Genetic relationship between milk dry matter and other milk traits in extended lactations of Polish Holstein cows

The objective of this research was to examine heritabilities and genetic, phenotypic and permanent environmental relationships between milk dry matter (DM) and milk traits such as milk, fat, protein and lactose yields, milk urea nitrogen (MUN) and somatic cell score (SCS) in extended (to 395 days) lactations of Holstein cows from a big farm in Poland. The data set consisted of 78 059 test day records from the first, second and third lactations of 3 792 cows, daughters of 210 sires and 1 677 dams. Single- or two-trait random regression models were used with fixed effects of calving year, calving month, dry period and calving interval and random additive genetic and permanent environmental effects. The last two fixed effects were not included in the analysis of first lactation data. The highest values of heritabilities for all traits, except DM, were observed in the second lactation. First lactation heritabilities for all traits - except milk yield and SCS - were smaller than those in the third lactation. Lactose yield was highly heritable, with average h2 equal to 0.25, 0.29 and 0.28 in lactations 1, 2 and 3, respectively. Heritability for DM was slightly lower than that for lactose (0.22, 0.26 and 0.28 for lactations 1, 2 and 3, respectively). In all lactations heritabilities for SCS were below 0.1. Genetic correlations between DM and milk yield (0.64-0.74) were lower than those between MUN and milk yield (0.67-0.79) as well as between lactose and milk yield (0.72-0.82). In general, DM was much more closely correlated with fat or protein yield (0.55-0.79) than with MUN or lactose (0.38-0.76). Only in the third lactation the correlation between DM and protein (0.72) was lower than between lactose and protein (0.76). For all lactations there were very high genetic correlations between DM and lactose (0.96-0.98) and high correlations between DM and MUN (0.63-0.83) and between lactose and MUN (0.70-0.85). The results suggest that further research is needed, focused on DM and its relationship with other traits in larger populations.