Summer heat doesn’t just make cows uncomfortable, reducing feed intake and subsequent production, it sets off a chain of biological events that affects colostrum quality, passive immune transfer, calf birth weight, and preweaning health outcomes that can persist well into first lactation. For dairy producers and veterinarians, understanding the full scope of heat stress and knowing when and where to intervene is one of the most impactful management decisions of the year when it comes to maintaining production and calf health.

Download the Cow & Calf Heat Stress Management Checklist

 

Heat Stress Impacts Calves Before They Hit The Ground 

There are 4 main routes where heat stress impacts colostrum and calf health: 

 

Most conversations about heat stress focus on lactating cows, and for good reason, but one of the most overlooked and consequential heat stress periods occurs in the dry cow pen.

These last 6 weeks of gestation are critical for calf development. The dry period coincides with the window of fastest fetal growth, when the calf accumulates approximately 60% of its birth weight. Dry cow heat stress during this period disrupts placental blood flow, reduces oxygen and nutrient delivery to the developing fetus, shortens gestation length, and impairs the mammary gland’s ability to concentrate immunoglobulins into colostrum. By the time the calf is born, the damage is already done, and it shows up in measurable ways that directly affect your bottom line and your calf health outcomes.

Literature has shown that calves born to heat-stressed dry cows weighed 2.4 kg less at birth and 6.6 kg less at weaning compared with calves born to cooled dams. This weight disadvantage persisted regardless of what happened postnatally. Additionally, heat-stressed cows have been found to produce 43% less colostrum at first milking (4.03 vs. 7.06 kg) with significantly lower IgG concentration (74.7 vs. 92.2 g/L) and reduced total protein and total solids. Most notably, these deficits were not simply explained by the reduced feed intake that typically accompanies heat stress. These were the direct physiological effects of heat on the mammary gland and on IgG transport mechanisms that were responsible for the reduced quality and quantity of colostrum.

The bottom line: a heat-stressed dry cow is producing less colostrum, of lower quality, setting calves up for reduced performance well before calving. Luckily, this can be managed to mitigate calf health issues by identifying the signs and being proactive.

 

1. Compromised Colostrum Quality 

What’s happening

IgG in colostrum originates in the cow’s bloodstream and is actively transported across the mammary barrier in the weeks before calving. Heat stress directly impairs this transport mechanism, reduces mammary epithelial cell proliferation, and compromises the integrity of the mammary barrier — resulting in lower IgG, lower protein, and lower total solids in first-milking colostrum, independent of how much the cow eats.

Shorter gestation length is associated with lower colostrum IgG concentration and an increased risk of retained placenta. Heat-stressed cows also calve earlier, by approximately six days in some studies, which increases risk and adds further postpartum complications to an already stressed animal.

What to watch for

1. First-milking colostrum Brix readings below 25% (indicating IgG below approximately 50 g/L)
2. Reduced colostrum volume at first milking, particularly in cows calving during or shortly after a heat event
3. Increased incidence of retained placenta in summer-calving cows
4. Shorter-than-expected gestation lengths

What to do

1. Brix test every colostrum sample at first milking — do not assume quality based on season or parity alone. Summer colostrum should be treated with added scrutiny.
2. Maintain a colostrum inventory of high-quality frozen colostrum (Brix ≥ 25%; 89% confidence that colostrum is at least 50 g/L) or a high-quality colostrum replacer year-round so that substandard maternal colostrum can be supplemented or replaced immediately.
3. Do not feed heat-stressed colostrum without testing. A colostrum that looks normal may have an IgG concentration well below the threshold for reliable passive transfer.
4. Cool your dry cows. Providing shade, fans, and water soakers to dry cows during the last 45–60 days of gestation is one of the highest-return interventions available. Research consistently shows this improves colostrum IgG, colostrum yield, calf birth weight, and passive transfer success.

2. Compromised Passive Transfer Even With Good Quality Colostrum

 

What’s happening

One of the most important and underappreciated findings in recent heat stress research is that calves born to heat-stressed dams absorb IgG less efficiently than calves born to cooled dams even when both groups receive the same high-quality colostrum in the same volume at the same time.

