Livestock Research for Rural Development 25 (3) 2013 Guide for preparation of papers LRRD Newsletter

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Growth, carcass and behaviour characteristics of castrated and intact male grasscutters (Thryonomys swinderianus)

S Y Annor, I Iddisah and K T Djang-Fordjour

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


Forty eight (48) male and twelve (12) female grasscutters, 4-5 months old and weighing between 0.69-1.94kg were used for the study. The male grasscutters were divided into two (2) groups of 0.69-1.00kg and 1.18-1.94kg live weights, constituting two blocks (blocks 1 and 2). Twelve (12) male grasscutters from each block were randomly selected and surgically castrated. The Randomized Complete Block Design (RCBD) was used. To find the effect of castration on growth and carcass characteristics, two treatments, namely, intact males (IM) and castrates (CS) were used. Carcass evaluation was conducted using eight (8) intact males and eight (8) castrated grasscutters. To find the effect of castration on behaviour of grasscutters, castrated and/or intact grasscutters were mixed in the same cage with or without a female. The treatment groups were intact males only (IM), castrates only (CS), intact males and females (IMF), castrates and females (CSF), intact males and castrates (IMCS), and intact males, castrates and females (IMCSF).

Castration of grasscutters between 4-5 months of age could not influence growth rate (P> 0.05) but led to improved feed utilization (P< 0.05). Aggressive behaviours of grasscutters in colony, and docility were also not influenced by castration (P> 0.05). Intact males mixed with castrates only (IMCS) and intact males mixed with castrates and females (IMCSF) co-existed peacefully than the other treatments.  Carcass weight (hot/cold), dressing percentages (hot/cold), fat deposition, chemical constituents of meat and organ weights were not affected (P> 0.05) by castration. Sensory panelists identified cooked grasscutter meat colours to range from ivory to yellow lemon for fore legs, and ivory to light ivory for hind legs and longissimus dorsi. Both intact males and castrated grasscutter meats were generally acceptable (P> 0.05) by sensory panelists.

Keywords: carcass composition, castration, docility, feed intake, organoleptic characteristics, rodent, weight gain


Grasscutter is a wild rodent. Its meat is a delicacy and it is the most preferred and perhaps the most expensive meat in West Africa (Mensah and Okeyo 2006). The meat attracts higher prices than beef, mutton, chevon, chicken and pork (Olomu et al 2003). The high demand for grasscutter meat has led to its domestication in the West African sub-region. NRC (1991) observed that successful domestication of grasscutter could bring about a reduction in Africa’s chronic protein shortage.  

The immense benefits of grasscutter production have led to the widespread promotion of grasscutter farming by individuals, governmental and non-governmental organizations in the West African sub-region. The progress in the domestication process has however been slow due to paucity of information on the biology of the grasscutter (Yeboah and Adamu 1995).  

The grasscutter has nervous temperament and are difficult to rear in captivity (Hemmer 1993). On attainment of puberty, male grasscutters in colony fight a lot, most often resulting in injuries and deaths (Mensah et al 2005). Traumatic injuries caused 32% of mortalities in a colony of grasscutters (Adu 2002). Aggression and hostility, leading to high mortality, is of great concern in the grasscutter industry, as it lowers productivity. Aggressive behaviour further reduces feed intake and this could translate into slow growth rate. One solution to this problem is to separate males into individual cages at puberty. This would however require many individual cages. However, buying many cages may be too expensive for the poor farmer (Annor et al 2009). The high cost of cages limits separating sexually mature males into individual cages. In this regard, there arises the need to research into other husbandry practices to find solution to the problem. Castration is an option to be considered.  

The objective of this study was to find the effects of castration on growth, carcass and behaviour characteristics of sexually mature male grasscutters. 

Materials and Methods

Experimental Site and Period of study 

The study was conducted at the grasscutter section of the Department of Animal Science Education of the College of Agriculture Education, University of Education, Winneba, Asante Mampong, Ghana. The vegetation and climate of the study area have been described by Annor et al (2012). The study took place between April-August, 2009 in three phases. The first phase was carried out from April to July, 2009. The second and third phases took place in August, 2009. 

