A meta-analysis and meta-regression to compare the production performance, feed efficiency, and milk N efficiency in...

A meta-analysis and a meta-regression were conducted to compare DMI, production performance, feed efficiency (FE), and milk N efficiency (MNE; milk N yield/N intake) of Holstein and Jersey cows. Only experiments in which purebred Holsteins and Jerseys enrolled in the same study, under similar experimental conditions, were used to build the datasets (n = 30 peer-reviewed articles). The weighted raw mean difference (WMD) between breeds was computed for each comparison, and weights were assigned based on the inverse of the variance. A meta-regression was run to explore the effect of continuous (year of publication, DIM, and dietary concentrations of NDF, CP, and forage) and categorical (forage type: corn silage, alfalfa/grass-clover mix, and other forage sources ["others"]) covariates on the WMD of different variables. Jerseys had lower DMI (-4.15 kg/d), milk yield (-10.8 kg/d), yields of milk fat (-61.6 g/d) and milk true protein (-211 g/d), and ECM yield (-5.11 kg/d) than Holsteins. In contrast, concentrations of milk fat (+16.1 g/kg) and milk true protein (+6.98 g/kg), DMI (% of BW [+0.44%] and % of metabolic BW [MBW = BW^0.75; +0.64%], and ECM yield (% of BW [+0.64%] and % of MBW [+1.63%]) were greater in Jersey than Holstein cows. Whereas FE (milk yield/DMI) was lower (-0.26 kg/kg) in Jerseys than Holstein cows, breed did not affect FE (ECM yield/DMI) and MNE. Year of publication widened the WMD of milk fat and milk true protein concentrations and ECM yield (% of MBW), and it narrowed that of DMI (kg/d), milk true protein yield, and N intake, favoring Jerseys over Holsteins in recent publications. In addition, DIM narrowed the gap between breeds for the variables yields of milk, ECM yield (kg/d), milk fat, and milk N yield, and widened the gap for DMI (% BW and % of MBW) and ECM yield (% of BW and % of MBW), all in favor of Jerseys as lactation progressed. Dietary concentrations of NDF, CP, and forage collectively affected the WMD of BW, yields of milk and milk lactose, milk true protein and milk lactose contents, ECM yield (% of BW and % of MBW), FE (milk yield/DMI), and N intake, but did not consistently benefit Holsteins or Jerseys. Corn silage affected all WMD, except milk fat yield, ECM yield (% of BW and % of MBW), FE (ECM yield/DMI), and MNE. Relative to corn silage, alfalfa-grass/clover mix diets affected the WMD of DMI (kg/d and % of MBW), milk yield, milk fat and milk true protein yields, ECM yield (kg/d), N intake, and milk N yield, favoring Holsteins over Jerseys. Compared with corn silage, the forage type others affected the WMD of milk fat and milk true protein concentrations, DMI (% of BW), and ECM yield (% of BW and % of MBW), benefiting Jersey versus Holstein cows. In conclusion, the WMD of FE (ECM yield/DMI) and MNE were similar between breeds and the continuous (year of publication and DIM) and categorical (forage type) covariates largely influenced the WMD of several variables.