Dado-Senn et al. (2020) found that despite each calf receiving 4L of quality colostrum and identical management, prenatally heat-stressed calves had significantly lower serum IgG at 24 hours. The deficit in passive transfer was not a colostrum problem; it was a calf problem. In utero heat stress appears to compromise the intestinal epithelium’s capacity to absorb macromolecules, reducing the efficiency of IgG uptake across the gut wall during the critical window of passive transfer.

This is a critical insight for veterinarians: a calf born to a heat-stressed cow in the middle of summer is biologically predisposed to failure of passive transfer, regardless of your colostrum management protocol.

What to watch for

1. Serum total protein below 5.5 g/dL or Brix below 8.4% at 24–48 hours of age in summer-born calves, even when colostrum management appears adequate
2. Higher-than-expected rates of failure of passive transfer (FPT) during summer months
3. Calves that are slow to stand, reluctant to nurse, or require esophageal tube feeding — prenatally heat-stressed calves were tubed approximately twice as often as calves born to cooled dams in research settings
4. Increased morbidity and mortality in summer-born cohorts despite consistent colostrum protocols

What to do

1. Measure passive transfer routinely using serum total protein or Brix refractometry at 24–48 hours of age on every calf, every season — but be especially vigilant in summer-born calves.
2. Feed more IgG mass to summer-born calves. Consider increasing the volume of colostrum fed or using a supplement to boost total IgG delivery at the first feeding, with the goal of compensating for reduced gut absorption efficiency. Targeting 200g of IgG mass at the first feeding may be warranted in high-risk periods.
3. Feed within two hours of birth. The window for intestinal IgG absorption closes within 24 hours and begins declining almost immediately. In summer-born calves with already-compromised absorption, time is even more critical.
4. Use a high-quality colostrum replacer as a supplement when maternal colostrum IgG is borderline or when a calf is identified as high-risk (slow to stand, difficult birth, twin, heat exposure at birth).

3. Heat Stress in Preweaned Calves

What’s happening

Even after birth, the battle isn’t over. Calves exposed to postnatal heat stress have elevated rectal temperatures, higher respiration rates, reduced feed intake, and tend to experience more medication events for fever and infection compared with calves provided active cooling. In research settings, postnatal heat stress pushed calf rectal temperatures above the physiological range in the evenings — a point at which normal organ function begins to be compromised.

Heat-stressed preweaned calves also eat less, which matters enormously during a period when nutrient intake drives immune system development, gut maturation, and future milk production potential. Research has demonstrated that every additional kilogram of preweaning average daily gain is associated with meaningful improvements in first-lactation milk yield — meaning that heat stress-driven reductions in preweaning growth have long-term production consequences.

What to watch for

1. Elevated respiration rate (>60 breaths per minute) in the afternoon — a reliable early indicator of heat stress in calves
2. Reduced milk replacer or starter intake during hot periods
3. Increased incidence of fever (rectal temperature ≥ 40°C) and infection in summer-housed calves
4. Calves spending extended time at the water source rather than the milk feeder
5. Sluggish, depressed calves in the afternoon hours

What to do

1. Provide active airflow at calf level. Shade alone is insufficient. Dado-Senn et al. (2020) demonstrated that two fans positioned at calf level in group pens reduced body temperature by 0.5–1°C throughout the day and reduced total medication events. A target wind speed of 2 m/s at calf level is a practical goal.
2. Adjust feeding timing. Offer milk replacer and starter during the cooler parts of the day such as the early morning and evening when calves are more likely to be motivated to eat.
3. Ensure constant access to clean, cool water. Water intake increases significantly during heat stress and must be met to avoid compounding the negative effects of heat on intake and gut function.
4. Monitor health scores daily during heat events, paying particular attention to rectal temperature, hydration status, and feed intake.
5. House calves with adequate space. Crowding increases heat load and pathogen pressure simultaneously, two things that compound each other rapidly in summer conditions.

 

4. The Compounding Effect: When Everything Works Together in All the Worst Ways

What’s happening

The most important insight from recent research is not any single finding occurs in isolation. Heat stress during the dry period and heat stress during the preweaning period compound each other in meaningful ways. Calves born to heat-stressed dams that are then housed under heat stress conditions consistently show the highest rectal temperatures and respiration rates, the worst growth trajectories around weaning (including a negative average daily gain) and the highest medication burden.

