**Background:** The rearing of replacement heifers represents a major expense for dairy farmers, with 30–35% of heifers dying or being culled before their second calving in Victoria, Australia. Accelerated preweaning nutrition has been shown to have long-term benefits on cow health and productivity through developmental programming, but the effects on immune and metabolic function are not well understood. This experiment investigated whether differences in growth, immune competence, and metabolic characteristics observed in calves fed high (8 L/day) versus low (4 L/day) volumes of milk in the preweaning phase were preserved after a period of no nutritional treatment.
**Methods:** Twenty Holstein-Friesian replacement dairy heifers previously enrolled in a preweaning experiment (Ockenden et al.) were used. At birth, calves were randomly assigned to receive either 4 L (Low) or 8 L (High) of whole milk daily from birth until weaning at 10 weeks of age. Postweaning, all heifers were managed identically under non-experimental conditions on a single property in Trafalgar, Victoria, Australia. At approximately 12 months of age, heifers underwent a repeat immune challenge using a commercially available bovine respiratory disease vaccine (Bovilis MH + IBR). Blood samples were collected on day 1 (pre-vaccination) and day 10 (post-vaccination) for analysis of immune biomarkers (white cell count and differential, IBR antibody titres) and metabolic biomarkers (beta-hydroxybutyrate, non-esterified fatty acids, glucose, insulin). Delayed-type hypersensitivity skinfold testing was performed on days 8 and 10. Bodyweight was measured monthly. Statistical analysis used linear mixed models fitted using restricted maximum likelihood, with birth bodyweight and estimated Balance Performance Index as covariates.
**Key Results:** At weaning, High treatment heifers were 19.0 kg (±2.3) heavier than Low treatment heifers (96.7 kg vs. 77.7 kg, p < 0.001). Postweaning, Low treatment heifers had slightly higher average daily gain (0.89 kg/day vs. 0.82 kg/day), and bodyweight differences between groups disappeared by 13 months of age. Prior to vaccination at 12 months, there were no significant differences in immune biomarkers between treatments. However, 10 days post-vaccination, white cell count was significantly higher in the High treatment group (10.9 vs. 8.8 × 10⁹/L, p = 0.038). Neutrophil count was approximately 30% higher in the High group (5.9 vs. 4.1 × 10⁹/L, p = 0.057), though this did not reach statistical significance. No significant differences were found in lymphocytes, monocytes, eosinophils, basophils, or IBR antibody titres post-vaccination. There were no significant differences in any metabolic biomarkers (BHB, NEFA, glucose, insulin) between treatment groups at 12 months of age. Corrected skinfold increase from delayed-type hypersensitivity testing was similar between groups (High: 6.0 mm, Low: 5.8 mm, p = 0.666). Notably, three of ten heifers in the Low treatment group failed to get in calf, whereas all ten High treatment heifers conceived.
**Clinical Implications:** These findings provide evidence that high-volume preweaning milk feeding (8 L/day) results in superior immune competence that persists until at least 12 months of age, even after 9 months of identical postweaning management. The preservation of immune differences suggests a form of immunological developmental programming from early-life nutrition. The superior immune response did not appear to come at a metabolic cost, as metabolic biomarkers did not differ between groups at 12 months. The results do not support restricted milk feeding of calves (4 L/day, the current industry standard). However, the study is limited by its small sample size (n = 10 per group), and larger studies are needed to validate these findings and determine effects on first-lactation milk yield. The observation that 30% of Low treatment heifers failed to conceive (mirroring industry culling rates) warrants further investigation into the effects of preweaning nutrition on reproductive performance.