Abstract
The effect of interaction between the farm and calving season, as well as the farm and group of lactations on milk performance traits (milk yield, yield of milk fat, 4% fat corrected milk and milk fat content) was analyzed on sample of 2805 Simmental cows with total 9718 standard lactations, housed on dairy farm ?Zlatiborski suvati? on Zlatibor (578 cows and 1968 lactations), dairy farm of the Agricultural holding Dobricevo in Cuprija (964 cows and 3237 lactations) and dairy animals housed on family farms/households in the region of Kotraza (1263 cows and 4513 lactations). The significance of the effect of interaction between the farm and calving season, as well as interaction between the farm and group of lactations on all milk performance traits was very high (P<0.001), which justified their inclusion into models for assessment of the breeding value of dairy cows. The share of variance of interactions in total variance of milk performance traits in standard lactations was low (below and around 4%), which clearly indicated even more systematic factors and their interactions which influence the total variability of observed traits of milk performance of cows in standard lactations.
Highlights
IntroductionThe cattle breeding procedures used today most commonly employ linear methods and models that combine fixed parameters (year, farm, season, lactation) and random variables (age at first conception or calving, genetic effect of sire, genetic effect of an individual animal, etc.) which can be mutually dependent (related) or independent, with or without interactions, depending on the trait analysed
The cattle breeding procedures used today most commonly employ linear methods and models that combine fixed parameters and random variables which can be mutually dependent or independent, with or without interactions, depending on the trait analysed
A somewhat lower percentage of non-genetic factors of 22.7% in the total variability of performance traits was found by Jovanovac (1987), whereas Hansen et al, 1983 reported a considerably higher percentage of about 45%
Summary
The cattle breeding procedures used today most commonly employ linear methods and models that combine fixed parameters (year, farm, season, lactation) and random variables (age at first conception or calving, genetic effect of sire, genetic effect of an individual animal, etc.) which can be mutually dependent (related) or independent, with or without interactions, depending on the trait analysed. Depending on the level of production, sample size and mathematical statistical model, non-genetic discontinuous factors (year, farm, season, lactation) can account for as much as above 50% of the total variations in milk production (Stojić et al, 1996). A somewhat lower percentage of non-genetic factors (farm, year, calving season and lactation number) of 22.7% in the total variability of performance traits was found by Jovanovac (1987), whereas Hansen et al, 1983 reported a considerably higher percentage of about 45% (in terms of the farm-year-season effect). The effect of season of birth of cows and their calving, i.e. beginning of lactation as systematic factor, on traits of milk performance is reflected through various climatic circumstances and nutrition throughout the year, so it is included into models for evaluation of breeding value of dairy animals
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