In other words, a calf that was already compromised in utero and then managed through a hot summer faces cumulative biological disadvantages that a standard colostrum or nutrition protocol alone cannot overcome.

What to do

• Take a whole-system view. Managing heat stress in calves starts in the dry cow pen, not at the colostrum bucket. Producers who invest in dry cow cooling will see measurable returns in colostrum quality, passive transfer success, calf birth weight, and preweaning health, before a single management decision is made at the calf level.
• Identify your highest-risk calves early. Summer-born calves, twins, calves born from difficult deliveries, and calves slow to stand are all at elevated risk of failure of passive transfer and preweaning morbidity and that risk is compounded by heat stress. These calves warrant additional intervention at birth, including guaranteed colostrum delivery by bottle or tube within two hours and verification of passive transfer at 24–48 hours.
• Implement a post-Day 1 colostrum supplementation program for high-risk calves. Extending colostrum delivery beyond the first feeding — using colostrum powder incorporated into milk replacer or whole milk — supports gut development, immune maturation, and resilience during the days when heat-stressed calves are most vulnerable. Research supports a meaningful return on investment for these programs, with reductions in morbidity and mortality and improved daily gain.

A Practical Summer Checklist To Mitigate the Effects of Heat Stress

 

1. Dry Cow Pen

• Are dry cows provided shade, fans, AND water soakers during heat events?
• Is the temperature-humidity index (THI) being monitored? Active cooling is warranted at THI ≥ 68.
• Are gestation lengths being tracked? A pattern of shorter gestations signals heat stress impact.
• Are retained placenta rates increasing? This may indicate shortened gestation and heat-compromised placental maturation.

2. Colostrum Management

• Is every first-milking colostrum sample being tested with a Brix refractometer?
• Is an inventory of frozen high-quality colostrum (Brix ≥ 25%) or a colostrum replacer maintained year-round?
• Are calves receiving a minimum of 150–200 g of IgG within two hours of birth?
• Are summer-born calves receiving additional IgG mass to compensate for reduced gut absorption?

3. Passive Transfer Verification

• Is serum total protein or Brix being measured at 24–48 hours on every calf?
• Is the target threshold set at ≥ 5.5 g/dL total protein or ≥ 8.4% Brix?
• Are failure of passive transfer rates being tracked by season and dam cooling status?

4. Preweaned Calf Housing and Management

• Is active airflow provided at calf level (target 2 m/s wind speed)?
• Is clean, cool water available ad libitum at all times?
• Are feeding times adjusted to cooler parts of the day?
• Are health scores being recorded daily during heat events?
• Are high-risk calves (twins, dystocia, slow-to-stand, summer-born) flagged for additional monitoring and intervention?

Closing Thoughts

Heat stress is not only a seasonal inconvenience it is a biological threat to colostrum quality, passive immune transfer, calf gut function, preweaning health, and ultimately, long-term productive potential. The science is clear that its effects begin weeks before calving and extend well into the preweaning period, compounding at every step when mitigation measures are absent.

The good news is that the problems associated with heat stress can be mitigated by management. Dry cow cooling, routine Brix testing, consistent colostrum management, targeted use of colostrum replacers and supplements, and active airflow in calf housing are all practical, evidence-based interventions that pay measurable dividends in calf health, survival, and performance.

Seasonal problems require seasonal adjustments. The calves born this summer deserve the same start as the calves born in October. With the right protocols in place, that standard is achievable.

 

Download the Heat Stress Management Checklist

 

References:

1. Dado-Senn, B. et al. (2020). Pre- and postnatal heat stress abatement affects dairy calf thermoregulation and performance. J. Dairy Sci. 103:4822–4837.
2. Seyed Almoosavi, S.M.M. et al. (2021). Effects of late-gestation heat stress independent of reduced feed intake on colostrum, metabolism at calving, and milk yield in early lactation of dairy cows. J. Dairy Sci. 104. https://doi.org/10.3168/jds.2020-19115
3. Tao, S. and Dahl, G.E. (2013). Invited review: Heat stress effects during late gestation on dry cows and their calves. J. Dairy Sci. 96:4079–4093. Monteiro, A.P.A. et al. (2014). Effect of heat stress during late gestation on immune function and growth performance of calves. J. Dairy Sci. 97:6426–6439.