How To Prevent Heat Stress In Dairy Cows And Calves

heat stress blog

Heat stress in dairy cattle and youngstock is a severe issue affecting productivity, health, fertility, and welfare. And with temperature and humidity levels continuing to rise in the UK, it’s a problem more farmers will already be facing or likely to face in their herds. In this blog, we look at how it can affect stock, symptoms to look out for, and how to prevent it to stay a step ahead.

Cattle are sensitive to factors which impact their thermal exchange with the environment. These factors include:

  • Air temperature
  • Radiant temperature
  • Air velocity 
  • Relative humidity

Heat stress occurs when cows and calves generate and absorb more heat than they can get rid of by respiration, sweating, and air blowing by them. This leads to increased respiration rates, body temperatures, sweating, and time standing and is a significant welfare concern.

Air and radiant temperature directly influence the heat exchange ability of the animal. Air velocity increases the amount of heat transfer from the surface of the cow whilst air movement can also improve evaporation, which assists in heat loss. However, humidity can decrease the heat exchange and have debilitating effects on the cow.

heat stress blog


When does heat stress occur? 

Dairy cows are most comfortable (thermoneutral zone) when temperatures are between -15°C (Lower Critical Temperature (LCT)) to +25°C (Upper Critical Temperature (UCT)). However, humidity plays a significant part because, as the relative humidity increases, the temperature at which a dairy cow exhibits signs of heat stress falls.

Cows begin to experience heat stress at much lower temperatures than humans. This will depend on the animal’s age, production, and humidity. Generally, mild heat stress in cows starts around 22°C with 50% humidity, though high-yielding cows are more susceptible to heat stress as they eat more and generate more heat. They can begin to experience heat stress in well-ventilated sheds at air temperatures as low as 18°C.

Calves will likely start to feel the effects of heat stress at 21⁰C. Above this, they will have to use additional energy to maintain a normal body temperature of 38 – 39°C.

Using the Temperature Humidity Index (THI), it’s possible to determine whether your cows/ calves are at risk of heat stress. THI considers both temperature and humidity to estimate the level of heat stress cows will experience based on environmental conditions.

Cow Plan has developed a nifty calculator to generate real-time THI .

Table 1 highlights when cows may be heat stressed based on THI.

Heat stress level Temperature humidity index (THI) Respiration (breaths per minute) Body temperature (centigrade)
No heat stress Less than 68 40-60 38.6-39.1
Mild 68-71 60-75 39.1-39.4
Mild to moderate 72-79 75-85 39.4-40
Moderate to severe 80-90 85-100 40-40.5
Severe 90-99 100-104 Over 40.5

Signs of heat stress

When a cow’s temperature exceeds the UCT, they have two main ways to maintain their thermal balance:

  1. Increasing heat dispersion– evaporation through panting, drooling, and increased subcutaneous blood flow.
  2. Limiting heat production– by reducing activity and decreasing her appetite.

Cows that are heat stressed will have reduced production, fertility, and health issues. Research suggests mild to moderate heat stress can reduce yields by 2.6L a cow a day and moderate to severe by almost 4L a day.

Heat stress can also affect fertility and embryo development. For example, according to one study in the Journal of Dairy Science1, cows subjected to heat stress were 63% less likely to get pregnant than those not.

Early growing embryos are also likely to die within the first three days of fertilisation from prolonged periods of heat stress. Heat detection also becomes more difficult due to a cow’s or heifer’s lack of activity or movement.

Calf production can also be severely impacted by heat stress. Youngstock can experience poor growth rates from reduced feed intakes and lethargy, which can cause strain on metabolic processes in the body and increase the risk of disease/illness.

Signs of heat stress in cows and calves include:

  • High respiratory rates (normal = 30-60 bpm for calves/ 40-60 bpm for cows)
  • Dehydration
  • Reduced feed intakes
  • Open-mouthed panting
  • Increased water consumption
  • Sweaty coats
  • Higher rectal body temperatures
  • Calves lying flat out/lethargic
  • Poor growth rates in growing heifers
  • Smaller breeding size
  • Poor fertility
  • Coughing/health problems
  • Reduced milk production
  • Rumination problems/acidosis


9 steps to managing heat stress 

Heat stress can be managed by simple system changes, including:

