Evaluation of models for estimation of genetic parameters for post-weaning body measurements and their association with yearling weight in Nellore sheep.

Objective
The objective of this study was to obtain (co) variance components and genetic parameter estimates for post-weaning body measurements such as body length, height at withers and chest girth recorded at six (SBL, SHW and SHG), nine (NBL, NHW and NHG) and twelve (YBL, YHW and YHG) months of age along with yearling weight (YW) in Nellore sheep maintained at livestock research station, Palamaner, Andhra Pradesh, India and also the association among body measurements with yearling weight was studied.


Methods
Data on 2,076 Nellore sheep (descended from 75 sires and 522 dams) recorded between 2007 and 2016 (10 years) were utilized in the study. Lambing year, sex of lamb, season of lambing and parity of dam were included in the model as fixed effects and ewe weight was kept as a covariate. Analysis were conducted with six animal models with different combinations of direct and maternal genetic effects using restricted maximum likelihood procedure. Best model for each trait was determined based on Akaike's Information Criterion.


Results
Moderate estimates of direct heritability were obtained for the studied traits viz., BL (0.02-0.24), HW (0.31-0.49) and CG (0.08-0.35) and their corresponding maternal heritability estimates were in the range of 0.00-0.07 (BL), 0.13-0.17 (HW) and 0.07-0.13 (CG), respectively. Positive direct genetic and phenotypic correlations among the traits and they ranged from 0.07 (YBL-YW) to 0.99 (SBL-SHG, SHG-YW and NBL-YBL) and 0.01 (SBL-YBL) to 0.99 (NBL-NHG), respectively. Further, the genetic correlations among all the body measurements and yearling weight were positive and ranged from 0.07 (YW and YBL) to 0.99 (YW and SHG).


Conclusion
Strong association of chest girth at six months with yearling weight. Further, it is indicated that moderate improvement of post-weaning body measurements in Nellore sheep would be possible through selection.


INTRODUCTION 48
Nellore is one of the recognized native sheep breeds of southern India and are reared for meat and 49 known for its superior growth rate. This sheep is tallest and characterized by white with black spots 50 around the lips, eyes, lower jaw and abdomen. They are predominantly distributed in south coastal 51 districts of Andhra Pradesh state. Its population was about 11.74 million and accounts for 19.17 per cent 52 of total sheep population of India [1]. These sheep possess better adaptability to hot and humid climatic 53 conditions and also resistant to most of endemic infectious diseases. 54 In sheep meat yield is an intricate quantitative trait found to be affected by several genetic and 55 non-genetic factors. However, body measurements or biometric traits were controlled by uncomplicated 56 genetic mechanisms. Hence, biometric traits could be used as a selection criterion in improving the farm 57 animals in terms of meat-yield. Biometric traits in sheep are majorly affected by individual's own genes 58 as well as maternal genes and maternal environment apart from the environment in which it is raised [2][3][4]. 59 Genetic evaluation of sheep for economically important traits is prerequisite and plays pivotal 60 role in formulating breeding objectives and helps in conservation of germplasm. Several investigations on 61 growth and its related traits stated that the traits are affected by both direct additive genetic effects and 62 maternal genetic effects and models which ignored maternal genetic effects resulted in biased estimation 63 of genetic parameters [5][6][7][8][9]. To implement better breeding plans, information on (co)variance components 64 and genetic parameters for these traits are prerequisite.

