Effects of non-genetic factors on the birth weight, litter size and pre-weaning survivability of West African Dwarf goats in the Accra Plains

B A Hagan, J K Nyameasem, A Asafu-Adjaye and J L Duncan

CSIR-Animal Research Institute, P. O. Box AH 20, Achimota, Accra, Ghana
bernardatohagan@yahoo.com   ;   ato.hagan@csir-ari.org

Abstract

A total of 675 records each on birth weight, birth type and pre-weaning survivability of West African Dwarf (WAD) goats kept on natural pasture over a period of 9 years were assessed to determine the effects of environmental factors on them.

The overall mean birth weight was 1.20±0.01 kg. Birth type, season of birth, sex, parity and year of birth all had significant  effect on birth weight of kids. Single born kids (1.28±0.02 kg) were heavier than twins (1.21±0.01 kg) and triplets (1.13±0.02 kg). Kids born in the major wet season (1.25±0.02 kg) were heavier than those born in the dry and minor wet seasons. Male kids (1.25±0.01 kg) were heavier than their female counterparts (1.15±0.01 kg). Parity 6 kids recorded the highest birth weight (1.25±0.03 kg) which was higher than those of parities 2 and 4 but not 1, 3 and 5. Mean litter size for does was 2.07±0.03. Litter size in the major wet season (1.96±0.05) was  lower than those of the dry (2.09±0.04) and minor wet (2.13±0.06) seasons. Litter size of parity 1 kids (1.52±0.05) was lower than those of subsequent parities. Litter size of year 2003 kids (1.33±0.21) was lower than those of subsequent years. The overall mean pre-weaning survivability was 79.9%. Birth type, parity of does and year of birth affected pre-weaning survivability whilst season of birth and sex had no effect. Environmental factors have substantial influence on birth weight, litter size and pre-weaning survivability of WAD goats. 

Keywords: birth type, coefficient of variation, environmental factor, Ghana, kids

Introduction

In the developing world, livestock production constitutes approximately 30% of the agricultural gross domestic product (GDP) (World Bank 2009). Goat is an important livestock species kept by most households in Ghana. The rearing of goats serves as source of income and store of wealth for smallholder keepers in most areas of the country. Goats are raised for their meat and skin with the meat from goat being a delicacy throughout the country. There are no religious barriers to the consumption of goat meat in Ghana compared to other livestock species.

The West African Dwarf (WAD) goat is an important breed of both savannah and forest zones of West Africa. It has a small body size (Ahulu et al 1995), high fecundity (Oppong and Yebuah 1981), short kidding interval (Tuah et al 1992) and well adapted to the harsh tropical environment (Hotse et al 1988; Ozoje 1998). The small body size of the WAD goat makes for easy management of the animal by even children whilst the short generation interval provides a faster turnover of capital for keepers of this breed. These positive attributes of the breed can be harnessed to increase the profitability of raising WAD goat as a business enterprise. Birth weight, litter size and pre-weaning survivability are important traits that can affect the profitability of the goat enterprise. In some livestock species, these traits are affected by a number of environmental factors. Odubote (1996) reported significant effect of parity and year of birth on litter size of WAD goats. Singh (2002) and Baiden (2007) have both reported significant effects of birth type on birth weight of kids. Whilst a number of studies have been done on the effects of environmental factors on some performance traits of WAD goats, relatively small data size have been used in these studies. The objective of this study therefore was to assess the effects of non-genetic factors on the birth weight, birth type (or litter size) and pre-weaning survivability of WAD goats kept under agro-pastoral system in the Accra plains of Ghana.

Materials and methods

Study area

The study was conducted at the Katamanso research station of Animal Research Institute of the Council for Scientific and Industrial Research (CSIR-ARI), Ghana. The area has a bimodal rainfall pattern with a major wet season from April to July and the minor one from September to November. The remaining months (January, February, March, August and December) constitute the dry season (Figure 1). The mean annual rainfall of the site ranges between 600-1000 mm whereas the mean temperature falls between 15-34ºC (Okantah et al 2005).

Figure 1: Monthly rainfall distribution for 12 year-period (1997-2008) at Animal Research Institute, Katamanso station

Management

Animals were housed in wooden walled and wooden pallet floor structures with aluminium roofing sheet. They were allowed to graze on natural pasture between 08:00 and 16:00 hours daily. Pasture comprised grass species (Panicum maximum, Sporobolus pyramidalis and Vertiveria fulvibarbis) and browse species (Griffonia simplicifolia, Baphia nitida and Milletia thoningii) (Oddoye et al 2002). Animals were not given any supplementary feed. Water was provided freely to all animals. Animals were routinely treated against ecto-parasites and endo-parasites using anti-helminth, Albendazole 10% once a month. Animals were also vaccinated against Peste des petits ruminants (PPR) once a year in May. All animals including breeding bucks, does and kids grazed together on the same natural pasture. At the birth of a kid, the birth weight was taken within 24 hours and the animal identified by ear tagging.   