Experimental Animals 

Forty eight (48) male and twelve (12) female sexually mature grasscutters, 4-5 months old and weighing 0.69-1.94 kg were used for the experiment. The male grasscutters were divided into two (2) live-weight groups of 0.69-1.0 kg and 1.18-1.94 kg, respectively. Each group constituted a block. Twelve (12) male grasscutters were randomly selected from each block and castrated. 

Castration of Grasscutters 

The surgical method of Andersen (2007) with anesthesia (Mensah et al 2005) was used in castrating the grasscutters. The males were restrained by covering the head and thorax regions with cloth bag (Rand 2001), and handled by two persons. Lidocaine concentration of 4% (Rand 2001) was injected intra-muscularly into the scrotal and pelvic regions at a rate of 1ml/kg live-body weight, after disinfecting the area with methylated spirit. The testicles were located by palpating the inguinal area and incised with a scalpel blade. The spermatic cord with its associated testicular blood vessels were located and ligated to prevent internal bleeding. Oxytetracycline aerosol was sprayed onto the incised tissues to disinfect and facilitate quick wound healing. There was 100% wound healing at 2 weeks after castration in all the animals.

Management of Experimental Animals

The grasscutters were housed in 3-tier wooden cages lined with wire mesh. In the first phase of the experiment (investigating the effect of castration on growth and carcass characteristics), the grasscutters were housed in individual cages with dimensions of 50cm x 50cm x 40cm. In the second phase (investigating the effect of castration on behaviour of grasscutters), each cage had a dimension of 100cm × 50cm × 40cm with a floor space of 94.92cm × 44.92cm, accommodating 2-3 animals. Each cage apartment was barricaded with iron roofing sheets to prevent grasscutters in one apartment from seeing and reaching others in adjacent apartments.

Health care 

Cages were disinfected with a disinfectant (Dettol solution) at a rate of 7.5ml/l of water before the start of the experiment. The cages, feeding and watering troughs were cleaned each day before feeding the animals. Grasscutters that sustained external injuries were treated with Oxytetracycline aerosol spray that contained 5mg of Oxytetracycline hydrochloride.

Feeding and watering   

The grasscutters were fed ad libitum with elephant grass (Pennisetum purpureum) as the basal diet and supplemented with concentrate, containing 14% crude protein, and fed at a rate of 50g per animal per day. Proximate analysis of the concentrate diet and that of elephant grass were carried out in the laboratory.  The composition of the concentrate diet and the results of the proximate analysis are shown in Tables 1 and 2, respectively. The concentrate was moistened with water at a rate of 6.12ml/50g (0.1224ml/g) to reduce dustiness and avoid choking of the animals. The grasscutters were fed and provided with fresh drinking water ad libitum daily.

Table 1. Composition of concentrate diet





Wheat bran


Soya bean meal


Oyster shell




Common salt




**Vitamin-mineral premix composition: Vitamin A (8.000V.I.); Vitamin D3 (1.500 V.I.); Vitamin E (2.500 V.I.); Vitamin K3 (1.000mg); Vitamin B2 (2.000mg); Vitamin B12 (5.000mg); Nicotinic acid (8mg); Calcium panthotenate (2mg); Antioxidant (10mg); Folic acid (500mg); Choline cloruro (50mg); Manganese (50mg); Zinc (40mg); Copper (4.50mg); Cobalt (100mg); Iodine (1mg); and Selenium (100mg)

Table 2. Proximate composition of elephant grass and concentrate (DM Basis)

Chemical composition

Elephant grass (%)

Concentrate (%)

Crude protein



Crude fibre



Ether extract






Dry matter



Nitrogen free extractives



Data Collection 
Feed intake and live weight 

Grass, concentrate and left-over feed were weighed with electronic weigh master digital scale (Penn Scale Manufacturing Company, Philadelphia).  Daily feed intake was computed by calculating the difference between the amount of grass and concentrate fed and the left-over feed. The live grasscutters were weighed with electronic weigh master digital scale fortnightly and their weights recorded. Feed conversion ratio (FCR) was computed as ratio of feed eaten to gain in weight.     