  1. Monitoring shed temperatures daily – use a min/max thermometer and record temperature fluctuations. Recording humidity is also beneficial to calculate THI.
  2. Adjusting the diet – when feed intakes reduce due to heat stress, the nutrient concentration should be increased to maintain the equivalent nutritional level of the diet. This can be done by increasing the energy density, using high-quality forages and feeding more concentrates. However, care should be taken to balance diets properly to avoid digestive disorders such as acidosis and displaced abomasums.
  3. Changing feeding times – encourage higher intakes by feeding 60-70% of your herd’s diet between 8pm and 8am – the coolest part of the day.
  4. Provision of water –water intakes can increase by 10-20% in hot weather, so it’s essential that yards, buildings, grazing areas, and dispersal areas are well supplied with clean and fresh water. When cows are out in the field, consider adding additional troughs as their activity is reduced. Cows can consume 100L a day in hot weather2.
  5. Assessing air flow – by using smoke bombs. If the flow is inadequate, consider adjusting the ventilation by installing an open/protected ridge in the roof and increasing the number of air inlets. Mechanical interventions such as fans and/or sprinklers can also help. Airflows as low as 10km/hour can reduce respiration rates in heat-stressed animals by as much as 50%.
  6. Reduce sun exposure and improve airflow – look at opening inlets and outlets, prevent hot spots under sunlights, and consider mechanical ventilation, if necessary
  7. Handling – ensure any handling or routine work is carried out in the coolest part of the day
  8. Stocking rates – if possible, consider reducing stocking rates in the shed
  9. Increasing milk intakes – for calves, consider increasing milk by 0.5-1L per day


  1. -K. Schüller, O. Burfeind, W. Heuwieser,Effect of short- and long-term heat stress on the conception risk of dairy cows under natural service and artificial insemination breeding programs, Journal of Dairy Science, Volume 99, Issue 4, 2016, Pages 2996-3002.
  2. BCVA

TIME4TLC – Managing nematodirus in lambs

TIME4TLC – Managing nematodirus in lambs

Nematodirus is a disease caused by gastrointestinal worms which young lambs ingest through grazing, particularly on pasture grazed by young lambs the previous year. It can cause a high number of deaths within a matter of hours and poor growth rates.

This Q&A guide offers answers to common questions surrounding this disease including how to prevent it, how to quickly spot affected lambs, and how to treat them effectively.

What age are lambs most at risk of nematodirus?

Six to twelve week old lambs are most likely to be affected as they are likely to be eating enough grass that could be infected with the worm larvae to put them at risk.

Younger lambs can also be affected if they’re not receiving enough milk which causes them to start grazing earlier, for example, triplets. Those with a weakened immune system, or already presented with other challenges such as coccidiosis, are also more at risk of succumbing to the disease.

From three months-old, lambs are less likely to be affected when they ingest significant numbers of larvae.

Managing nematodirus in lambs

What time of year is nematodirus an issue?

A sudden cold snap that’s followed by a period of warm weather (10°C or higher) can trigger a mass hatch of eggs into larvae that are ingested by the lambs, causing their likelihood of exposure to significantly increase. Lambs are therefore most at risk of becoming infected between April and June due to the colder nights and warmer days that typically occur.

Although generally more of an issue in spring, unpredictable weather patterns experienced in recent years have caused nematodirus challenges to occur as early as February.

What are the signs of nematodirus?

 Knowing the symptoms of affected lambs will ensure prompt treatment can be administered quickly. These signs include:

  • Sudden onset of profuse diarrhoea
  • Dark or black faeces
  • Dull lambs
  • Lambs that stop sucking
  • Loss of body condition
  • Dehydration

How should I prevent nematodirus?

Prevention is better than cure because nematodirus strikes quickly. Faecal egg counts aren’t an effective preventative method because the damage is caused by larvae that don’t produce eggs, however, knowing the signs and taking the following steps will help to reduce the impact of nematodirus:

  • Where possible avoid grazing lambs in fields that were grazed by lambs the previous spring
  • Nematodirus risk varies year on year so keep an eye on the SCOPS nematodirus forecast to predict the likelihood of your flock being affected and for recommended treatment decisions to help reduce the risk of wormer resistance. This useful tool takes data from weather stations across the UK to provide updates on the disease risk level across the UK based on the predicted hatch-date of eggs into larvae
  • Don’t forget your on-farm or in-store specialist can help provide guidance on how to prevent nematodirus, so make sure you get in touch if required

How should I treat nematodirus?

 An effective parasite treatment strategy should be part of a flock health plan that should be put in place by a vet as a priority, and will help to make sure nematodirus is treated at the right time, using the right product.

