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

Milk solids per kilogram of body weight (BW) is growing in popularity as a measure of dairy cow lactation efficiency. Little is known on the extent of genetic variability that exist in this trait but also the direction and strength of genetic correlations with other performance traits. Such genetic correlations are important to know if producers are to consider actively selecting cows excelling in milk solids per kilogram of BW. The objective of the present study was to use a large data set of commercial Irish dairy cows to quantify the extent of genetic variability in milk solids per kilogram of BW and related traits but also their genetic and phenotypic inter-relationships. Mid-lactation BW and body condition score (BCS), along with 305-d milk solids yield (i.e., fat plus protein yield) were available on 12,413 lactations from 11,062 cows in 85 different commercial dairy herds. (Co)variance components were estimated using repeatability animal linear mixed models. The genetic correlation between milk solids and body weight was only 0.05, which when coupled with the observed large genetic variability in both traits, indicate massive potential to select for both traits in opposite directions. The genetic correlations between both milk solids and BW with BCS; however, need to be considered in any breeding strategy. The genetic standard deviation, heritability, and repeatability of milk solids per kilogram of BW was 0.08, 0.37, and 0.57, respectively. The genetic correlation between milk solids per kilogram of BW with 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 for milk solids per kilogram of BW is expected to result in, on average, an increase in 16.1 kg 305-d milk solids yield, a reduction of 25.6 kg of BW and a reduction of 0.05 BCS units (scale of 1-5 where 1 is emaciated). The genetic standard deviation (heritability) for 305-d milk solids yield adjusted phenotypically to a common BW was 27.3 kg (0.22). The genetic correlation between this adjusted milk solids trait with milk solids, BW, and BCS was 0.91, -0.12, and -0.26, respectively. Once also adjusted phenotypically to a common BCS, the genetic standard deviation (heritability) for milk solids adjusted 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 BW adjusted phenotypically for differences in milk solids was 35.3 kg (0.61), which reduced to 33.2 kg when also phenotypically adjusted for differences in BCS. Results suggest considerable opportunity exists to change milk solids yield independent of BW, and vice versa. The opportunity is reduced slightly once also corrected for differences in BCS. Inter-animal BCS differences should be considered if selection on such metrics is contemplated.

Milk production of Holstein-Friesian cows of divergent Economic Breeding Index evaluated under seasonal pasture-based management

The objective of this study was to validate the effect of genetic improvement using the Irish genetic merit index, the Economic Breeding Index (EBI), on total lactation performance and lactation profiles for milk yield, milk solids yield (fat plus protein; kg), and milk fat, protein, and lactose content within 3 pasture-based feeding treatments (FT) and to investigate whether an interaction exists between genetic group (GG) of Holstein-Friesian and pasture-based FT. The 2 GG were (1) extremely high EBI representative of the top 5% nationally (referred to as the elite group) and (2) representative of the national average EBI (referred to as the NA group). Cows from each GG were randomly allocated each year to 1 of 3 pasture-based FT: control, lower grass allowance, and high concentrate. The effects of GG, FT, year, parity, and the interaction between GG and FT adjusted for calving day of year on milk and milk solids (fat plus protein; kg) production across lactation were studied using mixed models. Cow was nested within GG to account for repeated cow records across years. The overall and stage of lactation-specific responses to concentrate supplementation (high concentrate vs. control) and reduced pasture allowance (lower grass allowance vs. control) were tested. Profiles of daily milk yield, milk solids yield, and milk fat, protein, and lactose content for each week of lactation for the elite and NA groups within each FT and for each parity group within the elite and NA groups were generated. Phenotypic performance was regressed against individual cow genetic potential based on predicted transmitting ability. The NA cows produced the highest milk yield. Milk fat and protein content was higher for the elite group and consequently yield of solids-corrected milk was similar, whereas yield of milk solids tended to be higher for the elite group compared with the NA group. Milk lactose content did not differ between GG. Responses to concentrate supplementation or reduced pasture allowance did not differ between GG. Milk production profiles illustrated that elite cows maintained higher production but with lower persistency than NA cows. Regression of phenotypic performance against predicted transmitting ability illustrated that performance was broadly in line with expectation. The results illustrate that the superiority of high-EBI cattle is consistent across diverse pasture-based FT. The results also highlight the success of the EBI to deliver production performance in line with the national breeding objective: lower milk volume with higher fat and protein content.