Injuries and mortalities 

All injuries and deaths were recorded as and when they occurred. Post-mortem examination was conducted on each dead animal.  


Docility was defined as the ability of an animal to accept human presence (Annor et al 2011). The capacity of an animal to accept human presence was scored on a scale of 1 to 4 (Mensah and Okeyo 2006; Annor et al 2011). The scores were as follows: 

Carcass evaluation 

Sixteen (16) grasscutters comprising 8 intact males and 8 castrates were randomly selected and slaughtered for carcass evaluation at the end of the experiment. Carcass weights were recorded using an electronic weigh master digital scale. After scalding with hot water at a temperature of 80oC (Omole et al 2005), the carcasses were weighed again using an electronic weigh master digital scale to obtain the defurred weights. The carcasses were dissected with a sharp knife and the internal organs were removed. Whole and individual viscera (heart, liver, kidneys, lungs and spleen) and fat from the visceral organs were weighed on an electronic weigh master digital scale and their weights recorded. The weights of the eviscerated carcasses, hot and cold, were also recorded and the dressing percentages calculated using the formula: (Dressed carcass weight/Slaughter weight) x 100 

Chemical composition of grasscutter meat 

Castrate and intact grasscutter meat samples were taken for chemical analysis. Two meat samples of 100 g each were taken from each treatment. Each sample comprised 100 g meat from the longissimus dorsi muscles, loins, fore legs, hind legs, thorax, neck and abdomen, respectively. The meat samples were put in plain polythene sheets separately and frozen for 24 hours in a deep freezer. The frozen samples were transported in cold flask containers to the laboratory. Samples were kept frozen in a deep freezer and analyzed 48 hours after slaughter. Moisture, energy, protein, lipid, pH and ash contents of meat were determined according to the procedure of AOAC (1980).

Organoleptic/sensory evaluation of grasscutter meat

Sensory evaluation was conducted on the meat of intact and castrated grasscutters, as described by Van Heerden et al (2007) and Osman and Aldosari (2006). Sensory panelists were given 4 hour training on sensory evaluation procedures at the Science Laboratory of the University of Education, Winneba, Asante Mampong. Ten (10) trained sensory panelists took part in the sensory analysis. Meat was taken from the animals’ fore legs, hind legs, Longissimus dorsi and skin.  

The meat samples were frozen in a deep freezer for 36 hours and cut into 1.5cm3 sizes, each weighing about 8g. Ten meat cubes from each part were cooked separately for 10munites in 65ml of water with 3g of salt. 

The meat cubes were served warm to 10 trained panelists in panel booths. Five minutes break was allowed between each of the 8 sessions for mouth rinsing to reduce sensory fatigue and halo effects. The meat cubes were coded and served randomly to ensure that each panelist evaluated every sample at the end of the test.  

Each panel member tasted 8 different samples of meat from intact and castrated animals.  The panelists evaluated meat colour, meat colour intensity, aroma, juiciness, texture, flavour and over all acceptability (Omole et al 2005) using a score sheet with 7 point hedonic scale (Osman and Aldosari, 2006). Panelists used colour charts and designed meat attribute description chartsin rating the meat (Van Heerden et al 2007). A meat coding chart was used to conduct the sensory evaluation.

Experimental Design and Treatments 

The experiment was conducted using the Randomized Complete Block Design (RCBD). The live-weight groups, 0.69kg-1.0kg and 1.18kg-1.94kg of grasscutters constituted blocks 1 and 2 respectively.  The experiment was conducted in three (3) phases. In the first phase, there were 2 treatments (intact males and castrates) using RCBD. Each treatment was replicated 6 times in a block. This phase lasted for 8 weeks and sought to determine the effect of castration on growth and carcass characteristics of grasscutters.  