Traits studied

A total of 675 birth weights, birth type and pre-weaning survivability records on WAD goats were collected from 2003 to 2012 except 2009 where the data quality was questionable hence removed from the analysis. Birth weight (BWT) was defined as the weight of a kid within 24 hours of birth using an electronic weighing scale. Birth type was the number of kids born by a doe at each parturition. Pre-weaning survivability was defined as the percentage of kids alive at the time of weaning at 3 months of age. 

Statistical analyses

Data were analysed using the Generalised Linear Model procedure of R (R Core Team 2012) and differences between means of a trait for different factors were tested using Duncan Multiple Range Test. The statistical model for birth weight and pre-weaning survivability was as follows: Y_ijklmn = µ + L_i + S_j + X_k + P_l + T_m + e_ijklmn

Where:

Y_ijklmn       =          birth weight and pre-weaning survivability

µ              =          overall mean

L_i             =          fixed effect of the i^th birth type (3 levels)

S_j             =          fixed effect of the j^th season of birth (3 levels)

X_k            =          fixed effect of the k^th sex of kid (2 levels)

P_l             =          fixed effect of the l^th parity of doe (6 levels)

T_m            =          fixed effect of the m^th year of birth (9 levels)

e_ijklmn         =          random error term associated with each observation

For birth type, the model used was as follows:

Y_ijkl      =          µ + S_i + P_j + T_k + e_ijkl

Where:

Y_ijkl          =         birth type (3 levels)

µ              =          overall mean

S_i             =          fixed effect of the i^th season of birth (3 levels)

P_j             =          fixed effect of the j^th parity of doe (6 levels)

T_k             =          fixed effect of the k^th year of birth (9 levels)

e_ijkl           =          random error term associated with each observation

Results were considered statistically significant when P<0.05.

Results

Birth weight and birth type

Birth weight generally declined with increase in litter size (Table 1).. Kids born as single were significantly heavier than twins and triplets. The average birth weight of twins (1.21±0.01 kg) was higher that of the triplets (1.13±0.02 kg). The mean birth weight of kids born in the major season (1.25±0.02 kg) was heavier than those born in the dry (1.20±0.01 kg) and minor (1.14±0.02 kg) seasons. Male kids (1.25±0.01 kg) were heavier than females (1.15±0.01 kg) at birth. Mean birth weight of parity 4 kids (1.13±0.02 kg) was lower than those of the other parities. There was no difference between the mean birth weights of parities 1, 3, 5 and 6 kids. Similarly, the mean birth weights of kids in parities 1, 2, 3 and 5 were similar. The mean birth weight of kids in year 2004 was similar to those of 2006 but lower than those of the other years.

The mean birth type of does that littered in the major wet season (1.96±0.05) was lower than those of the dry (2.09±0.04) and minor (2.13±0.06) seasons. Mean birth type of primiparous does (1.52±0.05) was lower than those of the other parities. The highest birth type was recorded at parity 4. Generally, birth type increased with increase in parity number. 

Pre-weaning survivability

Birth type, parity of doe and year of birth affected pre-weaning survivability (Table 2). The mean pre-weaning survivability of singles (79.2%) and twins (86.3%) were both shigher than triplets (66.6%). Season of birth and sex of kid did not have any effect on pre-weaning survivability.  

Discussion

The high variability of birth weight, birth type and pre-weaning survivability presents an opportunity for genetic improvement of these traits in the WAD goat. Pre-weaning survivability is the most important trait amendable for genetic improvement as it had the highest coefficient of variation.

The mean birth weight of kids (1.20±0.01 kg) in this study agrees favourably with the report by Akusu and Oyeyemi (2000).  However, the mean found here is lower than those means reported by Adebambo et al (1994) and Baiden (2007) and higher than those of Arthur (2000) and Tuah et al (1992). The significant effect of birth type on birth weight agrees with works by Adebambo et al (1994), Akusu and Oyeyemi (1998), Baiden (2007) and Tuah et al (1992) for the same breed of goat. Husain et al (1997) also reported significant effect of birth type on birth weight in Black Bengal goats in Bangladesh. Birth weight generally decreases with increase in the litter size. In lambs, Robinson et al (1977) reported that as the number of foetuses increases, the number of caruncles attached to each foetus decreases thus reducing the feed supply to the foetus and hence the birth weight of the lambs. 

