Genetic and Non-Genetic Factors Affecting Post-Weaning Growth and Morphometric Traits in Assam Hill Goat

Goat rearing is the backbone of economy of small and landless farmers in the state of Assam. It is an insurance against crop failure and provides alternate source of livelihood to farmers all the year round. Goats are among the main meat producing animals in the state where chevon is the meat of choice and has huge domestic demand. The indigenous goats of Assam known as ‘Assam Hill Goat’ are famous for their excellent meat quality, high prolificacy and adaptability to local environment. It is a small sized animal in which the predominant coat colour is white, but, black, brown and grey are also found.
        
Growth and development are important factors for production of meat animals. Body weight and body measurements are important parameters to describe growth (Salako, 2006). Post-weaning body weight is primarily affected by hereditary factors along with plane of nutrition, prevailing meteorological conditions and ability of the animal to adapt to its environment. In meat producing animals, external body measurements could be a reliable indicator of its future performance with respect to live body weights, if and only if a correlation has been identified among these traits of interest (Tomar et al., 2001). The potential of genetic improvement of the trait of interest is largely dependent on its heritability value and genetic relationship with other traits of economic importance upon which some selection pressure may be applied. Information on heritability is essential for planning efficient breeding programmes, and for prediction of response to selection (Falconer, 1989). The meat production of Assam Hill goat could be increased by augmenting the growth performance of the animals.  Therefore, in the present study, an attempt was made to study the genetic and non-genetic factors affecting post-weaning body weights and morphometry of Assam Hill goat along with the genetic parameters. 

Location and data collection
 
The animals used in this study were reared by the farmers belonging to the field units under “All India Coordinated Research Project on Goat Improvement”, Goat Research Station, Assam Agricultural University, Burnihat. The location lies between 90.48° and 92.22° (East) longitude and 20.09° and 26.95° (North) latitude. The data pertaining to 960 animals were utilized in the present investigation. Measurements of four morphological characters viz. body weight (BW), height at withers (HW), heart girth (HG) and body length (BL) were recorded at 6, 9 and 12 months of age. Measuring tapes were used to record the length and heart girth in cm, while heights were measured using graduated stick. The body weights were measured by using spring balance of 50 kg capacity in the morning hours. The data were classified into three locations viz., Batabari (L₁), Nahira (L₂) and Tetelia (L₃); four seasons of birth viz. pre-monsoon  i.e March-May (S₁), Monsoon i.e. June-September (S₂), post-monsoon i.e. November-December (S₃) and winter i.e. December-February (S₄) and two sexes as male and female.
 
Management practices
 
The goat sheds were made of locally available materials i.e. bamboo, wood, thatches etc. The slotted floors of the houses were raised by about 3 feet. The walls of the shed were made of bamboo and covered up to 3/4^th of the total height of the wall and rests of the portions were kept open for ventilation. The animals were kept under semi-intensive management system. Selective breeding was practised where the breeding does were mated with superior bucks supplied under the project. All the male kids except those kept for breeding purpose were castrated before 3 months of age. The animals were immunized regularly against enterotoxaemia, peste des petits ruminants and goat pox. Regular deworming was practised in all the field units under study.
 
Statistical methods
 
The data were analyzed by using least-squares technique as described by Harvey (1990). The statistical model used to analyze the data was Y_ijko = µ + L_i + M_j + S_k + e_ijko, where, Y_ijko is the value of the o^th animal in the (ijk)^th sub-class, µ is the overall population mean, L_i is the effect of i^th location (i=1, 2, 3), M_j is the effect of j^th season of birth (j=1, 2, 3, 4), S_k is the effect of the sex of the kid (k=1, 2) and e_ijko is the random error associated with Y_ijko assumed to be NID (0, σ²_e). Duncan’s Multiple Range Test (DMRT), modified by Kramer (1957) was applied to make pair wise comparison among the means wherever significant differences exist.
        
The estimates of heritability (h²), phenotypic (r_P) and genetic (r_G) correlations were calculated from the sire component of variance and covariance using methods described by Becker (1975). The statistical model used to estimate the genetic parameters was Y_ij = µ + S_i + e_ij, where, Y_ij is the observed value of j^th progeny on i^th sire, µ is the overall population mean, Si is the effect of i^th sire common to all its progeny (i = 1,2,3.....) and e_ij is the random error component assumed to be NID (0, σ²_e). Standard errors of heritability were estimated as per Swiger et al., (1964), while standard errors of phenotypic and genetic correlation were estimated as per the methods of Panse and Sukhatme (1969) and Robertson (1959), respectively.

