A meta-analysis and a meta-regression were conducted to compare DMI, production performance, feed efficiency (FE), and milk N efficiency (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 data sets (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 impact of continuous (year of publication, DIM, 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 observed and estimated 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. Contrarily, 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 = BW0.75; +0.64%), and ECM yield (% of BW [+0.64% and % of MBW +1.63%) were greater in Jersey versus 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 milk N efficiency. 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 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. Dietary concentrations of NDF, CP, and forage collectively impacted 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 (milk yield/ECM), and MNE. In comparison with 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 across breeds and the continuous (year of publication and DIM) categorical (forage type) covariates largely influenced the WMD of several observed and estimated variables.
Almeida et al. (Sun,) studied this question.