Estimation of genetic trends for body weight traits in Markhoz goat at different ages

The objective of the present study was to estimate genetic trends for body weight at different ages in Markhoz goat, including birth weight (BW, n = 4758), weaning weight (WW, n= 3685), 6-month weight (6MW, n = 3420), 9-month weight (9MW, n = 3032) and 12-month weight (12MW, n = 2697). Data and pedigree information were collected from 1992 until 2014 at the Breeding Center of Markhoz goat, Sanandaj, Iran. The GLM procedure of SAS was used for selecting the variables and identifying significant fixed effects in the equation of model. Various animal models were applied for genetic analysis and the best model was determined based on Akaike information criteria (AIC). Breeding values of animals were predicted using Wombat program. Genetic trends were obtained by regressing the average predicted breeding values on birth year for each trait. Based on the best model, direct estimated genetic trends were positive and significance for WW, 6MW, 9MW and 12 MW were 15.51, 26.28, 58.36 and 76.70 g/year, respectively (p<0.001). Maternal genetic trend for BW and WW were 0.61 and 5.47 g/year, respectively (p<0.01). The low and moderate generic trends obtained in the present study, indicated the possibility of growth traits improvements through genetic selection at all ages in Markhoz goat. Additional keywords: breeding value; growth traits; animal model. Abbreviations used: AIC (Akaike information criteria); BW (birth weight); WW (weaning weight); 6MW (6-month weight); 9MW (9-month weight); 12MW (12-month weight). Authors’ contributions: Analysis of data: ML. Both authors wrote, read and approved the final manuscript. Citation: Latifi, M.; Razmkabir, M. (2019). Short communication: Estimation of genetic trends for body weight traits in Markhoz goat at different ages. Spanish Journal of Agricultural Research, Volume 17, Issue 1, e04SC01. https://doi.org/10.5424/sjar/201917113608 Received: 19 Jun 2018. Accepted: 11 Feb 2019. Copyright © 2019 INIA. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International (CC-by 4.0) License. Funding: The authors received no specific funding for this work. Competing interests: The authors have declared that no competing interests exist. Correspondence should be addressed to Meysam Latifi: m.latifi@agri.uok.ac.ir


Introduction
The Markhoz goat is an indigenous breed originating from Kurdistan Province and west Azerbaijan province in Iran.These goats are small-sized with a wide range in brown, white and black coat color.The range of coat color characters is a unique between goats in the world.Most income for farmers is obtained from sale of kids and meat production (Rashidi et al., 2011(Rashidi et al., , 2015)).
The profitability of goat production for meat and increasing of meat production in goats depend on kid's weight as the growth performance and production of more kids per doe.Accurate prediction of breeding value of animals is one of the best tools available to maxi mize response to selection programs and the success of a breeding program can be assessed by examining the actual change in breeding value expressed as a proportion of expected theoretical change of the breeding value mean for the trait under selection (Jurado et al., 1994).The accuracy of genetic evaluations depends on how well the assumptions of the model match the data (Wiggans & VanRaden, 1991).Selection methods and management can be evaluated by estimating genetic and environmental trend.However, few researches have reported evaluation of genetic trends for body weight traits in goat (e.g., Snyman, 2012;Hassan et al., 2013;Hasan et al., 2014).
Several studies have been conducted on Markhoz goat including the estimation of genetic parameters or inbreeding effects (Rashidi et al., 2008;Kheirabadi & Rashidi, 2016;Mahmoudi et al., 2017), but there is not any information about genetic trends for body weight traits at different ages in this breed.Therefore, the objective of this study was to determine the best models of genetic analysis for body weight of Markhoz goat kids at different ages and estimation 2 (Meyer, 2013).Six different animal models were used for genetics analysis for each trait, by ignoring or including maternal additive genetic effect, permanent environmental effect and covariance between directmaternal additive genetic effects.
where y is a vector of observations of the studied traits; b, a, m, pe and e are vectors of fixed effects, direct additive genetic effects, maternal, permanent environmental effects and the residual effects, respectively.X, Z 1 , Z 2 and Z 3 are incidence matrices relating these effects to the records.A is the additive numerator relationship matrix, and σ am is the covariance between direct additive genetic and maternal additive genetic effects.Assumptions for variance (V) and covariance (Cov) matrices involving random effects were: and Cov (a,m) = Aσ am where, I d and I n are identity matrices that have order equal to the number of dams and records, respectively, and σ a , σ m , σ pe and σ e are variance of additive genetic values, maternal additive genetic varian ce, maternal permanent environmental variance and residual variance, respectively.

Model comparison criteria
Akaike information criteria (AIC) were applied to choose the best-fit model amongst all six models (Akaike, 1974).It is defined as AIC = −2 log L + 2 p, where log L is the maximized likelihood and p is the number of parameters in the model.The model with the of genetic trends in body weight traits over a 23-year period.

