Document Type : Research Paper
Authors
1 Department of Animal Science, Faculty of Agriculture, University College of Agriculture and Natural Resources, University of Tehran, Karaj, Iran
2 Department of Animal Science, Faculty of Agriculture, University College of Agriculture and Natural Resources, University of Tehran, Karaj, Iran,
Abstract
The objective of this research was to estimate the relationship between length of productive life, incidence of clinical mastitis and somatic cell score with linear type traits in Holstein cows, through a meta-analysis study. In the dairy industry, type traits have indirect economic values due to genetic correlation with production, reproduction, clinical mastitis and length of productive life. A comprehensive search of the literature was conducted using the key words “genetic correlation”, “type traits”, “length of productive life”, “clinical mastitis”, “somatic cell score”, “Holstein cow” in Google Scholar search engine. Out of the 103 potentially relevant papers we found, 75 studies were excluded from the analysis, because these studies did not contain appropriate and relevant data. Finally, from the search results, we prepared three real data sets in relation to genetic correlation of linear type traits with length of productive life (6 studies), clinical mastitis (8 studies), and somatic cell score (17 studies) in Holstein cows. These studies involved the total of 263065, 4716267 and 922090 animals, corresponded to length of productive life, clinical mastitis and somatic cell score respectively. Analyses were performed using “metafor” procedure in R software package. We used random model to estimate key statistics in a meta-analysis such as effect size, 95% confidence interval and measure of heterogeneity (I^2). The obtained results showed that length of productive life showed the highest genetic correlation of - central ligament 0.19, -0.13 with chest width and 0.14 with udder depth. The genetic correlation of clinical mastitis was higher with udder depth (-0.28) and with teat length (-0.23). In addition, somatic cell score had high genetic correlation with body condition score of -0.31 and udder depth of -0.25. Test of heterogeneity (I^2 statistics) indicated that there was significant heterogeneity (variance among studies) at the highest level (more than 99%). Meta-analysis of genetic correlation between length of productive life, somatic cell score and clinical mastitis with linear type traits clarified that body condition score, chest width, udder depth and teat length could be used as indicator traits in Holstein cows breeding programs. Results suggest that udder depth as a main mammary system trait potentially might be taking into account in dairy breeding programs. Cows can produce higher amount of milk and spend higher productive period when they have achieved to ideal scores of type traits. Results of this study might indirectly be applicable for estimation of type traits’ economic values of Holstein cow’s selection indices.
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