In the second phase, there were 6 treatments and 4 replications. The experimental treatments were made up of colonies of intact males, castrates or intact males and castrates, with or without females as follows:  

IM = Intact males only

CSF = Castrates and females

CS = Castrates only

IMCS = Intact males and castrates

IMF = Intact males and females

IMCSF = Intact males and castrates

To further enhance easy identification, the experimental animals were fitted with ear tags of varying colours. Intact males, castrates and females were fitted with red, white and black coloured tags respectively. This phase lasted for 8 weeks and sought to determine the effect of castration on docility and aggressive behaviour of grasscutters. Injuries and mortalities were recorded at this phase.  

In the third phase there were three sub-phases. Sub-phase 1 was to evaluate carcass characteristics. That is, slaughter weight, dressed carcass weight (hot and cold), dressing percentages (hot and cold), organ weight and viscera fat weight of grasscutters. There were 2 treatments (intact males and castrates) with 8 replications. Sub-phase 2 was to determine the chemical constituents of grasscutter meat. There were 2 treatments (intact males and castrates), each with 12 replications. Sub-phase 3 was to conduct the sensory analysis of intact and castrated grasscutter meat. There were 2 treatments (intact males and castrates) with 10 replications. 

Data analysis

Data were analyzed using Analysis of Variance (ANOVA) and General Linear Model (GLM) procedures of Statistical Analysis System (SAS 2008). Data on docility were analyzed using Generalized Linear Mixed Models (GLMM) with GLIMMIX procedure of SAS (SAS 2008). Differences between means of significant effects were separated by the Probability of Difference (PDIFF) procedures of the Statistical Analysis System (SAS 2008). 

Results and discussion

Feed utilization and growth performance of grasscuters 

There were no differences between intact males and castrates for initial and final live weights, and total weight and daily weight gains (Table 3). However, daily dry matter intake was lower in castrates than in intact males which translated into a better feed conversion ratio by castrates. 

Table 3. Feed Utilization and Growth Performance of Grasscutters




Standard Error




Initial live body weight (g)





Dry matter intake (g/day)





Final live weight (g)





Total weight gain (g)





Daily weight gain (g/day)





Feed conversion ratio





(a and b): Means on the same row with different superscripts are significantly different (P< 0.05).

Mensah et al (2005) reported similar results to this study in feed conversion ratio. However, in that same study, castrates performed better than intact males in terms of live body weight and daily weight gain. Grasscutters castrated between 1-2 months of age grew 1.4 times faster than intact ones of the similar age (Mensah et al 2005). However, Alogninouwa et al (1999) reported that growth rate of male grasscutters castrated over four months of age was found to be lower than that of intact males. There have been contradictory reports in other species. In cattle, castration reduced feed intake (AVMA 2007) but in goats it improved it (Muhikambele et al 1994). Studies on the effect of castration on the growth rate of rams have also shown contradictory findings (Bani-Ismail et al 2007).  


Animals in colonies of intact males only (IM), castrates only (CS), intact males and females (IMF) and castrates and females (CSF) sustained body injuries (Table 4). Similar numbers of injuries were observed in intact males only (IM), castrates only (CS) and castrates and females (CSF) on one hand, and intact males (IM), intact males and females (IMF) and castrates and females (CSF) on another hand (P> 0.05). The castrates only (CS) sustained higher number of injuries than the intact males and females group (IMF). Injuries found on the animals were located around their snouts, loins, fore quarters and hind quarters. No injuries were recorded in colonies of intact males and castrates (IMCS) and intact males, castrates and females (IMCSF). 

