THE EFFECT OF INTERACTION OF SYSTEMATIC FACTORS ON MILK PERFORMANCE OF SIMMENTAL COWS IN STANDARD LACTATION

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 Dobričevo in Ćuprija (964 cows and 3237 lactations) and dairy animals housed on family farms/households in the region of Kotraža (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.


Introduction
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.The selected model results essentially in the breeding value of an individual animal (Bogdanović et al., 2003).
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).The same authors, Stojić et al., 1995, reported that farm, year and season of calving induced 35.7% of total variations in milk production.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.
Moat of domestic and foreign authors indicate significant and highly significant effect of order of lactation, i.e. groups of lactations on expression of production traits.Namely, maximum production of milk is achieved depending on the intensity of breeding in the period from the third until the fifth lactation.The lowest production of milk is in the first lactation, due to insufficient body development of animals, whereas drop in production occurring after the third i.e. fifth lactation often occurs due to health disorders.
Breeding region or farm usually have significant impact on milk performance traits due to different rearing methods, nutrition, housing, care, climatic conditions, age structure and herd size, as well as series of other influences related to work and farm management.

Material and methods
The effect of systematic, non-genetic factors and their interactions on traits of milk performance in standard lactations was analyzed on sample of 2805 Simmental cows with total 9718 standard lactations, housed on dairy farm "Zlatiborski suvati" on Zlatibor , dairy farm of the Agricultural holding Dobričevo in Ćuprija and dairy animals housed on family farms/households in the region of Kotraža.
Distribution of standard lactations according to classes of systematic influences is presented in Table 1.1989. 839 1990. 786 1991. 627 1992. 612 1993. 306 1994. 467 1995. 516 1996. 342 1997. 76 1998. 102 The impact of following systematic factors on expression of production traits in standard lactations, such as milk yield in standard lactation (MYSL, kg), milk fat content in standard lactations (MFCSL, %), milk fat yield in standard lactations (MFYSL, kg) and yield of 4 % fat corrected milk in standard lactations (4%FCMSL, kg), was observed: o Breeding area.The effect of three farm locations was studied (the farm on Mt. Zlatibor, Dobričevo farm and farms in the Kotraža region).o Lactation groups.Lactation groups were established in order to equalise the number of animals within different lactations as much as possible and reduce variability: Group I (first lactation), Group II (second lactation), Group III (third lactation), Group IV (fourth lactation), Group V (fifth lactation), Group VI (sixth and other lactations), o Breeding area x calving season interaction (3 breeding areas x 4 calving seasons).o Breeding area x lactation group interaction (3 breeding areas x 6 lactation groups).o Age at first conception.
For analysis of the mentioned non-genetic factors and their interactions on production traits in standard lactations, a general linear model was used based on application of the method of least squares of the statistical software Statistica for Windows Release 6.0, which in this case is: ) + e ijkl , where: y ijkl -an individual animal of the i-th breeding area, j-th lactation group, k-th group, l-th calving season, μ -population mean with equal participation of all classes of effects (B, L, G, Cs, BCs, BL), B i -fixed effect of the i-th breeding area (1-3), L j -fixed effect of the j-th lactation group (1-6), G k -fixed effect of the k-th group (year of birth x season of birth) (1-68), Cs l -fixed effect of the l-th calving season (1-4), BCs il -fixed effect of the i-th breeding area x l-th calving season interaction (1-12), BL ij -fixed effect of the i-th breeding area x j-th lactation group interaction (1-18), b 1 -linear regression coefficient of the effect of age at first conception, and e ijkl -other undetermined effects.

Results and discussion
In research results only the results of the effect of interaction between systematic factors on expression of milk performance traits in standard lactations according to applied model are presented.
Mean values of least squares, standard errors and significance of the influence of interaction farm and calving season on production traits of standard lactations according to applied model are presented in Table 2.The effect of interaction between the farm and calving season on all milk traits of standard lactations was highly significant (P<0.001),therefore, inclusion not only of these individual systematic factors but also their interactions into models for evaluation of breeding value of cows is justified.The effect farmsyear-season, according to research by Stojić (1996), was the most dominant effect for all studied milk performance indicators in standard lactation (P<0.01).Petrović M. M. et al., 1997 andPetrović D.M. et al. 2009, in their study of the active population of Simmental cattle in Serbia, indicated that interaction between breeding region and calving season contributed to significant variation in the milk yield and yield of milk fat (P<0.01).Very significant (P<0.01) and significant (P<0.05)effect of breeding region, as individual systematic factor, on production traits of Simmental cows is indicated by Petrović et al., 1997and 2006, Perišić, 1998, Rychen, 1999, Chladek and Kucera, 2000, Đurđević, 2001, Đurđević et al., 2002, Panić, 2005and Petrović D.M. et al., 2009.Share of interaction between farm and calving season in total variance of milk traits in standard lactations was very low and ranged from only 0.18% in case of milk fat yield to 0.49% in case of production of 4% fat corrected milk, which clearly indicated the high contribution of other systematic factors in variability of these traits which are included into presented model and were not considered in this study.Low level of share of variance of certain systematic factors in total variability of production of 4% fat corrected milk is stated in the research of Petrović D.M. et al., 2010.Results of the analysis of the effect of interaction between breeding region and groups of lactation on production traits of standard lactations, i.e. means of least squares (LSM), standard errors of means (SE LSM ) and significance of observed effects are presented in Table 3. Significance of the effect of interaction between breeding region and group of lactations on all milk traits of standard lactations was highly significant (P<0.001),which justified its inclusion into evaluation of breeding value of dairy animals, as confirmed by research of M. M. et al., (1997) and Petrović D.M. et al. (2009).
Share of variance of interaction between breeding region and group of lactations in total variance of production traits of whole lactations was slightly higher and ranged from 0.25% in milk fat content to 4.12% in milk fat yield in standard lactations.However, the share of variance in total variability unambiguously indicates even more systematic factors and their interactions which influence total variability of observed milk traits of standard lactations, as confirmed by Petrović M.M. et al., (1997) andPetrović D.M. et al. (2009 and2010).

Conclusion
Based on presented results the following can be concluded: • The significance of the effect of interaction between farm and calving season, as well as interaction between farm and group of lactations on all milk traits of standard lactations was very high (P<0.001),which justifies their inclusion into models for evaluation of breeding value of dairy cows.
• The share of variance of interactions in total variance of milk traits of standard lactations was low, which clearly indicated presence of more systematic factors and their interactions which have influence on total variability of observed milk traits of standard lactations.
Calving season, i.e. the onset of lactation (I-spring season (March through May), IIsummer season (June through August), III-autumn season (September through November), IV-winter season (December through February)).oYear of birth x season of birth interaction (cows that calved from 1982 to 1998 x 4 seasons (I-spring, II-summer, III-autumn and IV-winter seasons)).