Impact of passive immunity and quality of transition milk on preweaning dairy and dairy-beef cross calf health in a pasture-based block calving system: A cohort study

The objective of this observational cohort study was to evaluate how passive immunity, measured by serum IgG at 24 to 36 h of life, and transition milk quality (assessed via IgG intake) was associated with mortality, treatment incidence, antimicrobial use and growth in dairy calves reared within a pasture-based block calving system. This study was conducted between February and June 2024 on a 550-cow spring calving herd in Pembrokeshire, West Wales, United Kingdom. A total of 251 calves were included in the study which comprised 139 British Friesian dairy replacements, and 92 Wagyu, 19 Hereford, and 1 Simmental beef-on-dairy calf. Calves were left with their dam immediately after birth, which allowed them to potentially suckle maternal colostrum, before being separated and fed colostrum replacer. Excess colostrum from the dam (from the first milking after calving, after the calf had suckled) and subsequent transition milk from milkings 2 to 6 after calving was pooled, pasteurized, and fed to all calves from the second feeding up to 10 d of age. Every calf was blood sampled to assess the concentration of serum IgG at 24 to 36 h of age using radial immunodiffusion (RID) testing. The transition milk mixture was collected daily and tested using RID to determine the total quantity of IgG fed over the 10-d period to each calf. The incidence of mortality and morbidity was recorded using DairyComp305 (version, 23.11, release date Nov. 28, 2023;Valley Ag Software, VAS), whereas live weight gain was recorded using a weigh tape at birth and digital scales at weaning. The mortality rate from enrollment to weaning was 12.4%, with several associations identified, including the level of passive immunity, month of calving, and an interaction between the level of passive immunity and quantity of IgG fed through transition milk. Specifically, calves born in April and May had higher odds of dying in the preweaning period compared with calves born in February. In addition, for every 1 g/L increase in serum IgG measured at 24 to 36 h of age, the odds of mortality declined. Finally, lower mortality was observed in calves with lower serum IgG at 24 to 36 h when receiving higher IgG in transition milk. A total of 42 calves (16.7%) were treated during the preweaning period. A higher concentration of serum IgG at 24 to 36 h was also linked to lower treatment rates, as was month of birth, with those born in March and April having higher odds of treatment. The median defined daily dose for animals (DDDvet) was 0, with a range of 1 to 16.66. Similar to previous findings, serum IgG was associated with this outcome, where calves with serum >22.18 g/L having a lower DDDvet compared with those with a serum IgG of <9.30 g/L. Over the preweaning period, the average growth rate was 0.39 kg/d. Calves with serum IgG concentrations >22.18 g/L gained 0.08 kg/d more compared with those with a serum IgG of <9.30 g/L. Further, calves fed >470 g of IgG in transition milk gained 0.07 kg/d more than those that consumed <260 g of IgG in transition milk. However, no differences in ADG were found for calves consuming 260 to 325 g or 326 to 465 g compared with those in the <260 g group. This study demonstrated that higher levels of passive immunity were associated with reduced mortality, lower odds of disease treatment and antibiotic use, and improved growth rates, whereas increased IgG intake through transition milk was associated with reduced preweaning mortality, especially in calves with low passive immunity, and improved growth, underscoring the importance of passive immunity and transition milk feeding in pasture-based block calving systems.