Seasonal effects on preweaned calves in Southeast studied
Consistency in nutrition, feeding and management is critical for successful calf rearing, especially during inconsistent weather patterns.
By Dr. Al KertzPreweaned calves are quite susceptible to environmental conditions for several reasons. Their surface to body weight ratio is the greatest it will be in their lifetime. They have a nascent ability to regulate their body temperature. Their body fat content is low. And their housing varies considerably. Environmental effects were reviewed briefly in my book on pages 125-127 (Kertz, 2019). Within the last two years, I have reviewed four studies done with heat stress in dairy calves (Kertz, 2023a,b; 2024a,b).
Recently, a major calf study was done at the University of Georgia’s Tifton Dairy Research Center (Roper et al., 2025). “The aim of this study was to investigate the seasonal effects on growth, apparent total-tract nutrient digestibility and metabolic responses of preweaning dairy calves born and raised in either the summer or winter in Georgia,” the study report stated. “The same experimental procedure was followed during both the summer and winter months. The summer months included calves born from June through August 2018, and the winter months included calves born from November 2018 through January 2019.”
A total of 96 Holstein calves (48 per season; 23 heifers and 25 bulls in the summer, and 21 heifers and 27 bulls in the winter) were separated from their dams within six hours after birth, navel-dipped with 7% tincture of iodine and fed colostrum replacer containing at least 200 g of immunoglobulin g (IgG). The day of birth was considered one day of age (DOA). Calf management followed the protocols of the Tifton research center. All calves were housed in outdoor individual polyethylene hutches with no additional shade. Calves were bedded with sand, which was cleaned daily and replaced weekly, according to the report.
Calves were fed 1.43 lb. of dry matter (DM) per day of a milk replacer (MR) containing 26% crude protein (CP) and 17% fat twice daily (0700 and 1600 hours) from two to seven DOA. At eight DOA, calves were randomly assigned to one of the four treatments in a 2 x 2 factorial arrangement, including two feeding rates (1.43 or 1.61 lb. of DM/day of the 26:17 MR) and two feeding frequencies (2x at 0700 and 1600 hours or 3x at 0700, 1600 and 2200 hours; Orellana Rivas et al., 2022). The MR was medicated with 96 mg/kg lasalocid and provided (DM basis) 27.98% CP and 19.01% fat during summer and 26.22% CP and 18.37% fat during winter. Calves were fed MR via bottles initially and bucket trained at five DOA. When being reconstituted, MR was mixed with warm water (approximately 44°C) to 12.5% solids. From 43 to 49 DOA, the daily MR allowance was reduced by half and was only offered one time at 0700 hours for all calves. Calves were fully weaned at 49 days and kept in hutches for another two weeks.
Water and calf starter grain were offered ad libitum starting at two DOA. During summer, the calf starter (DM basis) contained 17.77% CP, 19.47% neutral detergent fiber (NDF) determined using amylase and corrected for ash (aNDFom) and 3.04 Mcal/kg of metabolizable energy (ME); during winter, the starter (DM basis) contained 18.43% CP, 18.09% aNDFom and 3.02 Mcal/kg ME. Calves remained in the experiment until 63 DOA, according to the report.
Average daily ambient temperature and relative humidity were 80.6°F ± 4.6°F and 83.4% ± 7.6%, respectively, during the summer months and 55.1°F ± 8.1°F and 81.5% ± 15.2% during the winter (Figure 1). Note: All figures were kindly provided by Dr. Shao Tao. The study findings included:
Temperature and relative humidity variability were greater in winter than during summer.
Several references were cited that temperatures over 68°F (20°C) increased calf respiration rate (NASEM, 2021; NRC, 2001).
Thus, calves raised during summer months experienced heat stress even at night at this southern Georgia calf site.
During winter months, the range of temperatures was greater than during summer – and the upper temperature during winter was often above heat stress.
Rectal temperatures and respiration rates followed a similar pattern in that each was greater in summer than winter (Figure 2).
