Effects of whole milk feeding rate and straw level of starter feed on performance, rumen fermentation, blood metabolites, structural growth, and feeding behavior of Holstein calves

Sixty Holstein female calves [41 ± 3.6 kg of body weight (BW)] at 4 d of age were housed individually and allocated to 1 of 6 treatments in a complete randomized design with a 2 × 3 factorial arrangement of treatments to examine effects of whole milk (WM) feeding rate and wheat straw (WS) content of starter feed (SF) on calves' performance. Thus, 6 treatments were evaluated: usually 4 L/d of WM (4 L) along with a SF without WS (CON; 4 L-CON), 4 L along with a SF containing 75 g/kg of DM WS (75S; 4 L-75S), 4 L along with a SF containing 150 g/kg of DM WS (150S; 4 L-150S), usually 8 L/d of WM (8 L) along with a CON (8 L-CON), 8 L along with a 75S (8 L-75S), and 8 L along with a 150S (8 L-150S). Calves in 8 L were gradually (in 2 steps) offered WM and both 4 L and 8 L calves were gradually (in 1 and 2 steps, respectively) weaned at 56 d of age. Calves had ad libitum access to water and SF containing ground concentrate and no or desired amount of chopped WS as total mixed ration during the trial (from 4 to 70 d of age). Overall, milk feeding rate had no effect on solid feed intake. Calves on 8 L had greater (P < 0.05) BW than calves on 4L from the third week until the end of study, however, 4L-75S calves showed similar BW with calves in 8L groups. Final body length, heart girth, and hip width were greater (P < 0.05) in 8L compared with 4L calves. Incremental increases in the WS content of SF increased (P < 0.05) post-weaning intake and average daily gain (ADG), and improved (P < 0.01) pre- and post-weaning rumen pH and total time of ruminating, all in a linear manner. Feeding 4L-75S resulted in numerical greater overall ADG (620, 519 and 553 g/d, respectively) and final BW (82.9, 77.6 and 79.8 kg, respectively) rather than 4L-CON or 4L-150S. Feeding 8L-150S increased numerically overall ADG (668, 621 and 596 g/d, respectively) and final BW (88.2, 84.4 and 82.8 kg, respectively) in calves compared with 8L-75S or 8L-CON. In conclusion, offering usually 8 L/d of WM increased growth performance of calves throughout the study, without altering SF consumption, and increases in the WS content of SF increased intake and ADG of calves after weaning.