Genotype by environment interaction was studied with 526 lactation milk records of Red Steppe dairy cows maintained at State Enterprise “Breeding reproducer “Stepove” (Mykolayiv region, Ukraine). The analyses in this study were based on the milk yields of cow per 1st–10th month (M1–M10) and per 305 day for complete lactations (Y305). We tested the hypotheses that milk performance were influenced by the sire (factor “Sire”), by number of lactation (factor “NoL”), by of cow’s year of born (factor “Generation”) and by the season of calving (factor “SoC”). The data were analysed with the “Variance components” and the “ANOVA/MANOVA” modules of statistical software STATISTICA (StatSoft Inc, USA). Experimental cows originated from five sires. The effect of the sire was significantly expressed in milk yield from the 2nd to 7th month of lactation (in all cases: P < 0.001–0.024) and Y305 (P = 0.011). The 12-year period studied (year of cow’s birth from 2001 to 2011) was classified into four periods as follows: G1 – 2001–2003, G2 – 2004–2006, G3 – 2007–2009 and G4 – 2010–2011. Year of birth (factor “Generation”) had significant (in all cases: P < 0.001–0.044) effect on all traits studied (but not on M7–M8). All cows were divided according to the season of calving (SoC): winter (December to February), spring (March to May), summer (June to August) and autumn (September to November). The production of milk for M1–M2, M4–M8 and M10 (but not for 305 day lactation) statistically differed according to the season of calving (in all cases: P < 0.05). From the study results, a significant relationship was found between the milk yield and lactation number, with the maximum milk yield occurring in the third lactation cows (pattern 1 < 2 < 3 = 4+). Milk yields from the M1 to M6 month of lactation (in all cases: P < 0.001–0.017) and Y305 (P < 0.001) were statistically different between cows according to the number of lactation. Cow’s lactation number, year of birth and calving season causes differences in the shape and persistency of lactation curve. Genotype by environment interactions for lactation number and cow’s year of birth can be result in re-ranking of sire between the different environments.

The analysis included data on the origin and milk productivity of 109 first-born red steppe breed, which were descendants of five bulls-offspring (Narcissus, Topol, Tangens, Neptune, and Orpheus) and were kept in SE “Plemproductor Stepove” (Mykolaiv region, Ukraine ) during the years 2001–2014. The purpose of this study was to analyze the fat content of milk during different months of lactation (MFP1, MFP2,…, MFP10) to determine latent variables that best describe the variability of dairy cows' productivity in this herd. High correlation estimates of fat milk scores in different lactation months have been established. According to the results of the Principal Component Analysis, based on the (co)variation matrix of fat content in milk, three new variables (PC1, PC2, and PC3) were identified, which accounted for about 82% of the variability of the original data. The First Main Component (PC1) explained 53.5%, Second (PC2) – 17.7%, and Third (PC3) – 10.6% of the variability of the original data, respectively. PC1 was highly correlated with MFP4-MFP10 and, thus, it distributed the animals according to their fat content level. PC2 was highly positively correlated with MFP8-MFP10 but highly negatively correlated with M FP1-MFP3 and thus it shows the rate of increase in fat content in milk during lactation. PC3 characterizes the variability of fat content in milk during the first and second half of lactation. The Linear Discriminant Analysis found that the MFP1-MFP2 and MFP9-MFP10 scores contributed most to the discrimination among the five subpopulations. The individual identification of the offspring groups of different bulls according to the cross-check classification ranged from 44.4% (Topol) to 87.5% (Orpheus) of cows, which were correctly assigned to their own group.