نوع مقاله : علمی پژوهشی- ژنتیک و اصلاح دام و طیور
نویسندگان
گروه علوم دامی، دانشکده کشاورزی، دانشگاه فردوسی مشهد، مشهد، ایران.
چکیده
کلیدواژهها
عنوان مقاله [English]
نویسندگان [English]
Introduction The purpose of dairy cattle breeding is improvement of productive and fertility performance. The breeding of dairy cows was done to increase milk production, longevity, disease resistance and fertility efficiency. In most countries, importation of bulls’ semen with highly breeding values was increased. Global sales of semen expose progeny of sires to climates and production systems vastly different from their original selection environment. This may cause sire re-ranking because the progeny of some sires are not expected to perform to their optimum in every different environment. The objective of this study was to investigate 305 d milk yield and 305 d fat percentage performance of Holstein dairy cows under different climates of Iran.
Materials and Methods In this study, the records of 191910 first lactation Iranian Holstein (305 d milk yield and 305 d fat percentage) were used. These records were collected from 1368 herds during 2000 to 2011 by the Animal Breeding Center of Iran. Records from cows without pedigree information were excluded. Age at first calving was required to be between 20 and 40 month and calving interval between 300 and 600 d. According to weather conditions, geographical location of herds was classified to 5 climate groups (arid, semiarid, Mediterranean, semi humid and humid) via De Martonne method.
In this study, the models were developed based on data availability, literature evidence, genetic evaluation models that are used in other countries, and available computing facilities. Single-trait model for these traits was as follows:
where yijk denoted 305 d milk yield and 305 d fat percentage, µ was overall mean, HYSi was a combination of fixed effect of herd by year of calving by season of calving, b1 and b2 were linear regression coefficients of Holstein percentage and age at first calving, respectively, HF was the effect of Holstein percentage, Age was effect of age at first calving, aj was a random animal genetic effect and eijk was a random error term. Variance and covariance components were estimated by restricted maximum likelihood method using DMU program. Genetic and phonotypic trends were computed as a linear regression of yearly means on year of birth using the REG procedure of SAS 9.1.
Results and Discussion The average of 305 d milk yield was the highest in arid climate (7350.34±1558.29 kg) and was the lowest in humid climate (5578.66 ± 1024.2 kg). The higher 305 d milk yield in the arid climate can be due to better management conditions in these regions compared to other climates. It seems that high humidity in Mediterranean, semi humid and humid climates can intensify higher environmental temperature and thus management and control of diseases can be difficult that result in milk production depression in these climates. The estimated heritability ranged from 0.11 (Mediterranean climate) to 0.29 (arid climate) for 305 d milk yield. Logar et. al., (17) reported that additive genetic variance in environments with high milk production was higher than environments with low milk production. Thus, observed differences in additive genetic variance in the climates can be due to several factors including difference in level of herd yield, environmental variation, data size and management strategies in herds. In arid, semiarid, Mediterranean, semi humid and humid climates, genetic trends of 305 d milk yield was 22.20, 17.37, -0.074, 0.92 and -0.71, respectively. According to the results, it seems that genetic gain in herds of Mediterranean, semi humid and humid climates was very low.
Results showed that average 305 d fat percentage in Mediterranean climate was the highest and in semi humid was the lowest (3.44 ± 0.34% vs. 2.98 ± 0.56%). The estimated heritability for 305 d fat percentage was 0.11 (Mediterranean climate) to 0.29 (arid climate). It is concluded that small size of data in Mediterranean, semi humid and humid climates can lead to lower heritability. Also, differences in the estimated heritabilities in different climates can be due to difference in mean and coefficient of variations of 305 d fat percentage. Genetic trends of 305 d fat percentage was -0.0012, 1.23×10-5, 5×10-5, 0.0002 and -0.0004 for arid, semiarid, Mediterranean, semi humid and humid climates, respectively.
Conclusion According to differences in genetic parameters of traits in different climates, it seems that performance of cows was different, that can be due to genotype by environment interaction.
کلیدواژهها [English]
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