Delayed weaning of Holstein bull calves fed an elevated plane of nutrition impacts feed intake, growth and potential markers of gastrointestinal development

• Delaying weaning reduces the decline in starter intakes and the rise in βHBA at weaning.
• Weaning at 12 vs. 8 weeks improved overall weight gain in calves fed an elevated plane of nutrition.
• Potentially beneficial for production and ruminal development of calves.

Feeding calves an elevated plane of nutrition pre-weaning improves growth. However, some controversy remains about the optimal time of weaning required to minimize the decline in growth commonly associated to milk withdrawal. This experiment aimed to determine the influence of weaning age on performance of calves fed milk replacer on an elevated plane of nutrition (8 L d−1, 150 g L−1, 1.2 kg calf−1 d−1) and to further understand the impacts of weaning age on gastrointestinal development. To meet these objectives, 108 Holstein bull calves (18.7 ± 2.69 d old) were blocked by BW and randomly assigned to one of three treatments: step-down weaning at 8, 10 or 12 weeks of age. Daily milk replacer supply and refusals, and weekly starter and hay intakes were recorded throughout the 10 week experiment. Average daily gain (kg d−1) and BW (kg) were measured weekly. Digestive tract development was assessed by sampling blood and fecal matter at 5, 7, 9, 11 and 13 weeks of age. Blood was analysed for βHBA as a marker of rumen development (ruminal ketogenesis) and fecal starch content was used to evaluate its total tract digestibility. Intakes of starter were higher in calves weaned at 8 weeks of age, from week 7 onwards, translating into differences in total starter consumption across the 10 week trial (P < 0.01). Weekly differences occurred in ADG (P < 0.05) during the weeks of weaning, resulting in differences in final BW and total weight gain between calves weaned at 8 and 12 weeks of age. No differences were observed in fecal starch content (P > 0.05), yet, βHBA presented smaller spikes at weaning with increasing weaning age (P < 0.01), indicating that older animals may experience a less abrupt activation of rumen function. Overall, the results suggest that delaying weaning of calves fed an elevated plane of nutrition may be beneficial to sustain growth performance and avoid stress during weaning.