Relationships between milk protein concentrations at different phases of first to fourth lactation and subsequent calving intervals in Holstein cows

Abstract So far, no study has been conducted on the effect of milk protein concentrations (MP%) in different time periods of lactation on subsequent calving intervals (CI). The purpose of this study was to evaluate the association between MP% at different stages of the first to fourth lactations and subsequent calving intervals in primiparous and multiparous Holstein dairy cows. Test day records (3X milking frequency) of milk yield measured on primiparous (n = 503793) and multiparous (n = 389237, 257347, 156035 and 87750 for parity 2, 3 and 4, and 5 respectively) Holstein dairy cows collected during 1984–2019 were obtained from Animal Breeding Center of Iran. To investigate the relationship between MP% at different time intervals of lactation and subsequent calving intervals (up to fifth parity), each lactation was divided into four time periods (≤50d, 50