Abstract
Early-life management, including nutrition and environmental modifications, can shape calf health and performance. We examined the effects of 2 different milk replacer (MR) feeding programs with and without heat abatement in Holstein heifers during a subtropical summer. Holstein dairy heifers (n = 167, BW = 36.6 ± 0.6 kg) were enrolled at 1.7 ± 0.1 d of age (DOA). Heifers that calved on the same or adjacent days were randomly assigned to 4 treatments in a 2 × 2 factorial arrangement of 2 MR (26% CP and 20% fat) feeding programs (enhanced: ENH = 9 L/d; or control: CON = 4 L/d) and heat abatement (NC = noncooled; or CL = cooled via a positive pressure tube ventilation system with daytime misting). Heifers were housed outdoors in individual wire hutches under shade cloth blocking 80% of solar radiation until 70 DOA, after which they were managed similarly on pasture until 180 DOA. All heifers were fed equal amounts of MR 3 times daily until 42 DOA; CON heifers were then fed once daily (0500 h, 2 L) from 43 to 56 DOA, whereas ENH heifers were fed twice daily (0500 and 1300 h, 6 L) from 43 to 49 DOA and once daily (0500 h, 3 L) from 50 to 56 DOA. Milk replacer intake was recorded daily. Calf starter was provided from 7 to 70 DOA ad libitum, with daily intake recorded from 21 to 70 DOA. Respiration rate, rectal temperature, and health score were recorded 3 times weekly until 70 DOA, and disease incidences were recorded until 180 DOA. Growth parameters were assessed at enrollment and at 28, 42, 56, 70, 120 and 180 DOA. A subset of heifers (62 ± 0.8 DOA; n = 6/treatment) was killed to assess carcass and organ weights. Providing heat abatement reduced respiration rate and rectal temperature, confirming the effectiveness of the cooling system in alleviating heat load. Cooling heifers in summer led to greater DMI after weaning, and improved body growth from 43 to 70 DOA. Compared with CON, heifers fed ENH had higher respiration rates but maintained rectal temperature during the preweaning period, suggesting increased metabolic heat production. Heifers fed ENH consumed more MR but less starter before weaning and achieved greater BW gains during the preweaning period. The greater BW of heifers fed ENH persisted through 180 DOA. From enrollment to 70 DOA, CL heifers had lower pneumonia incidence and tended to have lower incidence of scours relative to NC heifers, whereas MR feeding program had no effect on disease incidences. Compared with CON, heifers fed ENH had lower stomach (with contents) and liver weights, and had or tended to have greater whole udder, thymus, spleen, and kidney weights, relative to BW. Compared with NC, CL heifers had greater thyroid but lower whole udder weights, relative to BW. In conclusion, an enhanced MR feeding program improved heifer DMI and growth before weaning, whereas heat stress abatement reduces heat load, improved postweaning DMI, and supported better health outcomes during summer. Notably, no major interactive effects between nutrition and heat abatement were observed.