in the grasscutter industry.

Grasses such as Panicum maximum and Pennisetum purpureum are the main forages offered by most farmers on a daily basis (Adu et al 1999). However, these forages alone are unable to meet the animal’s requirement for efficient productivity (Adu and Wallace 2003). Supplementing with a formulated diet has proved successful. Some authors (Adeniji 2008; Karikari and Nyameasem 2009; Wogar 2011) have conducted various researches using compounded feed as supplements for grasscutters.

Feeding regime depends on the availability of labor. Grass can be given in the morning and afternoons, whilst supplement, if available, is fed in the afternoon. Since the animals are diel-active, with activity patterns mostly restricted to early morning and evenings (Schrage and Yewadan 1999), they eat very little in the afternoons, but eat more in the evenings and early mornings. It should therefore be ensured that feed is always available in the evening and early morning.

Constraints to domestic grasscutter production

Currently, the grasscutter is being developed as a model animal for poverty alleviation in countries such as Ghana and the Republic of Benin; and though it has shown great potentials in this regard (Baptist and Mensah 1986; Adu et al 2005), there is very little information on basic production parameters for efficient economic exploitation under captive breeding. The paucity of information on the biology of the animal has translated into poor production performance under captivity compared to the rabbit (Adu et al 2005). Among the major constraints are poor growth rates and low reproductive efficiency (Adu and Wallace 2003). The National Research Council (1995) reported that high growth rates and reproductive performance are two key indicators of dietary adequacy. It has therefore been speculated that nutrition underpins the production constraints in the grasscutter industry (Adu and Wallace 2003).

Reproductive constraints

The maintenance of reproductive competence is the main determinant of successful domestication of a species (Addo et al 2007). Currently, mating of the captive grasscutter is natural and has been associated with injuries and sometimes death of the female. Artificial insemination has been suggested as a potential means to improve reproduction efficiency of the captive grasscutter. This would, however, require the harmonization of estrous in the flock to ensure efficiency. Nyameasem et al (2015) observed that housing female grasscutters close to males such that there is visual cue between them could increase the rate of vaginal patency which is a sign of oestrus in hystricormoph rodents, but this practice may compromise feed intake thus the need for further research.

Hystricomorphs, as a rule, have long gestation lengths for their size (Weir 1974). The grasscutter exhibits an induced ovulatory mechanism (Addo 1998; Addo et al 2002) with a gestation length of about 152 days (Stier et al 1991), which culminates in an average litter size of four (4). This means that the grasscutter has a breeding potential of only two litters per year for a female (Asibey 1974; Baptist and Mensah 1986; Adu 1999) compared with a breeding potential of seven litters per year for a rabbit doe (Adu et al 2005).

Both Adu and Yeboah (2000) and Yeboah and Adu (2000) have reported average implantation sites of over six for wild grasscutters. However, due to the high embryonic resorption rate of about 42% (Adu and Yeboah 2000; Yeboah and Adu 2000) the realized average litter is significantly reduced. In economic terms, this translates into an annualized loss of GH¢ 268.80 (US$ 67.88) per breeding female, when the price of a weaned grasscutter is set at GH¢ 80.00 (US$ 20.20) as currently pertains on the markets in Accra.

Nutritional constraints

The poor reproductive performance of the grasscutter in captivity can partially be attributed to the poor understanding of the nutritional physiology of the animal. Adu and Wallace (2003) have shown that feeding the grasscutter solely on fresh Panicum maximum, as is currently done in Ghana (Adu et al 1999) does not support efficient reproductive performance.

Cecal fermentation is not sufficient to meet the nitrogen requirement for efficient economic exploitation of the grasscutter (Adu 2008). Again the nutrient concentration of the forage diets offered to grasscutter is not able to support its physiological functions (Adu 2003; Adu and Wallace 2003). This is aggravated by the fact that during the dry season the nutritive value of the available forage declines. Feed dry matter intake could therefore be a major setback to productivity apart from obtaining the right balance of nutrients from the ingested material. Under such conditions farmers should modify the combination of the available feed resources by offering more cassava, sweet potato vines and maize stovers and less of grasses (Awotwi et al 2007).

