In order to find cows well-adapted to organic production systems, the present study compared the energy expenditure, grass intake, grazing behavior and physical activity of two different Holstein strains in an organic full-time grazing system without concentrate supplementation. Twelve pairs of Swiss (HCH; 613 ± 34 kg BW) and New Zealand Holstein-Friesian (HNZ; 570 ± 54 kg BW) cows in mid lactation were investigated during a 7 d data collection period. The energy expenditure was analyzed using the 13C bicarbonate dilution technique in combination with an automatic blood sampling system during 6h per day. Although HCH and HNZ differed in bodyweight and seize, physical activity and energy expenditure only differed numerically between the two cow strains. As the grass intake and grazing behavior did not differ between strains, HCH could not use their full genetic potential for milk production. This short-time study indicates that cow strains adapt their behavior, bodyweight and production level to the environment of forage-based production systems. Consequently differences in energy expenditure become harder to detect