Data collection and management
The data used in this study were collected between 1992 and 2014 from Breeding Center of Markhoz goat in Sanandaj, Kurdistan, Iran (1373 m asl and 35 • 20ˈ N latitude and 47º E longitude).The studied traits were birth weight (BW), weaning weight (WW), 6-month weight (6MW), 9-month weight (9MW) and 12-month weight (12MW).Details on the climatic conditions and herd management are presented by Rashidi et al. (2015).Briefly, in the station, does were mated with selected bucks at about 18 months of age in breeding pens.Each buck was randomly mated to ~20 does from early October to late November.Kidding was in February and March and kids were weaned until ~4 months of age.The kids were weighed and ear tagged after birth.The number of records, mean, standard deviation, coefficient of variation, number of sires and dams for the body weight traits are presented in Table 1.

Statistical and genetics analysis
The GLM procedure (SAS, 2001) was used for identifying fixed effects that had significant influence on the investigated traits.This was performed on a model including fixed effects of year of birth (23 levels from 1992 to 2014), age of doe (6 classes from 2 to 7 years old), kid's gender (2 classes of male and female) and birth type (singles, twins and triplets).All of these fixed effects were significant (p ≤ 0.001) for all body weight traits (BW, WW, 6MW, 9MW and 12MW) and therefore these effects were considered in the model.Age of kid at weighing (in days) was considered as covariate for all analyzed traits except BW.
(Co)variance components and corresponding ge netic parameters were estimated using Wombat program smallest AIC is considered as the best model for each trait.
Predicted breeding values of individual animals were obtained with Wombat (Meyer, 2013).Genetic trends of the studied traits were obtained by regression of average predicted breeding values on birth year of animals for each trait.Genetic trend analyses were carried out with the regression procedure (Proc Reg) of SAS program (SAS, 2001).

Results and discussion
The most appropriate model for each trait and parameter estimates are given in Table 2.The results obtained in this study indicate that the most appropriate models for BW, WW, 6MW, 9MW and 12 MW were ( 6), ( 5), ( 1), ( 2) and ( 1), respectively.Therefore, these models were selected for parameter estimates and prediction of breeding value for each trait.
The average of direct and maternal predicted breeding values by year of birth during the study period  are illustrated in Fig. 1.The estimates of genetic and maternal trends (g/year) for investigated traits are reported in Table 2.As illustrated in Fig. 1, the direct genetic trend of BW between 1992 and 2014 appeared to be constant, but generally showed an increase over time for the other studied traits (WW, 6MW, 9MW and 12MW).The average of annual genetic trends were positive from 1998 onward, and had irregular variation over the years.The annual direct genetic trends were positive and significant (p<0.001) for all growth traits except BW over the years (Table 2).Fig. 1 shows that the maternal genetic trend appeared to be constant and fluctuated for BW and WW traits, respectively.The annual maternal genetic trends were positive and significant for BW (p<0.01) and WW (p<0.001) over the years (Table 2).
Estimates of direct genetic trends for BW, WW, 6MW, 9MW and 12 MW were 1. 08, 15.51, 26.28, 58.36 and 76.70 (g/year), respectively.The direct genetic trend for birth weight in the current study (1.08 g/year) was low (p>0.05)and generally agrees with that reported by Ghavi Hossein-Zadeh (2012) in Moghani sheep (1.63 g/year), Hassani et al. (2009) in Baluchi sheep (0.7 g/year) and Zishiri et al. (2010) in Ile de France sheep (1 g/year).However, higher  Also, maternal genetic trends for BW (0.61 g/year) and WW (5.47 g/year) were positive (p<0.01 and p<0.001, respectively).Results of the current study showed that maternal effects influences on preweaning traits in Markhoz goat and this effect need to be considered in selection programs.Maternal genetic trend estimates for BW and WW are rarely reported in literature.The maternal genetic trends for BW in current study were low and generally in agreement with estimates of 1 and 2.36 g/year reported by Snyman (2012) and Ghavi Hossein-Zadeh (2012), respectively.Estimate of maternal genetic trend for WW was lower than those observed by Snyman (2012) in Angora goat (9 g/year) and Ghavi Hossein-Zadeh (2012) in Moghani sheep (49.2 g/year).The estimate of maternal trends for BW and WW were low and these values were lower than direct trends.These may be due to the larger direct effects on BW and WW than maternal genetic effects in Markhoz goat.Rashidi et al. (2008) reported lower maternal heritabilities estimates than direct heritabilities for BW and WW traits in Markhoz goat.
Results indicate the positive and significant genetic trends for weight traits of Markhoz goat.The low and moderate genetic trends obtained in Markhoz goat indicated that it can be possible the improvement of growth traits through genetic selection at all ages considered.

Figure 1 .
Figure 1.Direct and maternal genetic trends of mean breeding values by year of birth observed for body weight traits of Markhoz goat.

Table 1 .
Descriptive statistics for body weight traits in Markhoz goat.

Table 2 .
The most appropriate models, parameter estimates and estimates of genetic trends (g/year) for body weight traits in Markhoz goat.