Genetic evaluation of milk yield and milk composition of Saudi Aradi and Damascus goats

The aim of the present study was to evaluate the effects of various non-genetic factors on milk production traits and to estimates its' heritability and variance component in milk production trait among Saudi Aradi and Damascus goats. A total number of 213 (98 Aradi and 115 Damascus goats) was classified according to breed, season, year, and type of birth. Data were analyzed statistically. The variation in milk yield (MY) were moderate (32.80 %), while variations in fat (F %) and protein (P %) were low (fat 17.67 % and protein 19.75 %). The genetic groups had a significant (P≤0.05) effect on milk yield and highly significant on (F % and P %, (P≤0.01)). Damascus goats had higher than Aradi in milk yield, fat and protein. Milk yield and fat% during seasons were not significant differ. Milk protein% during Summer/Spring season (S2) had a significant higher (P≤0.01) than at during Winter/Autumn (S1). The highest fat% (P≤0.05) found in year 2009. Triplets had higher (P≤0.01) effect on milk yield than twins or single. However, there were no significant differences between type of births for protein% and fat%. Damascus had higher heritability estimates (0.45) than Aradi goats (0.29) for milk yield. Also, for protein%, the heritability estimates of Damascus had higher heritability estimates (0.69) than Aradi goats (0.37). But the heritability for fat % in Aradi (0.23) was similar to that in Damascus goats (0.22). In conclusions, the percentage of genetic variance component in Aradi goats is low so that the best programs for the genetic improvement in the Aradi goats could be upgraded through crossing with Damascus goats.


Introduction
Breeding programs depend on selecting does that have superior genetic to improve performances in dairy goats. The estimation of genetic and non-genetic factors affecting economically important traits is basics to select breeders for a certain traits. So, estimates the heritability of genetic and phenotypic traits are essential in developing efficient breeding schemes as well as in predicting breeding values accurately. Kendall et al. (2009) indicated that the breed affecting on level of milk production and there is variation in milk yield among different breeds and within the breed. The economic value of goat milk depends to a great extent on fat and protein contents as well as total milk yield (Montaldo and Manfredi, 2002). Several researchers have reported on the effects of breed on milk production and have also investigated milk yield traits in different breeds of goat. However milk production significantly varied in different breeds of goat and attributed it to the difference in genetic potentials of the animals (Zahraddeen et al., 2009). The exotic breeds like the Swiss breeds have much higher milk yields but lower milk composition than Indigenous goat breeds like the West African Dwarf and Red Sokoto (Park and Haenlein ., 2010). Scarcely, published estimates of genetic parameters for milk yield and content traits in goats, few multi-trait animal model-REML estimates of (co)variance matrices are available for milk yield and content traits (Analla et al., 1996;Gonçalves et al., 2002;Valencia et al., 2007). The present study was undertaken to evaluate the effects of various non-genetic factors on milk production traits (milk yield, fat % and protein %) and to estimates heritability and variance component for the same traits in Aradi and Damascus goats.

Location of study and animal management
Saudi goats (Aradi, A) was a native breed of Saudi Arabia and Syrian goats (Damascus, D) was carried out at Camel and Range Research Center (Al-Jouf province, Northern region of Saudi Arabia located at latitude of 29.97 °N and longitude of 40.21 °W and at 684 meters above sea level). All animals were raised under similar environmental, nutritional, and management conditions. The animals were housed in semishaded/open front barn and fed on a commercial concentrate and alfalfa hay. The amount of concentrate and hay were calculated according to the nutritional requirements for goats (NRC, 1981), which were dependent on animal ages and production status. Water, straw, salt and minerals supplemented in blocks were freely available to all animals.

Breeding plan of the genetic groups
This study used total number of 213 (98 Aradi and 115 Damascus goats) during the time period from 2008 to 2010 of the study. The measurement of milk production began after the 3 rd day post-partum to allow kids consuming colostrum. Milk yield was recorded by the ICAR (International Committee for Animal Recording) A4 method at 28-day intervals during the 240 days of lactation ICRPMA (1990). All goats were milked after weaning by an automatic milking system that can measure the milk yield automatically. Milk yields of the dams were measured using the alternate day system until weaning. In this system, goats were milked at 6 AM on the recording day. Then, kids were fed with dams' milk with the assistance of the stock person and suckled their dams until 6 AM the following day. Kids were isolated from their dams until the 6 PM milk recording. Sampling was started after weaning. Every 28 days, 200 ml milk samples were collected from the morning milk and immediately taken to the laboratory in an icebox. The samples were analyzed for fat by the Gerber method, and total protein was determined by the phenol titration method as described by James (1988).

