Contribution of genetic variability to phenotypic differences in on-farm efficiency metrics of dairy cows based on body weight and milk solids yield

摘要

Milk solids per kilogram of body weight (BW) isgrowing in popularity as a measure of dairy cow lactationefficiency. Little is known on the extent of geneticvariability that exist in this trait but also the directionand strength of genetic correlations with other performancetraits. Such genetic correlations are importantto know if producers are to consider actively selectingcows excelling in milk solids per kilogram of BW. Theobjective of the present study was to use a large data setof commercial Irish dairy cows to quantify the extentof genetic variability in milk solids per kilogram of BWand related traits but also their genetic and phenotypicinter-relationships. Mid-lactation BW and body conditionscore (BCS), along with 305-d milk solids yield(i.e., fat plus protein yield) were available on 12,413lactations from 11,062 cows in 85 different commercialdairy herds. (Co)variance components were estimatedusing repeatability animal linear mixed models. The geneticcorrelation between milk solids and body weightwas only 0.05, which when coupled with the observedlarge genetic variability in both traits, indicate massivepotential to select for both traits in opposite directions.The genetic correlations between both milk solids andBW with BCS; however, need to be considered in anybreeding strategy. The genetic standard deviation, heritability,and repeatability of milk solids per kilogram ofBW was 0.08, 0.37, and 0.57, respectively. The geneticcorrelation between milk solids per kilogram of BWwith milk solids, BW, and BCS was 0.62, −0.75, and−0.41, respectively. Therefore, based on genetic regression,each increase of 0.10 units in genetic merit formilk solids per kilogram of BW is expected to resultin, on average, an increase in 16.1 kg 305-d milk solidsyield, a reduction of 25.6 kg of BW and a reduction of0.05 BCS units (scale of 1–5 where 1 is emaciated). Thegenetic standard deviation (heritability) for 305-d milksolids yield adjusted phenotypically to a common BWwas 27.3 kg (0.22). The genetic correlation between thisadjusted milk solids trait with milk solids, BW, andBCS was 0.91, −0.12, and −0.26, respectively. Oncealso adjusted phenotypically to a common BCS, thegenetic standard deviation (heritability) for milk solidsadjusted phenotypically to a common BW was 26.8 kg(0.22) where the genetic correlation with milk solids,BW and BCS was 0.91, −0.21, and −0.07, respectively.The genetic standard deviation (heritability) of BWadjusted phenotypically for differences in milk solidswas 35.3 kg (0.61), which reduced to 33.2 kg when alsophenotypically adjusted for differences in BCS. Resultssuggest considerable opportunity exists to change milksolids yield independent of BW, and vice versa. Theopportunity is reduced slightly once also correctedfor differences in BCS. Inter-animal BCS differencesshould be considered if selection on such metrics iscontemplated.