Effects of male-female exposure period on some reproductive parameters of grasscutter (Thryonomys swinderianus) and the pre-weaning growth performance of their young ones

B A Hagan, A Asafu-Adjaye, K A Darfour-Oduro and K Boa-Amponsem

CSIR-Animal Research Institute, P. O. Box AH 20, Achimota, Ghana
achiamaa1@yahoo.com

Abstract

The effect of male-female exposure period on some reproductive parameters of grasscutter was undertaken at the Grasscutter Unit of CSIR-Animal Research Institute, Pokuase between June 2008 and December 2009. Twenty-eight female grasscutters and 10 males were paired in the first round of mating whilst 29 females and 15 males were used in the second round of mating. Mating colonies of grasscutter were subdivided into three groups based on male-female exposure periods (M-FEP) of 4, 6 and 8 weeks.

The mean litter size, time of exposure to parturition (TEP) and litter sex ratio were 3.96±0.32, 164.7±1.45 days and 0.43±0.05 respectively. The time to conception (TC) was 12.7±1.45 days. Male-female exposure period did not affect (P>0.05) TC, litter size, TEP, and litter sex ratio.  Conception rate and percentage littering were higher for females that were exposed for 6 weeks. Sixty-four percent (64%) of all females that conceived did so within two weeks expo  sure to the males. The pre-weaning average daily gain of the young grasscutters was 5 g/day. This study suggests that male-female exposure periods did not have any significant effect on reproductive traits of grasscutters. Grasscutter farmers could therefore separate their females after 2 weeks of exposure and the extra males can be fattened and sold for income generation.

Keywords: conception rate, gestation length, litter size, mating, time to conception

Introduction

Ghana imports about 80% of its meat requirement (New Horizons 2005). A survey conducted by Ntiamoah-Baidu (1998) indicated that “bush meat” (meat from the wild) is the second most preferred animal protein in Ghana after chicken. Grasscutter meat is one of the most popular “bush meat” sources in Ghana. It was consumed by the rural poor and “lower class” citizens (Asibey 1971), but is now a delicacy for many Ghanaians, and commands a high price per kilogram (Asibey and Addo 2000). According to Falconer (1992) and Ntiamoah-Baidu (1998), grasscutter meat is an important source of animal protein; and it is the most expensive meat in West Africa (Asibey and Addo 2000).

The performance of grasscutter bred in captivity has been documented (Adu 1999; Adu et al 2000a and Adu and Wallace 2001). Addo et al (2007) reported that grasscutter females are reflex ovulators, breed all year round and would conceive irrespective of the status of the vaginal membrane and consequently could be mated at the convenience of the farmer without knowing the oestrous cycle. The grasscutter has a gestation period of 152 days and this makes two parturitions in a year quite difficult. It has been recommended (Adu 1999) that nursing the young should be limited to a maximum of 4 weeks which will permit re-servicing of the females and consequently obtaining two deliveries per year.

Coupled with this challenge is that early diagnosis of pregnancy in grasscutters is a problem among grasscutter farmers. Most farmers in Ghana therefore keep males with females for a longer period, sometimes over two months, before separating them because they are uncertain as to whether the female grasscutters are pregnant or not. This could result in pre-mature abortion as a result of the males’ forcibly attempting to cross females even when they are pregnant. Asibey (1974) reported that grasscutters with their status as unfamiliar species, has led to the common practice of leaving breeding pairs together for too long, resulting in the cannibalism of neonates by the male and eventual failure of some breeding programmes.

Early separation of females from males will enable the use of few males for breeding since males can be used on new females for mating after they have been separated. The extra males in the breeding population can thus be fattened and sold for income generation. Farmers will also save money by not having to keep both males and females on high plane of nutrition for an extended period because females have not been separated from their male counterparts. The objectives of the study were to determine the effect of male-female exposure period on some reproductive traits of grasscutters and pre-weaning growth performance of young grasscutters. 

Materials and methods

Animals, location, housing and management

The study was carried out at the Grasscutter Unit, Pokuase Research Station of the Council for Scientific and Industrial Research-Animal Research Institute (CSIR-ARI), Accra between June, 2008 and December, 2009.

Twenty-eight female grasscutters and 10 males were paired in the first round of mating whilst 29 females and 15 males were crossed in the second round of mating. Mating colonies of grasscutters were subdivided into three groups based on male-female exposure periods (M-FEP) of 4, 6 and 8 weeks with 8 replicates each for 4 and 8 weeks M-FEP and 9 replicates for the 6 weeks M-FEP. After these exposure periods, the animals were separated into individual metal cages measuring 60×60×40 cm. Animals were identified with ear tags and records on each animal was kept in excel spreadsheet. The female and male grasscutters weighed between 1.45-2.53 kg and 1.80-2.87 kg respectively. After parturition the young grasscutters were weaned from their dams at 42 days of age.

Feed and Feeding

Animals were fed twice daily on freshly cut Panicum maximum at 09.00 and 15.00 hours. Cereal-based formulated feed of 14% crude protein was given as supplements between 09.00 and 15.00 hours.  Water was provided ad libitum. Leftover feed and water were changed the next day.

Data collection and analysis

Conception rate was calculated as number of grasscutter females that conceived as a percentage of females serviced within each male-female exposure period.

At parturition, litter size, time of exposure to parturition (TEP) and litter sex ratio were recorded. Littering rate was calculated as the number of females that littered as a percentage of females serviced.  Time to conception, which was used as a measure of the time taken to achieve successful mating after a male-female pairing, assuming a gestation length of 152 days (Schrage and Yewadan 1999).

Sex ratio was calculated as proportion of males to total number of young ones in a litter.