Data collection for statistical analysis
Data were recorded for all animals during season 1 (winter / Autumn), season 2 (spring / summer), and years from 2008 to 2010.They were classified according to breed, season, year, and type of birth. Data were analyzed using PROC GLM procedure (SAS, 2008) for the leastsquares means and standard errors of the fixed factors. Heritability and genetic parameters were estimated with derivative-free restricted maximum likelihood (REML) procedures using the MTDFREML program of Boldman et al. (1995).

Statistical model used for data analysis
The following statistical model was used for data analysis:

Yijklm= μ + Bi +Sj+ Yk+ Tl+ eijklm
Where; Y ijklm : The record of MY, F % or P % measured on does had milked at k th year, i th breed, j th season and l th type of birth; μ: Overall mean when equal subclass numbers exists; Bi: effect of i th breed (i = 1 and 2); S j : Fixed effect of j th season (j = 1 and 2); Y k : Fixed effect of k th year (k = 1, 2and 3); T l : Fixed effect of l th type of birth (l = 1, 2, and 3); e ijklm : Random error particular to the ijklm th observation assumed to be independently and normally distributed with mean zero and variance of δ 2 e. The assumed model for heritability, genetic correlations and breeding values was: Where; y: a vector of observations, b: a vector of fixed effects with an incidence matrix X; u: a vector of random animal effects with incidence matrix Z; e: a vector of random residual effects with mean equals zero.

Results and Discussion
The obtained results articulated wide phenotypic variations in all traits of the milk yield, fat and protein %. The percentages of variation for milk yield were moderate (32.80 %) but the difference between minimum and maximum were high ranged from 54.24 to 479.00, while these variations for fat and protein % were low (fat 17.67 % and protein 19.75 %) but the difference between minimum and maximum were high ranged from 2.16 to 6.34 for fat % and from 1.46 to 5.64 for protein % (Table 1). The genetic groups had a significant (P≤0.05) effect on milk yield and highly significant on fat and protein % (P≤0.01). Damascus goats had higher than Aradi in milk yield, fat and protein ( Table 2). The importance of the various environmental effects on milk yield, fat% and protein% has been also examined in other studies. Rabasco et al. (1993) in the Spanish Verata goats found that birth type and production year were significant effects for milk yield. It was recorded that milk yield significantly affected by season of kidding, litter size and year (Valencia et al., 2002;.  and Boichard et al. (1989) for Alpine (0.29). It was higher than that reported by Ribeiro et al. (1998) for Saanen goats (0.09), Kominakis et al. (2000) for Skopelos dairy goats (0.14 ± 0.04), Torres-Vázquez et al. (2009) for Saanen goats from Mexico (0.17 ± 0.04), Pesce Delfino et al. (2011) for Maltese Goat (0.21 ± 0,04), Valencia et al. (2007) for Saanen (0.22±0.07), Rabasco et al. (1993) for Verta goats (0.24). It was lower than that reported by Kala and Prakash (1990) for Jamunapari goats (0.40), and Grossman et al. (1986) for Toggenburg goats, Saanen goats and Nubian goats (0.41, 0.44 and 0.45 respectively). The heritability estimates for milk yield of Damascus goats (0.45) are similar to the estimate of Mavrogenis et al. (1989) worked with the same breed in Cyprus goats (0.46) and Hermiz et al. (2002) for Iraqi local goats (0.46). It was higher than that found by Constantinou (1989) for Damascus Goats (0.35) and Kala and Prakash (1990) for Barbari goats (0.36). It was lower than that reported by Voutsina et al. (1990( for Alpine (0.54), Thilagor et al. (1992, Other estimates were reviewed for Pinto et al. (1993) for Anglo-Nubian (0.55), Badamana et al. (1990) for British Sannen (0.68), Banda (1992( for Malawi goat (0.68) andFaiz et al (1994) for Beetal and Nachi (0.68). The heritability estimates in our study for fat % (0.23 & 0.22) and for protein% (0.37 and 0.69) among Aradi and Damascus goats was within the range of values estimated by other authors which ranged (from 0.14 to 0.73) for different dairy goat populations (Analla et al., 1996;Andonov et al., 2007;Bömkes et al., 2004;Ilahi et al., 1998;Torres-Vázquez et al., 2009). Generally, increasing the percentage of environmental variance component indicates that the trait is highly affected by environmental conditions. The moderate percentage of genotypic variance gives hope for the possibility of improving the trait through different breeding programs. In this study, the percentage of genetic variance component in Aradi goats is low so that the best programs for the genetic improvement and upgrading in the Aradi goats could be done through crossing with Damascus goats which has superior genetic characteristics.
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