The estimates of least-squares means (LSM) and standard errors (SE) for body weight, height at withers, heart girth and body length at 6, 9 and 12 months of age are presented in Tables 1, 3, 5 and 7, respectively. Least-squares analyses of variance showing the effect of different factors on the traits are presented in Table 2, 4, 6 and 8, respectively.
 
Body weight
 
The overall LSM ± SE for body weight at 6, 9 and 12 months of age were 7.557 ± 0.049 kg, 9.934 ± 0.044 kg and 12.549 ± 0.046 kg, respectively (Table 1), which are in concordance with the findings of Nahardeka (1994) in Assam Local goats. Lower value (5.96 ± 0.49 kg at 6 months of age by Sarma, 1997) and higher values (8.02 ± 2.17 kg, 10.92 ± 2.72 kg and 14.47 ± 3.55 kg, respectively at 6, 9 and 12 months of age by Mia et al., 2013) were reported respectively in Assam Local and Black Bengal goats.
 

 
Effect of location
 
Location had significant effect on body weight in all age groups (Table 2). Goats of Tetelia unit exhibited comparatively higher body weights at 12 months of age. Better health care and management, availability of feed, better financial condition of the farmers and awareness might have aided to the rise in the body weights of the goats of Tetelia unit.
 

 
Effect of season of birth
 
Non-significant effect of season of birth was observed on body weight at 6 months of age (Table 2), which is in agreement with the observations of Singh (1991) in Assam Local goats. However, significant effect was reported by Paul et al., (2014) in Black Bengal goats. Differences in the body weight at 9 and 12 months of age due to season of birth were found to be significant (P<0.05) in the present study (Table 2), which are supported by the findings of Paul et al., (2014) in Black Bengal goats. The higher body weight at 9 and 12 months of age for the goats kidded during pre-monsoon season might be due to exposure to better environmental conditions in terms of feeds, green fodder, low heat stress and other management practices. However, non-significant effect of season of birth was obtained by Singh (1991) in Assam Local goats on both the traits.
 
Effect of sex
 
Analysis of variance revealed that body weight at 6 months of age was not significantly affected by sex of kids though the weights of male kids were higher than the females (Table 2). Similar observations were obtained by Naik et al., (1985) in Ganjam goats. However, significant effect (P<0.01) was reported by Mehta et al., (1997) in Sirohi goats. On the other hand, the present study revealed significant differences (P<0.01) in body weight at 9 and 12 months of age due to sex (Table 2). These confirmed the earlier reports of Paul et al., (2014) in Black Bengal goats. The increase in body weight of male goats might be attributed to the early activation of male gonads than the females. Male sex hormone testosterone, responsible for the anabolic effect could have caused the faster growth in male animals.              
        
However, non-significant effect of sex on body weight at 9 and 12 months of age were reported by Naik et al., (1985) in Ganjam goats.
 
Height at withers, heart girth and body length
 
The overall LSM ± SE for height at withers, heart girth and body length were 41.231 ± 0.121 cm, 43.902 ± 0.120 cm and 46.791 ± 0.112 cm at 6 months; 44.621 ± 0.115 cm, 48.890 ± 0.099 cm and 52.765 ± 0.090 cm at 9 months; and 50.778 ± 0.172 cm, 55.552 ± 0.170 cm and 58.392 ± 0.207 cm at 12 months of age, respectively (Table 3, 5 and 7). These findings were in accordance with the findings of Pan et al., (2015) in Black Bengal goats for height at withers and heart girth and Saikia et al., (1985) in Assam Local goats for body length. However, much higher values were recorded in Sirohi goats by Dudhe et al., (2015).
 

 

 

 
Effect of location, season of birth and sex of kids on morphometric traits
 
Location affected height at withers and heart girth significantly (P<0.01 at 6 months and P<0.05 at 9 and 12 months of age in case of height at withers; and P<0.05 at 6 and 12 months and P<0.01 at 9 months of age in case of heart girth) in all age groups, but body length at 6 (P<0.05) and 12 months (P<0.05) of age only (Table 4, 6 and 8).
 