Nutritional strategies of the grasscutter

There is paucity of information on the nutrition of the grasscutter. However, a comparison of the digestibility coefficients of the grasscutter with data from various hystricomorph rodents such as the Cape porcupine (Hystrix africaeustralis), North American porcupine ( Erethizon dorsatum), guinea pig (Cavia porcellus), and the degu (Octodon degus), as well as the rabbit, indicates that the grasscutter is highly efficient at digesting dietary fiber, with a digestibility coefficient for Neutral Detergent Fiber (NDF) and protein higher than the rabbit, and more similar to hystricomorph rodents (Table 5).

Table 5 also shows that the coefficient of NDF digestibility for the grasscutter range between 36% and 80%, which is comparable to figures reported for the North American porcupine (46.6%-96.5%; Felicitti et al 2000).

The porcupine’s ability to efficiently digest fiber has been attributed to a number of mechanisms including (1) its lengthy mean retention time (MRT) of particles (38.3±0.56 h, Felicitti et al 2000), (2) the presence of a relatively large cecum and colonic separation mechanism, which supplies 16% of its basal metabolic energy requirements (Johnson and McBee 1967), (3) the presence of a distal colon four times longer than that of a similarly sized beaver (Castor canadensis, Vispo and Hume 1995), and (4) low metabolic fecal nitrogen (MFN) excretion (0.9-2.8 g N/kg dry matter intake (DMI), Fournier and Thomas, 1997; Felicitti et al 2000), which is indicated to be at the lower end of the normal range of 1-9 g N/kg DMI (Robbins 1993).

These mechanisms result in porcupines achieving nitrogen balance at relatively low levels of nitrogen intake (346.0-389.4 mg N kg ^-0.75 d^-1, i.e. on 3-5% crude protein; Fournier and Thomas 1997; Felicitti et al 2000). Though there are no comparable data for grasscutters, they have equally been reported to maintain their body weight on a low protein diet of 4.6% (Van Zyl et al 1999a) indicating that they were in energy and nutrient balance. The nutritional physiology of the grasscutter may thus be similar to that of the porcupines. However, like the guinea pig which though is able to survive on poor quality diets but requires a diet of crude protein content of 18% for optimum productivity, foods of dietary crude protein content of 17-18% has been recommended for efficient economic exploitation of the grasscutter (Adu et al 2005; Kusi et al 2012).

Productivity and economic return

The productivity of the grasscutter can be described as medium, within the range of species like chicken, ducks and pigs, and much higher than sheep and cattle (Table 7). However, the economic return is comparable to that of a cow, much higher than most livestock species, and only lower than the pig (Table 7). The economic potential of the grasscutter is thus equal to the cow. With the animal requiring minimal space for breeding as well as its attribute of being bred at most backyard the grasscutter can comfortably be described as a poor man’s cow.

Conclusion

• The grasscutter can be described as a small mammal similar to the rabbit but different from species like sheep, goat, pig and cattle. It thrives well in African grasslands.
  
  
• Its ability to feed on a wide range of grasses, leafy materials, tubers, fruits, grains and other kitchen left overs, enhances its production by the less privileged in the society. This, coupled with its productivity, demonstrates that the grasscutter if efficiently harnessed, will contribute significantly to reducing or alleviating poverty in most African countries where the animal occurs by providing alternative source of income for farmers and increase farmers’ access to, and utilization of animal protein for dietary needs.
  
  
• Grasscutter meat is highly accepted by all social classes in the urban and rural communities of the West African sub-region and a niche of Southern Africa serving as an important source of the highly priced animal protein because of its leanness and unique organoleptic properties.
  
  
• The grasscutter also has the capacity to be raised as a backyard venture. This attribute, as well as the comparable economic return from a female grasscutter to a cow makes attempts at its captive breeding worth supporting by development agencies and non-governmental organization interested in reducing poverty, particularly in countries in the West African sub-region where the meat of the animal is a delicacy.
  
  
• Further research is however required with regards to the nutrition of the grasscutter as well as the constraints associated with growth rates and reproductive efficiency in captivity. Genetic improvement of the grasscutter is another area that needs extensive research in order to improve upon its docility.

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Received 10 January 2017; Accepted 13 January 2017; Published 1 March 2017

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