Castration of grasscutters between ages 4-5 months could not eliminate injuries caused by the aggressive behaviour of the animals because both castrates and intact males were injured to the same degree. The result is in agreement with report by Manharth and Harris-Gerber (2002) that significant aggressive fighting and injuries were observed in colonies of castrated Rock hyrax (Procavia capensis). The result is however contrary to report that castration of male rats decreased aggression significantly (DeBold and Miczek 1981). The injuries in this study were observed to have been caused by aggressive behaviours, particularly fighting. The animals fought by using their claws and teeth to scratch and bite respectively, thereby resulting in external injuries. No aggressive fighting was observed in the colonies of intact males and castrates (IMCS) and intact males, castrates and females (IMCSF), and hence no injuries were sustained by animals in these treatments. Therefore, intact males and castrates mixed in colony, with or without females (IMCS and IMCSF) co-existed peacefully than other treatments. As to why such phenomenon happened is still not well understood and requires further investigations. 

Table 4. Injuries (%), Mortalities (%) and Docility Scores.



Treatment (T)

Standard Error



































(a, b and c): Means on the same row with different superscripts are significantly different (P< 0.05).

IM = Intact males only; CS = Castrates only; IMF = Intact males and females;  CSF = Castrates and females; IMCS = Intact males and castrates; IMCSF = Intact males, castrates and females


The proportion of animals that died in all treatment groups was similar, with the exception that higher proportion of animals was lost in the intact males and females groups (IMF) than the intact males only group (IM) (Table 4). This observation is in agreement with reports presented by Mensah and Okeyo (2005) and Adu (2002) that male grasscutters at puberty can fight to death in the presence of females. Djang-Fordjour et al (2012) also noted that when males are reared together from puberty they become constantly aggressive and hostile towards each other. From the post mortem findings, internal bleeding as a result of fighting was one major cause of deaths in all treatments. Septicaemia was responsible for the deaths recorded in colonies of intact males and castrates (IMCS) and intact males, castrates and females (IMCSF). Septicaemia has been observed to cause about 13.6% (Djang-Fordjour et al 2012) and 12.0% (Jori and Cooper 2001) of deaths in grasscutters, as a result of injuries. 


Animals in all the treatments had similar docility scores (Table 4). The current finding contrast previous reports that castration promotes docility in cattle (Turk 2007; Stafford and Mellor 2005). Based on the results, it could be determined that castration alone was not enough to ensure success in significantly reducing inter-male aggressive behaviours of grasscutters in colony. Other factors such as live weight and growth rate also influence docility. For example, it has been reported that cattle with higher body weight are more docile than those with lower body weights, and grew faster during fattening than aggressive animals (Fordyce et al 1988). These other factors need to be investigated in combination with docility to find their combined effect on aggressive behaviour of grasscutters.

Carcass weight, organ weight and dressing percentage of grasscutters  

Castration had little influence on carcass weight (hot/cold), dressing percentages (hot/cold), internal offals (plus or less fat), and heart, liver, kidneys, lungs and spleen proportions (Tables 5). 

Table 5. Carcass Parameters of intact male and castrated grasscutters








Slaughter weight (g)


1989 a



Defurred weight (g)


1918 a



Hot carcass weight (g)


1502 a



Dressing %, hot carcass


74.6 a



Cold carcass weight (g)


1493 a



Dressing %, cold carcass


74.1 a



Internal organs plus fat (%)1


8.25 a



Internal organs less fat (%)1


7.56 a



Viscera (%)1


0.69 a



Heart (%)1

0.56 a

0.49 a



Liver (%)1

1.44 a

1.28 a



Kidneys (%)1

0.45 a

0.44 a



Lungs (%)1

0.64 a

0.52 a



Spleen (%)1

0.09 a

0.11 a



(a and b): Means on the same row with different superscripts are different (P< 0.05). 1Internal offals and organs were expressed as % of pre-slaughter weight.

Similar findings were reported in Awassi lambs (Haddad et al 2006). The result may be due to the late castration as delayed castration was reported to confer no benefits in terms of carcass weight (Fisher et al 2001). Castration did not influence eviscerated weight and dressing percentage at 32 weeks of age of Pheasants (Severin et al 2007). Fat content, which is often a great concern as a public health issue was also not significantly affected by castration (Table 5). In goats, it was observed that early castration is recommended for thick fat deposition in the meat (Kebede et al 2008). Other reports also show that early castration of Damascus goats produced fatter carcasses in castrates than in intact males (Louca et al 1977).

