Effects of Heat Stress and Increased Protein Fed in Milk Replacers on the Health and Growth Parameters of Neonatal Holstein Bull Calves

Objectives of the study were to evaluate if calves fed 6 L of high protein milk replacer (HPMR; 1135 g/d, 28% crude protein (CP), 20% fat) had improved performance and health as compared to calves fed 4 L of a conventional milk replacer (CMR; 454 g/d, 20% CP, 20% fat) in heat stress and non heat stress environments. Holstein bull calves (n=52) <3 d of age were assigned to a 2 x 2 factorial trial based on initial BW, physical health score, and total serum protein levels.

One half of each nutrition group was housed indoors with temperature control, non-heat stress (NHS) environment and one half was housed outside under a shaded barn in subjecting them to a heat stress (HS) environment. The study was conducted for 56 d from June 18 to August 13, 2010. Average thermal heat index (THI) was calculated for each day using the average of 24 recorded temperatures and relative humidity (RH%). The 56 d average, low, and high range THI for the HS was 79, 67, and 86, respectively, while THI for the NH was 69, 66, and 74, respectively. Weekly measurements of body weight (BW) in kg, body length (BL), hip width (HW), wither height (WH), heart girth (HG), and hip height (HH) in cm were collected and average daily gain (ADG) was calculated. Water consumption (WC) in mL and starter intake (SI) in grams was measured daily. Feed conversion (FC) was also calculated for each nutritional treatment and environment. Fecal scores (FS) of 1 to 4 (1=hard, firm, 2=soft, firm, 3=no form, and 4=watery) were recorded daily. Calves with a FS of >3 were considered to have diarrhea and required treatment. Respiration rates (RR) were recorded at 0630 (AM) and 1830 (PM) to monitor respiratory challenges while rectal temperatures (RT) were also measured using a digital thermometer daily in AM and PM to monitor febrile events. If RT was greater than 39.2 degrees C for NHS calves and 39.7 degrees C for HS calves, they were treated for febrile events (FE). Data was analyzed using PROC MIXED (SAS 9.2). HPMR had a greater (P < 0.01) WH, HG, BL, HH, ADG, WC, and FS than the CMR (0.15 vs. 0.11, 0.37 vs. 0.28, 0.27 vs. 0.22, 0.21 vs. 0.14, 0.82 vs. 0.58, 4235 vs. 2656, and 2.05 vs. 1.73, respectively). HS had a greater (P < 0.01) WC than NHS (4365 vs. 2526, respectively). CMR had a greater SI and FC (P < 0.05) than HPMR (0.942 vs. 0.437, and 1.99 vs. 1.78, respectively). HS had a higher RT AM, RT PM, RR AM, and RR PM (P<0.01) than NHS (38.87 vs. 38.77, 39.03 vs. 38.79, 35.79 vs. 32.77, and 55.73 vs. 38.57, respectively. Calves in NHS had a higher FE (P<0.01) than the HS calves (6.24 vs. 2.33). There was no significant difference in growth parameters in HS or NHS in calves of like feeding strategies. The results show calves in HS experienced higher RT AM, RT PM, RR AM, and RR PM. The increased protein and energy fed to the HPMR calves resulted in greater FS and increased growth.