 

 
  
 
Least-squares analysis of variance revealed significant effect of season of birth (P<0.05) on height at withers at 12 months of age only, but on body length (P<0.01) at all age groups (Table 4 and 8). The values for height at withers were found to be comparatively higher in animals born during monsoon season and values for body lengths were comparatively higher in goats born during post-monsoon season. The results are in accordance with the findings of Dudhe et al., (2015) in Sirohi goats. On the other hand, observations contradictory to the present findings were reported by Das et al., (1989) in Barbari goats. However, in the present investigation, non-significant effect of season of birth was observed on heart girth at all age groups (Table 6), while, significant effect was observed by Dudhe et al., (2015) in Sirohi goats.
        
Sex of kids’ exerted significant effect (P<0.05 at 6 months and P<0.01 at 9 and 12 months of age) on height at withers at all the age groups; while on heart girth (P<0.05 and P<0.01) and body length (P<0.01) the significant effects were observed at 9 and 12 months of ages (Table 4, 6 and 8). The values of the least-squares means for male animals were higher than those for the females. These results were supported by the reports of Dudhe et al., (2015) in Sirohi goats. However non-significant effect was reported by Kharkar et al., (2014) in Berari goats.
 
Genetic parameters
 
The estimates of genetic parameters viz., heritability (h²), phenotypic correlation (r_P) and genetic correlation (r_G) along with the standard errors among body weights and body measurements at 6, 9 and 12 months of age are presented in Table 9-12.
 

 

 

 

 
Heritability
 
The heritability estimates for body weights under study were moderate (Table 9). These estimates are in agreement with the estimates of Nahardeka et al., (2001) in Assam Local goats for body weight at 6 months and Moghbeli et al., (2013) in Raini Cashmere goats for body weights at 9 and 12 months of age. The estimates of heritability obtained in the present study indicated the scope for improvement of the traits through selection. However, lower (0.070 ± 0.020, 0.120 ± 0.040 and 0.130 ± 0.040 at 6, 9 and 12 months of age, respectively) and higher (0.450 ± 0.070, 0.490 ± 0.060 and  0.470 ± 0.080 at 6, 9 and 12 months of age, respectively) estimates of heritability for these traits were obtained by Gowane et al., (2011) in Sirohi goats and Mia et al., (2013) in Black Bengal goats.
        
Similarly, moderate estimates of heritability for height at withers, heart girth and body length at 6, 9 and 12 months of age were obtained in the present study which also indicated scope of improvement by selection (Table 10, 11 and 12). However, Dudhe et al., (2015) reported higher estimates (0.709 ± 0.144, 0.699 ± 0.179, 0.708 ± 0.188 for height at withers; 0.563 ± 0.138, 0.676 ± 0.184, 0.503 ± 0.197 for heart girth and 0.589 ± 0.150, 0.571 ± 0.172, 0.605 ± 0.192 for body length) of heritability for morphometric traits in Sirohi goats as compared to the present estimates.
 
Phenotypic and genetic correlation
 
The phenotypic and genetic correlations among body weights were found to be positive and high at all age groups (Table 9). It has considerable significance in the selection programme for body weight, since body weight at early ages is a good indication of body weight at 12 months of age. The findings are in conformity with the findings of Mia et al., (2013) in Black Bengal goats.
        
The phenotypic and genetic correlations among body weights and various body measurements were found to be positive and high at all age groups (Table 10, 11 and 12). This indicated that the linear body measurements could reliably play a predictive role for body weight at any given age. Body weight appeared to be a function of these body measurements. Further, it also indicates the possibility of improving body weight as a correlated response by exerting selection pressure on the body measurements. Dudhe et al., (2015) in Sirohi goats also reported positive and high phenotypic and genetic correlation among body measurements as obtained in the present study.

The present study has revealed the importance of some non-genetic factors for growth performance which could be utilized for genetic evaluation. The effects of location, season of birth and sex were found to be significant for most of the parameters; heritability estimates of growth traits were moderate and phenotypic and genetic correlations were also positive and high. Therefore, it could be concluded that there is scope of selection based on morphometry and animals with higher body weights at initial phases of growth will perform better with respect to growth traits at later stages. These results, therefore, provide an important perspective on the selection objectives of Assam Hill goats by considering different environmental factors. Since body weight and body measurements are important parameters to describe growth, so, selective breeding of Assam Hill goat based on these parameters needs to be given more emphasis in breeding programmes for greater productivity.

The authors acknowledge the support of AICRP on Goat Improvement, Goat Research Station, Burnihat for conducting the research work.