Current Progressive Dairy digital edition

Impact of heat stress on dairy calves

Sha Tao and Ruth M. Orellana Rivas for Progressive Dairyman Published on 04 May 2018

Calves are future producers in a dairy farm, and many studies demonstrate the animal’s health and growth in early life influence its future productivity. The impacts of both pre- and post-natal heat stress on pre-weaning calves need to be recognized because they significantly influence calves’ growth, health and future performance.

During the dry period, heat stress negatively regulates placental development and shortens the dam’s gestation length, resulting in impaired fetal growth. When compared with cows that receive evaporative cooling during the dry period, non-cooled, heat-stressed cows normally have a gestation length shorter by four days and give birth to calves about 10 pounds lighter.



The influence of dry period heat stress on calf growth during the pre-weaning period is inconsistently reported.

A previous study conducted at the University of Florida found calves born to heat-stressed dry cows had similar growth rate from birth to weaning relative to calves born to cooled dry cows. However, two recent studies conducted at the same location reported a lower pre-weaning average daily gain of calves from heat-stressed cows than those from cooled cows.

Nevertheless, calves that experience in utero heat stress during the dry period maintain a lower bodyweight at least until 1 year old compared to in utero-cooled calves.

Multiple studies reported calves born to dry period heat-stressed cows had reduced efficiency to absorb immunoglobulin G (IgG) from colostrum, resulting in lower serum IgG concentrations during the first month of life. While the reduced quality of colostrum collected during summer compared with colostrum collected during other seasons has been reported, in controlled studies, the impact of late-gestation heat stress on colostrum IgG concentration were inconsistently reported.

This suggests heat stress imposed on late-gestating cows has little effects on colostrum IgG content. In addition to the passive immunity, several lines of evidence suggest heat stress during late gestation negatively influences the innate and acquired immunity of pre-weaning calves. Therefore, prenatal heat stress during the dry period reduces the overall immune competence of calves during their postnatal life.


Taking all factors together, it is not surprising calves born to dry period heat-stressed cows have poorer performance than those from cooled cows in their postnatal life. Heifers born to heat-stressed dams have a greater chance to leave the herd before puberty for several reasons including malformation, growth retardation and health issues, compared with those born to cooled dams.

Furthermore, heifers that experienced dry period maternal heat stress produced about 11 pounds per day less milk during their first lactation. Thus, heat stress during the dry period not only impairs cow performance in the next lactation, it also reduces their offspring’s future productivity. Therefore, proper heat abatement should be implemented during the dry period.

The direct impact of heat stress on dairy calves is often overlooked. It is a common notion heat stress affects pre-weaning calves to a lesser extent compared with mature cows because of the lower total surface area and heat production per unit of surface area. However, high environmental temperature adversely impacts pre-weaning calves’ performance as well.

Similar to adult cows, calves raised in an environment with elevated ambient temperature have increased body temperature, suggesting heat accumulation. In fact, calves initiate heat dissipation through evaporative heat loss when the ambient temperature reaches 68ºF. Above this temperature, the calf will shift additional energy from growth to maintain normal body temperature.

Coupled with reduced starter intake due to heat stress, calves raised in summer have lower average daily gain and weaning bodyweight.

The immune system is compromised by heat stress as well. Studies conducted in environmental chambers suggest an impaired cellular immunity of neonatal calves under heat stress relative to thermo-neutrality. Moreover, calves born during the summer have lower serum total protein concentration compared with those raised in other seasons.


This is not only attributed to the effect of prenatal heat stress but also a direct impact of heat stress on the calf after birth. Taken together, calves raised during high ambient temperature have an increased mortality rate.

Different management practices can be applied on farms to lessen the negative impact of heat stress. Providing shade to hutches in open areas has shown to reduce hutch internal temperature and calf body temperature and respiration rate. Spacing hutches to improve air movement and maintaining clean bedding will improve calf comfort and reduce disease incidences. These management practices will eliminate additional stress to the calf during summer.

If the facility allows, cooling with fans should be considered. In fact, a study conducted in Ohio found providing fans to calves housed in a nursery barn during a summer day increased calf average daily gain and feed efficiency before weaning by 20 percent.

Nutritional strategies to improve growth during summer have not been extensively explored. Water is an essential nutrient, and providing fresh and clean water frequently has shown to improve average daily gain in calves, regardless of the season. Because calves consume more water as ambient temperature rises, and water intake encourages grain intake, it is more important to manage water intake during summer when grain consumption is depressed.

Since heat-stressed calves have lower starter intake, it is logical to hypothesize feeding more milk or milk replacer will increase the total energy intake during summer, thereby improving growth. Indeed, according to a study in Ohio, increasing the allowance of a 20:20 (fat-to-protein) milk replacer from 1 to 1.5 pounds of solid per day during summer improves body growth.

To examine whether further increases in milk replacer intake will improve growth, a recent study was conducted at the University of Georgia – Tifton’s research dairy. In this study, calves fed 1.5 and 1.75 pounds of solid per day of a 16:27 milk replacer twice daily during summer had higher bodyweight compared with those fed 1.25 pounds of solid per day of a 20:20 milk replacer. However, increasing feeding rate from 1.5 to 1.75 pounds of solid per day of the 16:27 milk replacer did not have further improvement in bodyweight and average daily gain.

These data are inconsistent with studies conducted in temperate environments where feeding large quantity of milk replacer (2 pounds of solid per day or more) improved body growth and suggests heat-stressed calves may have altered nutrient and energy utilization relative to those under temperate environments.

Increasing fat content in milk replacer is an alternative approach to increase dietary energy content. However, a recent study conducted in New York demonstrated supplementation of additional fat in milk replacer failed to improve calf body growth during summer. Clearly, more research is needed to determine the appropriate milk replacer feeding program during summer.  end mark

Ruth M. Orellana Rivas is a graduate student in the department of animal and dairy science, University of Georgia. Email Ruth M. Orellana Rivas

Sha Tao is an assistant professor with the department of animal and dairy science ath the University of Georgia – Tifton. Email Sha Tao.