Association of body weight at first calving with growth, health, and reproductive performance during rearing and with subsequent milk production, health, and culling risk in dairy cows in the first and second lactation from a single commercial dairy farm.

The objective of this retrospective cohort study was to evaluate the association between BW at first calving and growth, health, and reproductive performance during the rearing period, as well as subsequent milk production, calving-related events, transition cow health, and culling risk in the first and second lactation in dairy cows from a single commercial dairy herd in northern Germany. The final dataset consisted of 3,891 primiparous cows that calved between 2017 and 2021, 1,894 of which were followed until their second lactation. Data were extracted from a commercial dairy herd management software system, including information from (1) the heifer rearing phase: growth, calfhood morbidity, and reproductive performance as nulliparous heifers, and from (2) first lactation and second lactation: including age at first calving, calving-related events, transition cow health, milk yield, culling risk, and growth from lactation 1 until 2. The animals were categorized into 4 quartiles based on their BW at first calving, BW quartile 1 to 4 (BW_Q1): 394 to 564 kg (n = 1,019), BW_Q2: 566 to 588 kg (n = 996), BW_Q3: 589 to 612 kg (n = 923), and BW_Q4: 614 to 768 kg (n = 953). Mixed effect models were used for continuous outcomes, multiple logistic regression for binary risk outcomes, and Cox proportional hazard models were used for time-to-event outcomes. Cows in BW_Q1 were already lighter at birth by 1.5 kg compared with cows from BW_Q4 and experienced higher preweaning morbidity (BW_Q1: 40.5% vs. BW_Q3: 34.3%) without showing a difference in failed transfer of passive immunity. Although lighter heifers in BW_Q1 had reduced preweaning growth, and, therefore, lower BW at weaning and at d 360, they showed better reproductive performance as nulliparous heifers, resulting in an 18-d reduction in age at conception and a 19-d reduction in age at first calving compared with heavier heifers in BW_Q4. In the first and second lactation, lighter cows had ∼2 kg/d lower milk production during the first 8 wk of each lactation than heavier cows. Average daily gain in lactation 1 was significantly higher in BW_Q1 (0.359 ± 0.006 kg/d) compared with BW_Q4 (0.253 ± 0.006 kg/d), suggesting some compensatory growth. Further, in lactation 1, lighter cows from BW_Q1 had ∼43% greater risk for culling within 300 d in milk, compared with the heavier cows from all other quartiles. Overall, BW at first calving was associated with subsequent performance, with lighter cows producing less milk but exhibiting greater postcalving growth, whereas reproductive performance differed during rearing but not after calving.