Factors associated with the time and magnitude of the nadir body condition score in early lactation and its subsequent effects on fertility of Holstein cows

Lower BCS at dry-off, greater milk yields up to 30 DIM, and occurrence of disease were associated with lower nadir BCS, while sick and high producing cows had increased DIM at nadir. Cows in the low nadir BCS category and cows with large loss in BCS between calving and nadir had smaller odds of P/AI1. The objective of this study was to identify variables associated with the time and magnitude of nadir BCS (nBCS) during early lactation and to characterize the effects of these two measurements on the subsequent pregnancy at first artificial insemination (P/AI1) and pregnancy loss of Holstein cows. A retrospective observational study was completed using data collected from 12,042 lactations in 7,626 Holstein cows calving between April 2019 and January 2022 in a commercial dairy operation located in Colorado, USA. Scores generated by an automated BCS camera system at dry-off (BCSdry), calving (BCS1), nadir (nBCS; defined as the lowest daily BCS from calving to 100 DIM), and 21 d post nadir were selected for the analyses and subsequently categorized into quartiles (Q1 = lower nBCS). Changes in BCS were calculated from dry-off to calving (multiparous cows); from calving to nadir; and from nadir to 21 d post nadir and assigned into quartile categories considering Q1 as the 25% of cows with greater loss (pre nadir) or smaller gain (post nadir) of BCS. Data were examined using logistic regression, ANOVA, and time to event analyses. Initial univariable models were followed by multivariable models that considered parity category, calving season, BCS1, and average milk yield up to 30 DIM (M30) as covariables. Sick cows and high producing cows had increased DIM at nadir, while lower BCSdry, greater M30, and occurrence of disease were associated with lower nBCS. Cows that conceived at AI1 had their nBCS earlier (45.1 +/- 0.32 d) than cows that remained open after first AI (46.4 +/- 0.35 d). On the contrary, DIM at nBCS did not differ by pregnancy loss status. The logistic regression analyses reported smaller odds of P/AI1 in cows in the lower nBCS categories. Likewise, cows with larger loss in BCS between calving and nadir had smaller odds of P/AI1. Although cows that lost their pregnancy had lower nBCS, this association was not consistent across the analyses. The time to and magnitude of nBCS, together with the increment BCS from calving to nadir could be useful measures to evaluate the transition and early lactation status of dairy cows. Lay Summary The transition from gestation to lactation requires adjustments in nutrient consumption that are challenging for the postpartum dairy cow. As milk energy output increases more rapidly than energy intake, most cows mobilize fat and labile protein from body energy reserves, which results in reductions in body condition score during early lactation. The objective of this study was to identify variables associated with the extent of the lossin body condition score (BCS) during early lactation and to characterize the effects of the nadir BCS on the subsequent pregnancy at first artificial insemination and pregnancy loss in Holstein cows. Parity, disease presentation, and milk yield were associated with the time and magnitude of nadir BCS. Sick cows and high producing cows had increased DIM at nadir, while lower BCS at dry-off, greater milk yield, and occurrence of disease were associated with lower nadir BCS. Overall, cows with later and lower nadir BCS and greater loss of BCS between calving and nadir had poorer fertility. Monitoring nadir BCS could be a useful tool for evaluating the transition and early lactation status of dairy cows.