Effect of weaning age and pace on blood metabolites, cortisol concentration, and the mRNA abundance of inflammation-related genes in gastrointestinal, adipose, and liver tissue of Holstein dairy calves.

The objective of this study was to evaluate the effects of weaning age and pace on blood metabolites, cortisol concentration, and mRNA abundance of inflammation-related genes in Holstein dairy calves. Seventy-one day-old calves [38.8 ± 4.4 kg, body weight (BW) ± sd] blocked by gender and birth BW, were randomly assigned to a 2 × 2 factorial arrangement of treatments. The first factor was weaning age [6 weeks (early) vs. Eight weeks (late)], the second factor was weaning pace [abrupt (4 step-down over 3 d, the initial milk replacer was 7.6 L, which was reduced 1.9 L in each step-down) vs. gradual (7 step-down over 14 d, the initial milk replacer was 7.6 L, which was reduced 1.09 L in each step-down)], generating early-abrupt (EA), early-gradual (EG), late-abrupt (LA), and late-gradual (LG) treatments. All treatments had 10 female and 8 male calves, except EA that had 1 fewer male calf. Milk replacer (MR; 24% CP, 17% fat) was bottle-fed, up to 1,200 g/d, twice daily (0600h and 1800h). EA and EG calves received 46.2 kg MR while LA and LG calves received a total of 63 kg MR. The study had 2 cohorts (2020, n = 40; 2021, n = 31), and each cohort included all treatments. Blood was collected from the jugular vein at 0900h on d 3 and d 7 of age, a day before starting and a day after weaning completion; male calves were humanely killed a day post-weaning. Rumen, jejunum, large intestine, liver, omental adipose and perirenal adipose tissues were sampled to determine the mRNA abundance of inflammation-related genes. Weaning pace, age, and pace × age, birth BW, and sex were included as fixed and cohort was included as random effects in the model. Blood metabolites and cortisol were analyzed as repeated measure, and sampling day, pace × sampling day, and age × sampling day were also included as additional fixed effects. Significance were noted at P ≤ 0.05 and tendencies when 0.05 <P ≤ 0.10. EA calves showed a tendency to have the greatest nonesterified fatty acid (NEFA) concentration compared with all other treatments. There was a pace × day effect on serum NEFA and β-hydroxybutyrate (BHBA); calves weaned in abrupt pace had an increased NEFA post-weaning compared with that for gradual weaning. Calves weaned in the gradual pace showed the greatest serum BHBA post-weaning. Most mRNA abundance of inflammation-related genes affected by treatments showed a similar pattern; downregulated by the abrupt (liver IL-1β) and early weaning (jejunum TNF-α and ICAM), and in some cases the interaction intensified the effect, demonstrating a weakened immune response in calves experiencing more stressful conditions (EA: IL-6 in the liver and NF-κB in the perirenal adipose tissue). Overall, the downregulation of mRNA abundance of inflammation-related genes in EA calves may be attributed to the suppression of the immune system and an immature immune response. Furthermore, the greater NEFA in EA calves could be attributed to a reduced starter intake, less developed rumen, or shorter time during the weaning transition.