Generally, under extensive and semi-intensive system of management where animals spend some time outdoors to graze, season has appreciable influence on a number of economic traits of farm animals. In this study, season of birth had significant effect on both birth weight and birth type. The birth weight of kids born in the major wet season was significantly heavier than those of the other seasons. The availability of unlimited quality forage during this period of the year could have contributed to this observation. Baiden (2007) also reported heavier birth weight for kids born in the major wet season. Kids born in the dry season also had  heavier birth weight (1.20±0.01 kg) than those in the minor wet season (1.13±0.02 kg). The dams of these kids might have conceived during the latter part of the major wet season allowing for the latter stage of pregnancy to fall within the minor wet season (September-November) where there is a considerable amount of high quality forage prior to kidding in the dry season. Hammond et al (1971) as cited in Baiden (2007) reported an increased growth of lambs when dams are fed extra feed during the second half of pregnancy.  

The effect of sex of kid and year of birth on birth weight in this study is consistent with Husain et al (1997) and Raza et al (1998). Baiden (2007) did not, however, observe any  effect of sex and year on this trait although the males were slightly heavier than the females at birth. The low birth weight of kids in 2004 and 2006 compared to the other years could partly be attributed to challenge with availability of adequate feed during these years. The significant effect of parity on birth weight in this study is contrary to the report by Baiden (2007). Kid’s birth weight generally increased with increase in parity number.  

The mean litter size at birth (2.07±0.03) in this study is higher than reports by Akusu and Ajala (2000), Baiden (2007) and Odubote (1996). Good management practices at the station could indirectly explain the higher litter size in the flock. The effect of season of birth on litter size is consistent with Baiden (2007). In this study, the low litter size in the major wet season could be due to does conceiving in the dry season where there is nutritional stress resulting in decreased ovulation rate. Litter size was also affected by parity and year of birth. Litter size generally increased with increase in parity of doe. This may be attributed to the improvement in the reproduction efficiency of does as they mature (Levasseur and Thibault 1980). Variation in feed availability in the form of annual pasture growth might explain the differences in litter size from year to year. Litter size is known to be influenced by both genetic and environmental factors (Wilson et al 1989).

The overall mean pre-weaning survivability of 79.9% in this study compares favourably with 79.1% reported by Baiden (2007), but lower than values reported by Turkson et al (2004) and Adenaike and Bemji (2011) for the WAD goat.  High litter size had been reported to negatively affect kid survival (Baiden 2007; Adenaike and Bemji 2011). A similar observation was found in this study with twins recording the highest survivability though not different from kids of single birth. The lower survivability observed with increased litter size can be attributed to reduced birth weight of kids (Snyman 2010) as litter size increases reducing their chances of survival.  

In the current study, pre-weaning survivability continued to decrease after the second parity.  Similarly, Debele et al (2011) observed a linearly decreasing survivability with increasing parity in Arsi-Bale kids. In other studies (Awemu et al 1999; Chowdhury et al 2002); however, pre-weaning survivability was found to increase linearly with increasing parity in Black Bengal and Sokoto goats. Awemu et al (1999) observed maximum kid survivability at the sixth parity. In this study, maximum pre-weaning survivability was observed in the second parity. Breed × parity interaction could thus be existent in pre-weaning survivability of goat kids.  Akusu (2003), Springer (2004), Turkson et al (2004) and Snyman (2010) have all reported that female kids had higher survivability than male kids. In this study, however, the effect of sex on pre-weaning survivability was similar for both sexes. It should, however, be noted that the previous studies were conducted over a shorter duration as compared to the current one.

No systematic trend, in relation to year of kidding, was observed for pre-weaning survivability, therefore, the defined differences among the years might be the effects of diseases and climatic conditions (Al-Najjar et al 2010).

Pre-weaning survivability was similar for kids born in the wet and dry seasons. Baiden (2007) however reported season of kidding had an effect on kid survivability. The observed similarity for pre-weaning survivability in this study in relation to season of birth could be attributed to the good health care given to the flock throughout the seasons. Also the availability of feed, especially during the dry season might have enhanced the dam’s nutrition and consequently milk yield, thus leading to heavier and stronger lambs (Hammond et al 1971; Chowdhury et al 2002).

Conclusion

• Though the performance of the WAD goat kept by the CSIR-ARI is encouraging, environmental factors have considerable influence on birth weight, birth type and pre-weaning survivability. These traits, however, need further improvement. There is also the need to improve the management, particularly nutrition at the station to enhance the birth weight of the breed further.

Acknowledgements

Special thanks to all the staff of the Small Ruminant Unit of the CSIR-ARI Katamanso station for the collection of data for this work.

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Received 29 September 2013; Accepted 10 December 2013; Published 1 January 2014

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