Data on litter size, TEP, TC and litter sex ratio were analyzed using the ANOVA procedures of SPSS in a completely randomized design and fixed effects model with male-female exposure period (M-FEP) as the main factor. The χ²-square statistic was used for the analysis of conception and littering rates.

Pre-weaning weekly weight of young grasscutters was recorded and used to plot a pre-weaning growth curve. Pre-weaning average daily gain of the young was computed as:

(Weaning weight (g) – birth weight (g))/Weaning age (days) 

Results

Litter size, time of exposure to parturition, litter sex ratio and time to conception

The mean (± standard error) litter size of grasscutter under the study was 3.96±0.32 with no significant difference between treatments for male-female exposure period (Table 1).  

The mean litter sex ratio and TEP were 0.43±0.05 and 164.7±1.4 days respectively. Time to conception for the three groups of M-FEP (4, 6 and 8 weeks) was 10.2±4.5, 14.0±2.1and 12.3±2.1 days respectively whilst the overall mean TC was12.7±1.4 (not shown in Table). There was no significant difference (P>0.05) among the litter sex ratios and TEP for the male-female exposure periods. The mean litter sex ratio of 0.43±0.05 indicates that slightly more females were given birth to than male sex.

Conception rate and percentage littering

Male-female exposure period did not have any significant effect (P>0.05) on conception rate and percentage littering (Table 2). However, 6 weeks exposure tended to improve conception rate and littering percentage (63.2%). 

At 4 and 8-week male-female exposure periods, conception rates were lower than 50% and percentage littering fell below conception rates. The growth pattern of the young grasscutters from week 1 to 6 depicts a normal growth curve (Figure 1).

Discussion

Time of exposure to parturition, time to conception, litter size and litter sex ratio

The mean litter size of 3.96 in this study corroborates reports by Adu (1999), Addo et al (2007) and Yeboah and Adu (2000), who reported means of 4.00, 3.80 and 3.33, respectively. However, they are lower than the means (4.47, 4.59 and 4.63 respectively) reported by Chardonnet and Bonnet (1996), Jori (1998) and Adu et al (2000b). The variation in litter size could be due to nutritional differences, reproductive management and embryonic resorption (Robinson 1990 and Owusu et al 2010). Good reproductive management also involves good nutrition, which can result in increased litter size. Robinson (1990) reported that the increased litter size might have been due to better nutrition of the animals. Embryonic resorption rate in the grasscutter, which can occur at all stages of pregnancy, could also have an effect on litter size (Owusu et al 2010). The size of the dam could also determine the litter size. According to Addo et al (2003), the larger females tend to produce larger litters.

Time of exposure to parturition (TEP) among all the three treatments were statistically similar (P>0.05). This indicates that male-female exposure periods (M-FEP) did not alter the gestation period of grasscutters neither did it affect the sex ratio of young grasscutters at birth, implying that females could be exposed to males for four weeks to achieve fertile mating. Opara (2010) is of the view that at birth the sex ratio of the young ones is well balanced in grasscutters. Factors such as maternal nutrition, body condition of the dam, season, parity, litter size, litter birth weight and age of dam can also affect the sex ratio in rodents (Rosenfeld et al 2003). According to Pratt et al (1989), mammals usually produce approximately equal numbers of male and female offspring except where adverse effect causes skewing towards a particular sex. In their study, they reported a higher susceptibility of male embryos to intrauterine death in the golden hamster when the dam was physiologically stressed.

The mean time of conception of 12.7±1.4 days in this study is an indicator of the relatively short conception period of the female grasscutters after exposure to males.

Evidence from this study indicates that fertile matings resulting in conception occurred within one week (28%) of exposure to the males and that majority of the fertile matings (64%) took place within 2 weeks. Only 12% of fertile matings were recorded on the 4^th week of exposure (Figure 2). The practice of farmers keeping grasscutter mating pairs for over 4 weeks (Ngo-Samnick 2012) could therefore be discouraged.

Conception rate

Even though conception rate was 63.2% at 6 weeks M-FEP, it was still lower than 87.1% reported by Addo et al (2007) who used only females which were in estrous and also practiced repeated mating until females were diagnosed to be pregnant.  The present study simulated farmers’ practice in Ghana where they usually leave the males with the females for stipulated period (usually 8 weeks), thus avoiding the high labour cost associated with hand-mating.

Pre-weaning average daily gain

The average birth weight of the young ones in the study was 90 g. This compares favourably with Adu (2003) but is lower than reports by Yewadan and Schrage (1992), Jori (1998) and Ntsame-Nguema and Edderai (2000) who obtained weights of between 133 g and 138 g. The differences in birth weight could be due to management systems of the grasscutters, litter size and the nutrition of the dam. The birth weight of pups decreased with increase in litter size, (Addo et al 2007, Wogar and Agwunobi 2012). This inverse relationship between birth weight and litter size has also been reported by Odubote and Akinokun (1991) and Henry (2011).

Pre-weaning average daily gain was 5 g/day. This figure appears to be lower than reports by Jori and Chardonnet (2001) who reported pre-weaning average daily gain of between 8 and 13 g/day. Ntiamoah-Baidu (1998) also reported a pre-weaning average daily gain of 8.5 g/day. The average daily gain of 5g/day recorded in this study can be explained by the fact that the crude protein content of the diet used in this study fell below the recommended dietary crude protein of 18% for growing grasscutters (Kusi et al 2012).

Conclusion

• Male-female exposure period did not significantly affect litter size, time of exposure to parturition, litter sex ratio and time to conception. This study suggests that fertile matings resulting in conception occurred within one week of exposure to males and that majority of the fertile matings (64%) occurred within the first 2 weeks of exposure. Further studies are underway to find out factors which delay fertile matings in the captive grasscutter colonies.

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Received 1 November 2012; Accepted 23 November 2012; Published 2 December 2012

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