Replacing concentrates with a high-quality hay in the starter feed of dairy calves: II. Effects on the development of chewing and gut fermentation, and selected systemic health variables.

Early development of the rumen, rumination, and fermentation is highly important in dairy calves. Yet, common rearing practices with feeding of concentrate-rich starters may jeopardize them because of lacking physically effective fiber (peNDF). The main objective of this study was to establish the influence of the composition of the calf starter feed (only forage with 2 different qualities or concentrate-rich starter diet) on chewing behavior, rumen development, rumen and hindgut fermentation, and selected systemic health and stress variables of dairy calves. The experiment was carried out with 40 newborn Holstein-Friesian calves, randomly assigned to 4 different solid feed treatments: MQH = 100% medium-quality hay (9.4 MJ metabolizable energy, 149 g of crude protein, and 522 g of neutral detergent fiber/kg of dry matter); HQH = 100% high-quality hay (11.2 MJ of metabolizable energy, 210 g of crude protein, 455 g of neutral detergent fiber/kg of dry matter); MQH+C = 30% MQH + 70% starter concentrate; HQH+C = 30% HQH + 70% starter concentrate). All calves were up to 14 wk in the trial and received acidified whole milk ad libitum in the first 4 wk of life, thereafter in reduced quantity until weaning on 12 wk of age. Water and the solid feed treatments were available ad libitum throughout the trial. Chewing activity was measured in wk 4, 6, 10, and 12 using RumiWatch halters. Until wk 3, rumen fluid, feces and blood were sampled weekly, thereafter every 2 wk. Rumen mucosal thickness (RMT) was measured on the same days with rumen fluid samples. Data showed that calves fed the HQH diet consumed more peNDF and this was associated with longer rumination time (591 min/d) and more ruminating boli (709 boli/d) than calves fed concentrate-rich diets (MQH+C: 430 min/d, 518 boli/d; HQH+C: 430 min/d, 541 boli/d), whereas the MQH group was intermediate (539 min/d, 644 boli/d). Ruminal and fecal pH were higher in calves fed only hay (especially MQH) compared with calves with concentrate supplementation. In both hay-fed groups, ruminal and fecal short-chain fatty acids were shifted toward acetate, whereas only the HQH diet increased the butyrate proportion in the ruminal short-chain fatty acids profile. Ruminal ammonia concentration was at a high level only in the first 3 wk and decreased thereafter. Feeding HQH tended to increase ruminal ammonia, likely because of its high crude protein content and ruminal degradability as well as lower assimilation from rumen microbes. The RMT similarly, though nonlinearly, increased in all groups over the course of the experiment. When using RMT as an indicator of rumen development in dairy calves in the practice, our data suggest an RMT of 1.7 mm and >2 mm at wk 5 and 10 of life, respectively. Feeding did not affect the blood levels of aspartate aminotransferase, gamma glutamyl transferase, glutamate dehydrogenase, and cortisol. In conclusion, feeding high-quality hay, instead of concentrate-rich starter feeds, resulted in improved rumination and ruminal fermentation profile, without affecting ruminal pH and systemic and stress health variables.