Phenotypic and Genetic Correlations among Milk Energy Output, Body Weight, and Feed Intake, and their Effects on Feed Efficiency In Lactating Dairy Cattle
Continued improvements in feed efficiency are essential for a thriving and sustainable dairy industry. Gross efficiency (GrEff) is defined as the energy captured in milk and body tissues as a percentage of gross energy intake. Our objective was to characterize the relationships among component traits for feed efficiency in lactating Holsteins and to determine their relationships to GrEff. Milk energy output per day (MilkE), calculated from milk, fat, protein, and lactose yields, dry matter intakes (DMI), body weights (BW), and body condition scores (BCS) were collected on 4452 lactating Holstein cows ranging from 50 to 200 days in milk from Scotland, the Netherlands, and the United States. The first 42-day records were analyzed with multivariate animal model in ASREML 3.0. Daily body energy change (dBE) was estimated from body weight change and BCS. Metabolic BW (MBW) was BW to the 0.75 power. GrEff was calculated as (MilkE + dBE) / Gross Energy intake, assuming all diets were 4.5 Mcal/kg. For these cows, MilkE was 26 ± 6 Mcal/d, BW was 600 ± 70 kg, DMI was 22 ± 5 kg/d, and intake as a multiple of maintenance was 3.9 ± 0.6. Genetic correlations for feed efficiency traits were 0.07 ± 0.04 for MilkE and MBW, 0.73 ± 0.03 for MilkE and DMI, and 0.40 ± 0.03 for MBW and DMI. Phenotypic correlations for feed efficiency traits were 0.16 ± 0.02 for MilkE and MBW, 0.60 ± 0.01 for MilkE and DMI, and 0.37 ± 0.01 for MBW and DMI. All correlations were reasonably consistent across countries. Genetic correlations of GrEff with MilkE, MBW, and DMI were 0.61 ± 0.04, -0.14 ± 0.05, and 0.04 ± 0.06, respectively. Phenotypic correlations of GrEff with MilkE, MBW, and DMI were 0.47 ± 0.01, -0.05 ± 0.02, and -0.17 ± 0.01, respectively. We conclude that, for Holsteins at a multiple of maintenance around 4, selection for milk component yield remains the overwhelming determinant of feed efficiency, and that selection for smaller body size may benefit feed efficiency but its impact will be slight compared to selection for more milk. Moreover, selection for greater milk will have a greater impact on profitability than would selection for BW. We suggest that that direct selection for body size (either larger or smaller) is likely not warranted as a means to enhance milk production or feed efficiency.
lactating dairy cow, feed efficiency, body weight