How does stage of lactation and breeding worth affect milk solids production and production efficiency of grazing dairy cows?

The study investigated the effect of stage of lactation and Breeding Worth (BW) index on estimated dry matter intake (DMI), milk solids (MS) production, energy use efficiency (EUE) and feed conversion efficiency (FCE) of grazing cows. Two hundred crossbred cows with similar calving date (14 August ± 9.97 days), live weight (471.5 ± 44.02) and age (7.5 ± 1.25 years) were separated into five groups (n = 40) based on New Zealand BW index: Low BW (BW = 63.1); Medium Low BW (BW = 88.2); Medium BW (BW = 19.1); Medium High BW (BW = 128.9); and High BW (BW = 146.9). Milk samples were collected in early, mid and late lactation and herbage samples were taken the day before milk sampling. The DMI was estimated by back-calculation based on metabolizable energy requirement for maintenance and production. The MS production, herbage DMI, EUE and FCE declined from early to late lactation. The overall results suggest regardless of the stage of lactation, cows with higher BW had a higher DMI, MS production and FCE. [ABSTRACT FROM AUTHOR]