Delayed First Milking in Unassisted Overnight Calving Did Not Affect the Quality of Colostrum but Influenced Serum Brix Refractometry in Holstein Calves at Two Days of Life.

Timely administration of good-quality colostrum represents the first farm strategy to avoid the failure of passive transfer (FPT). However, calves born during the night are likely to be fed later than recommended. Our aim was to evaluate whether night-occurring calving and delayed first milking affected colostrum quality and immune passive transfer. The dataset included 463 calvings. Four liters of colostrum were administered by an esophageal tube feeder. The mean Brix% of colostrum was 27.43%, while serum Brix% at two days of life in calves was 10.19%. According to the Generalized Linear Model, parity ≥ 4, calving months of March, April, and from September to November positively influenced the quality of colostrum. Dams carrying a male calf produced lower quality colostrum compared with those carrying a female calf (−2.78 ± 1.04 Brix%, p = 0.008); heavier female calves were associated with greater colostrum quality (0.29 ± 0.05 for each kg increase, p < 0.001). Night- or day-calving had no effect on the quality of colostrum. The only factor influencing the serum Brix% of female Holstein calves at two days of life was the day- or night-occurring birth (−0.386 ± 0.188 Brix% in calves born during the night, p = 0.04). Our results showed that calves born overnight and fed the day after had decreased serum Total Protein concentrations as indicated by reduced Brix refractometer readings, compared with calves born during the day and fed quickly after birth. However, the administration of 4 L of high-quality colostrum likely improved their serum Brix% at two days of life. Alternatively, where the prevalence of good-quality colostrum is lower, improving calving supervision and ensuring timely feeding are important to reduce the risk of FPT.