Figure 1

Overview of the animal experiment. A total of 64 calves (n = 16 calves/treatment, 8 male and 8 female) were randomly assigned to 4 treatments in a 2 × 2 factorial arrangement with age at group housing (early = d 28 ± 2, EG vs. late = d 70 ± 2, LG) and age at incremental hay supplementation in calf starters from 7.5 to 15% of DM (early = d 42 ± 2 d, EH vs. late = d 77 ± 2, LH) as main factors (marked with different colors). Treatments included late group housing-late hay increment (LG-LH), late group housing-early hay increment (LG-EH), early group housing-late hay increment (EG-LH), and early group housing-early hay increment (EG-EH). Calves received 4 L/d from day 4 to 9, 6 L/d from day 10 to 20, 8 L/d from day 21 to 49, 6 L/d from day 50 to 55, and 4 L/d from day 56 to 59. Weaning was initiated by limiting milk intake to morning feedings (2 L) from day 60 to 62 of life. Calves were fully weaned at 63 days of age. All calves (female and male) were observed until they were 90 days of age, and heifers were observed until the first calving. Figure created using a web-based program BioRender (https://app.biorender.com).