4
yearling age, which is suitable for the slaughter. With this Hypothesis, Hence, the present study was 74 undertaken to estimate the variance components and genetic parameters for post-weaning body 75 measurements, to estimate phenotypic, genetic and environmental correlations among the traits and also 76 to know the association of body measurements with the yearling weight and to identify the major non-77 genetic factors affecting the traits. 78

Data collection 80
In this study, data were obtained from the Nellore sheep flock maintained at the Livestock Research 81 Station; Palamaner, Andhra Pradesh, India (located at13º.20'E latitude and 78º.75'N longitude at an 82 altitude of 683 m above mean sea level) over a period of ten years (2007 to 2016). A total of 2076 lambs 83 descended from 75 sires and 522 dams were considered for analysis. Different body measurements 84 considered in the analysis were body measurements recorded at 6, 9 and 12 months of age viz., body 85 length, height at withers and chest girth and along with yearling weight of lambs and the body 86 measurements were recorded [[123]. 87

Animal Management 88
All the animals in the flock were reared under semi-intensive system of management. Four 89 hundred females were maintained in the flock with male to female ratio for breeding was around 1:25. 90 Ten to fifteen sires were kept for breeding per year. Sires used for breeding were retained in the flock for 91 at least two years, males were selected based on their six months weight and their progeny performance 92 was also considered for their selection. Breeding in the flock was confined to major (March to May) and 93 Weaning of lambs was done at an age of three months. Lambs were fed with concentrates ad 100 libitum from ten days after birth till weaning. From six months of age, animals were sent for grazing for a 101 period of 8 to 10 hours. In addition to this, 300 g of concentrate mixture was provided during post-102 weaning period. Fodder tree loppings and hay of Stylo hamata, cow pea, horse gram and alfalfa were also 103 fed to animals. Seasonal differences were observed in the growth patterns of animals during March to 104 June due to limited grazing resources. Grazing area consisted deciduous vegetation and fodder trees like 105 Subabul (Leucaena leucocephala), Neem (Azadirachta indica) and Avisa (Sesbania grandiflora). 106

Statistical Methods 107
Initially data were analyzed to know the fixed effects to be included in the model by least-108 squares analysis of variance [134]. The model included the fixed effects of year of lambing (nine levels), 109 season of lambing (two levels), sex of the lamb (two levels) and parity of dam (seven levels). Ewe weight 110 at lambing was kept as a covariate. Only significant effects (p ≤ 0.05) were included in the models which 111 were subsequently used for genetic analysis. Convergence of the Restricted Maximum Likelihood 112 (REML) solutions was assumed when the variance of function values (-2 log-L) in the simplex was less 113 than 10 -8 . To ensure that a global maximum was reached, the analysis was restarted. When estimates did 114 not change up to two decimals, convergence was confirmed. Six models which accounted for the direct 115 and maternal effects were fitted and are as follows: 116 y = Xb+ Z a a + e model 1 117

y = Xb+ Z a a + Z m m + e with Cov (a m, m o ) =0
model 2 118  Estimates of (co)variance components were obtained by REML using wombat software program 143