Chemical composition  

The chemical composition of the meat of intact males and castrates were similar with regards to moisture content, energy, protein, fat, pH and ash (Table 6). However in other studies, castration had effect on fat and moisture content of meat. In pheasants, fat and moisture in the meat were reported to be higher in castrates than in intact males and protein content was also lower in castrates than intact males (Severin et al 2007). Higher intramuscular fat deposition was reported in castrated goats than in intact males (Koyuncu et al 2007). In cattle, higher protein content with lower moisture contents were observed in castrates than in intact males (Destefanis et al 2003).  

Table 6. Chemical Composition of Grasscutter Meat



Standard Error




Moisture (%)


77.3 a



Energy (J)

360 a

356 a



Protein (%)

19.4 a

19.7 a



Fat (%)

0.96 a

1.17 a




6.28 a

6.34 a



Ash (%)

1.82 a

1.80 a



(a): Means on the same row with different superscripts are significantly different (P< 0.05).

Organoleptic/sensory characteristics  

Among all variables evaluated on sensory characteristics of meat, panelists were able to determine differences between only longissimus dorsi muscle colour intensity of intact males and castrates (Table 7). Castrates produced lighter longissimus dorsi muscle colour than intact males. Castration had little influence on cooked meat colour. The meat colour for both intact male and castrates ranged from ivory to yellow lemon for fore legs, and ivory to light ivory for hind legs, longissimus dorsi muscle and skin. The cooked meat aroma, juiciness, texture and flavour were also slightly influenced by castration. Similar reports were presented by Morgan et al (1993) in cattle.  

Generally, the meat of intact males and castrates were both acceptable as sensory panelists could not detect any differences in both (Table 7). Significant differences did not occur between cooked meat attributes of intact males and castrates perhaps because of the late castration of the animals. Late castration confers no benefits in terms of taste/flavour (Shahidi 1994) and fat deposition (Kebede et al 2008).

Table 7. Sensory Characteristics of Cooked Grasscutter Meat



Standard Error



Fore leg meat colour


1.73 a


Hind leg meat colour

2.74 a

3.14 a


Longissimus dorsi meat colour

2.69 a

3.32 a


Skin colour

3.81 a

2.81 a


Fore leg meat colour intensity

3.41 a

2.91 a


Hind leg meat colour intensity

3.23 a

2.83 a


Longissimus dorsi meat colour intensity

4.31 a

2.31 b


Skin colour intensity

4.25 a

3.75 a


Fore leg meat aroma intensity

4.55 a

3.95 a


Hind leg meat aroma intensity

4.59 a

3.59 a


Longissimus dorsi meat aroma intensity

4.59 a

3.79 a


Skin aroma intensity

4.38 a

4.68 a


Fore leg meat juiciness

4.08 a

4.98 a


Hind leg meat juiciness

4.00 a

4.00 a


Longissimus dorsi meat juiciness

3.86 a

3.96 a


Skin juiciness

4.94 a

4.94 a


Fore leg meat texture

4.24 a

4.94 a


Hind leg meat texture

3.87 a

4.57 a


Longissimus dorsi meat texture

4.51 a

5.31 a


Skin texture

5.15 a

4.85 a


Fore leg meat flavour

4.5 a

5.00 a


Hind leg meat flavour

5.31 a

5.31 a


Longissimus dorsi meat flavour

4.80 a

4.70 a


Skin flavour

5.30 a

5.70 a


Fore leg meat overall acceptability

2.49 a

2.39 a


Hind leg meat overall acceptability

5.73 a

5.83 a


Longissimus dorsi meat overall acceptability

5.94 a

5.44 a


Skin overall acceptability

5.67 a

5.27 a


(a and b): Means on the same row with different superscripts are significantly different( P<0.05). 




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Received 24 November 2012; Accepted 16 February 2013; Published 1 March 2013

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