Calves during the summer initially ate less MR, but after week 2, they ate more MR than in winter. There was no difference in starter intake during winter and summer.
The following is a summary of the results shown in Table 1:
Calves gained more in summer prior to the initiation of weaning at 42 days and tended to gain more overall, too.
Gain in heart girth was greater in winter prior to initiation of weaning at 42 days and overall, too. This was the opposite of daily gain.
Wither height gain was greater in summer than in winter at the initiation of weaning at 42 days but otherwise did not differ from winter.
There was no difference in hip height gain between winter and summer. Hip height is usually 2 in. greater than wither height.
There were no differences in body length increase between winter and summer.
Apparent nutrient digestibilities were lower in summer than winter for DM, organic matter and CP before weaning and lower for CP and fat after weaning. The authors indicated that this may have been because starter intake was negligible earlier prior to weaning and may be attributed to impaired nutrient digestion and absorption of MR in the lower gut compared to winter-fed calves.
Figure 3 shows lower plasma glucose and similar insulin levels, especially during days 14 to 49. Non-esterified fatty acids also tended to be greater during summer than winter, indicating some mobilization of limited body fat in these young calves.
Comments:
The southeastern U.S. has not only hotter but also more humid weather than any other part of the U.S. This combination was also evident in another study (Kertz, 2023a) that compared the California versus Florida weather impact on calves and heifers.
The authors also speculated that calves housed in barns (if well-ventilated) may not be subjected to as much within-day temperature variation than in hutches.
The Bottom Line
In this study, although there were continual heat stress conditions during summer, the growth and efficiency of calves raised in summer was more beneficial than in winter. In the winter, it is important that calves be kept warm, but also be aware that temperature fluctuation within a day impairs calf performance. However, the inconsistent weather pattern across days in the summer was associated with negative calf performance. Consistency in nutrition, feeding and management is critical for successful calf rearing, but these results strongly suggest that consistent weather patterns either across days or within a day are also significant for optimal calf performance.
References
Kertz, A.F. 2019. Dairy Calf and Heifer Feeding and Management — Some Key Concepts and Practices. Outskirts Press, July 31. 166 pages. https://outskirtspress.com/dairycalfandheiferfeedingandmanagement.
Kertz, A.F. 2023a. Heat stress responses in calves and heifers to the SLICK1 allele. Feedstuffs. May 18.
Kertz, A.F. 2023b. Detecting heat stress in hutch-raised dairy calves. Feedstuffs. June 1.
Kertz, A.F. 2024a. Summer active ventilation of calf hutches. Feedstuffs. May 7.
Kertz, A.F. 2024b. Ventilation or not for heat stressed pair housed dairy calves. June 26.
National Research Council (NRC). 2001. Nutrient Requirements of Dairy Cattle. 7th rev. ed. National Academies Press, Washington, D.C.
National Academies of Science Engineering Medicine (NASEM). Nutrient Requirements of Dairy Cattle: 8th rev. ed. The National Academy Press, Washington, D.C.
Orellana Rivas, R.M., T. Rodrigues, J.S.E. Sousa, V.H.L.R. Melo, J. Gao, D. Ferreira de Araújo, J. Goncalves de Souza, T.N. Marins, P. Melendez, J.K. Bernard and S. Tao. 2022. Effects of milk replacer feeding rate and frequency of preweaning dairy calves in the southeastern United States: Performance, abomasal emptying and nutrient digestibility. J. Dairy Sci. 105:1150-1169.
Roper, A.M., R.M. Orellana Rivas, J. Gao, T.N. Marins, C.G. Savegnago, T. Rodrigues, V.H.L.R. Melo, D. Ferreira de Araújo, J. Goncalves de Souza, P. Melendez, *J.K. Bernard and S. Tao. 2025. Seasonal effects on growth, digestibility and metabolism of preweaning dairy calves in the southeastern United States, J. Dairy Sci. 108:4318-4331.
View our interview with Dr. Al Kertz on this topic.