RESULTS 151
Number of records, pedigree structure, summary statistics and different sources of variation for 152 post-weaning measurements in Nellore sheep are shown in Table 1. The least-squares means obtained in 153 our study for body lengths at 6, 9 and 12 months of age were 57.62 ± 0.12, 59.72 ± 0.13 and 62.51 ± 0.13 154 cm, respectively. Likewise, heights at withers were 65.27 ± 0.14, 69.73 ± 0.13 and 74.43 ± 0.12 cm, 155 respectively, whereas the corresponding means for chest girth were 69.76 ± 0.15, 70.26 ± 0.13 and 75.50 156 ± 0.10 cm respectively. Least-squares means for weight at 12 months of age in Nellore sheep are found 157 to be 26.21±0.12. In general, the coefficient of variation for the studied body measurements were ranged 158 from 6.01 to 10.81 per cent. All the fixed effects incorporated in the model were found to be significant 159 on post-weaning body measurements with few exceptions. The coefficients of determination for fitted 160 models were ranged from 0.44 to 0.51 per cent. 161 Based on the AIC values, best model explaining the source of variation for body lengths at 6, 9 162 and 12 months of age were 3, 2 and 2 models, respectively. Whereas, 2, 2 and 6 were observed to be best 163 models for height at withers at these ages and the best model for the corresponding chest girth were 3, 4 164 and 6 and model 6 explained better variation for yearling. 165 The direct heritability estimates obtained for body measurements (SBL, SHW and SCG) at sixth 166 months were 0.13, 0.31 and 0.08, respectively. Heritability estimates observed at nine months of age were  (Table 2). In the present study, very low estimates of maternal heritability were obtained. Height at withers 242 at six months and nine months were affected significantly by maternal genetic effects with a proportion of 243 13 and 17 per cent to the total genetic variation. Similar results were reported in Moghani sheep and 244 Muzaffarnagari sheep, respectively [2,4]. Low and zero estimates of maternal genetic effects for body 245 measurements at various stages of growth suggested that maternal genetic effects were not much 246 important in these traits. Although, the maternal heritability estimate for NBL is zero, the best model 247 explaining the variation is found to be model2. This could be explained by the fact that the maternal 248 genetic variance for NBL is very less in magnitude but not zero. negative. The estimates of total heritability were observed to be low, if negative correlation exists 267 between additive direct and maternal genetic effects and higher estimates of total heritability were 268 observed with low r am values (Table 2). This finding was in agreement with reports for Moghani and 269 (SHG) is most reliable body measurement among the others to predict the yearling weight and it may be 292 taken into account while implementing genetic selection program based on biometric characters as the 293 correlation magnitude between heart girth and yearling weight is highest. The genetic correlations among 294 the traits at six months were higher than the correlations between the traits at nine months and yearling 295 age. This could be explained by that selection of lambs is based on the six months body weight as there is 296 existence of greater additive genetic variability, further, the growth and biometrical traits at six months 297 age are mostly affected by common genes than traits at nine and twelve months age. In summary, the direct heritability estimates obtained in the study indicated a moderate to high 310 genetic variability for body measurements and suggested the possibility of genetic improvement through 311 selection. Few traits were also affected by maternal genetic/environmental effects. However, the effects 312 were not much significant. Based on the correlation estimates, it is suggested that heart girth at six months 313 of age is a reliable indicator of yearling weight from all perspectives (genetic, phenotypic and 314 environmental). Hence, it should be considered as a selection criterion by selecting the genetically 315 superior lambs for chest girth at an early age which enables breeder to bring better genetic improvement 316 in body weights at later stage. Further, it is advised to consider the negative environmental correlations 317 while implementing the breeding decisions. Based on the correlation estimates between body 318 measurements and yearling weights, the biometric traits may be incorporated in reliable selection index 319 and it may have relevance in other Indian sheep breeds. 320

CONFLICT OF INTEREST 322
We certify that there is no conflict of interest with any financial organization regarding the material 323 discussed in the manuscript.  Table 1 Characteristics of the data structure, summary of statistics and significance of the source of variation for post-weaning live body measurements and yearling weight in Nellore sheep Parity of dam ** NS ** ** NS ** * NS NS NS 1) SBL, SHW and SHG -body length, height at withers and heart girth at six months; NBL, NHW and NHG -body length, height at withers and heart girth at nine months; YBL, YHW and YHG body length, height at withers and heart girth at twelve months; YWyearling weight 2) Indicates the significance of the source of variation. * p < 0.05, ** p < 0.01. NS -non-significant (P > 0.05). A c c e p t e d A r t i c l e ơa 2 direct additive genetic variance,ơm 2 maternal additive genetic variance, ơam additive direct-maternal genetic covariance, ơpe 2 maternal permanent environmental variance, ơe 2 environmental variance, ơp 2 phenotypic variance, h 2 heritability, m 2 maternal heritability, r am additive direct-maternal genetic correlation, pe 2 = ơpe 2 /ơp 2 ; h t 2 total heritability, t m repeatability of the ewe performance, and log L log-likelihood expressed as a deviation from the model with highest likelihood; AIC Akaike's Information Criterion.
A c c e p t e d A r t i c l e