Abstract
The objective of this study was to assess variance components and genetic parameters for fat and protein content in Tsigai sheep using multivariate animal models in which fat and protein content in individual months of lactation were treated as different traits, and univariate models in which fat and protein content were treated as repeated measures of the same traits. Test day measurements were taken between the second and the seventh month of lactation. The fixed effects were lactation number, litter size and days in milk. The random effects were animal genetic effect and permanent environmental effect of ewe. The effect of flock-year-month of test day measurement was fitted either as a fixed (FYM) or random (fym) effect. Heritabilities for fat content were estimated between 0.06 and 0.17 (FYM fitted) and between 0.06 and 0.11 (fym fitted). Heritabilities for protein content were estimated between 0.15 and 0.23 (FYM fitted) and between 0.10 and 0.18 (fym fitted). For fat content, variance ratios of permanent environmental effect of ewe were estimated between 0.04 and 0.11 (FYM fitted) and between 0.02 and 0.06 (fym fitted). For protein content, variance ratios of permanent environmental effect of ewe were estimated between 0.13 and 0.20 (FYM fitted) and between 0.08 and 0.12 (fym fitted). The proportion of phenotypic variance explained by fym effect ranged from 0.39 to 0.43 for fat content and from 0.25 to 0.36 for protein content. Genetic correlations between individual months of lactation ranged from 0.74 to 0.99 (fat content) and from 0.64 to 0.99 (protein content). Fat content heritabilities estimated with univariate animal models roughly corresponded with heritability estimates from multivariate models: 0.13 (FYM fitted) and 0.07 (fym fitted). Protein content heritabilities estimated with univariate animal models also corresponded with heritability estimates from multivariate models: 0.18 (FYM fitted) and 0.13 (fym fitted).
Highlights
The objective of this study was to assess variance components and genetic parameters for fat and protein content in Tsigai sheep using multivariate animal models in which fat and protein content in individual months of lactation were treated as different traits, and univariate models in which fat and protein content were treated as repeated measures of the same traits
There can be found two different strategies of using of daily milk yields grouped by month of lactation (fat test day measurements of sheep milk (Oravcová, 2014): contents and protein contents, respectively) which tend to repeatability animal models in which daily milk yield is be higher between adjacent months (Ali and Schaeffer, treated as repeated measures of the same trait and analyses 1987; Komprej et al, 2011)
The estimated genetic parameters and variance ratios are discussed and compared with estimates from repeatability univariate animal models in which fat content and protein content were treated as the same effect, regardless of month of lactation
Summary
The objective of this study was to assess variance components and genetic parameters for fat and protein content in Tsigai sheep using multivariate animal models in which fat and protein content in individual months of lactation were treated as different traits, and univariate models in which fat and protein content were treated as repeated measures of the same traits. The animal model treating fat or protein content in modelling is known for daily milk yield, and individual months of lactation as a different trait has not for separate analyses of protein content (Serrano et al, 2001) been applied in Slovak sheep until now. Sci. 29:170-175 content in individual months of lactation using a multivariate animal model (each trait analysed separately) using alternative strategies of treating flock-year month effect: a fixed and a random effect. The estimated genetic parameters and variance ratios are discussed and compared with estimates from repeatability univariate animal models in which fat content and protein content were treated as the same effect, regardless of month of lactation. The overall means and standard deviations (regardless of month of lactation) are presented
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