Due to renewed interest in feeding acidified milk replacer (MR) for ad libitum consumption within North America, two 112-d trials were conducted using Holstein calves obtained from one dairy farm at 2 to 3 d of age. In trial 1, a 27% crude protein (whey protein), 17% fat MR powder was reconstituted with water to 14% solids and fed at 0.66 kg of dry matter (DM) per day for 42 d (CON) or acidified to pH 4.2 using citric acid and offered 24h/d for d 0 to 35, then switched to 0.66 kg of DM without acid per d for 7 d (A4). In trial 2, the same MR was fed as CON, ad libitum pH 5.2 MR offered from 0600 to 0800 h and 1600 to 1800 h for d 0 to 21 (R5), ad libitum pH 5.2 MR offered 24h/d for d 0 to 35 (A5), and ad libitum pH 4.2 MR offered 24h/d for d 0 to 35 (A4). After the ad libitum period, calves on R5, A5, and A4 were switched to CON until d 42. Growth measures were made until d 112 in each trial. Data from each trial were analyzed as a completely randomized design with repeated measurements over time using an autoregressive type 1 covariance structure when appropriate. In each trial, calves fed CON had lower average daily gain (ADG) from 0 to 42 d and greater ADG from 42 to 56 d than calves fed the other treatments. In both trials, ADG from 0 to 56 d and 56 to 112 d were not greater for A4 or A5 versus CON. From 56 to 112 d in trial 2, ADG was greater for R 0025 versus A5. Some calves completely rejected the MR at pH 4.2. Calves consumed more of the MR at pH 5.2 than 4.2. Calves fed A4 in trial 1 and treatments A5 and A4 in trial 2 consumed >95% of their MR between 0600 to 0800 h and 1600 to 1800 h. Time spent standing did not differ by treatment in trial 1 and increased from 312 to 332 min/d from d 6 to 56. Under the conditions of these trials, results to 112 d showed no advantage to feeding calves MR acidified to pH 4.2 or 5.2 ad libitum from 0 to 35 d with gradually weaning by 42 d compared with calves fed the control MR fed at 0.66 kg of DM/d. Additionally, when calves were fed MR ad libitum, they consumed >95% of the MR between 0600 to 0800 h and 1600 to 1800 h and standing behavior was not greatly affected.
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