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

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 (8L d, 150 g L, 1.2 kg calf d) 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) 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 (