Taking the following steps when managing nematodirus will also help to prevent wormer resistance, one of the biggest health risks to UK sheep farming at the moment[1]:

  • If just one lamb shows symptoms of nematodirus, treat every lamb within the group as quickly as possible using a white (1-BZ) drench
  • It’s extremely important to dose for the correct weight of the lamb – follow the product guidance for correct dosing measures
  • Conduct a faecal egg count seven to ten days after the drench is administered to ensure treatment is effective. Ensure all groups of lambs are tested separately because the level of infestation will vary between fields. Interpretation of the FEC is complex, so it’s best to discuss this and any next steps with your vet
  • Severely affected lambs may require further treatment which your vet will be able to advise on
  • Consult your vet if you’re unsure as to how to treat lambs

[1] Layout 1 (

TIME4RATIONING – Rationing advice for spring calving suckler cows

Rationing advice for spring calving suckler cows

It’s essential that spring calving suckler cows are fed balanced diets to maintain tight calving patterns and improve the chances of calving success, which in turn impact herd profitability.

Working out daily feed requirements will help ensure target body condition scores (BCS) are met to optimise cow health and fertility, as well as improve feed efficiency.

Read on to learn what these target BCS are and how to ensure herd energy and mineral requirements are met.

Target body condition scores

Condition score for spring calvers (QMS 2020)

Condition score for spring calvers

Ideally, suckler cows should be calving and bulling at a target BCS of 2.5 to help reduce the incidence of calving difficulties while promoting positive calving intervals. If cows are overweight at calving the assisted calving rate is more likely to increase. There is evidence to suggest that cows with a BCS of 3.5 typically have an assisted calving rate around double that of cows with a BCS of 2.5.

Aim to get pre-calving suckler cows to the correct condition score during mid-pregnancy by grouping cows with a similar BCS and feeding accordingly. Be aware that increasing the amount of energy in late pregnancy to achieve the target BCS can reduce calf vigour at birth.

Heifers and second time calvers are often the leanest cows on-farm so they, along with shy feeders, will need to be monitored more closely to make sure they reach the target pre-calving condition score.

Energy requirements

Autumn calvers require significantly more energy than spring calving beef cattle to maintain body condition while lactating throughout the colder winter months.

The energy requirements for a spring calver per day are 10% of their body weight plus 10MJ. In addition, dry matter requirements are 2% of total body weight. For example, a 600kg suckler cow would require 70MJ/energy and 12kg of dry matter per day.

It’s worth noting that genetics impact the amount of energy required to achieve target pre-calving condition scores. For example, cows put to a Charolais are likely to require more energy than those put to an Aberdeen Angus because larger breeds typically produce larger calves, meaning the cow will require more energy during pregnancy to sustain calf growth.

Generally, suckler cow energy requirements will be met through feeding forage alone and, unless cows are fed straw or have straw mixed into their diets, it’s advised to avoid feeding large amounts of concentrates.

If cows are fed a straw-based diet, aim to feed a good quality concentrate that contains 18% protein such as Lifetime Rearer or Unigold nuts. Bespoke blends can also be created by your local on-farm specialist, and also consider adding molasses to straw to encourage intakes.

Freshly calved cows will require double the energy needed by a dry cow for milk production and to promote recovery after birth. Energy is the key to ovulation and keeping a tight calving pattern, so ensuring the correct energy levels are fed during early lactation will also promote optimum fertility.

Mineral requirements

Mineral requirements

The mineral requirements of pre-and post-calving suckler cows are often overlooked, yet it is crucial the correct levels are fed to promote cow and calf health.

Consider conducting forage mineral analysis and then taking blood tests at least six weeks before calving to pinpoint any deficiencies in the following minerals:

Essential suckler cow minerals[1]

Mineral Impact Solution
Iodine Cows that are deficient in iodine in the last four to five weeks pre-calving are more likely to have weaker calves. Consider bolusing six weeks pre-calving with a high iodine bolus to be sure requirements are met. For pregnant and lactating stock this is 0.5Mg/kg DM.
Selenium A lack of selenium can cause white muscle disease in calves and retained cleansing in cows. Selenium can be administered via a bolus. For pregnant and lactating stock, the recommended level is 0.1Mg/kg DM.
Magnesium Magnesium deficiencies can increase the risk of staggers or hypomagnesaemia post-calving. Pregnant cows require 0.12% of total ration DM while lactating cows require 0.20% of total ration DM. Consider feeding magnesium rolls to achieve this.

Providing supplementary pre-and post-calving mineral blocks such as Vitalyx super suckler or Megastart pre-calver is another option to make sure suckler cow mineral requirements are met. Megastart contains Safmannan®  which will help to increase the immunoglobulins in the colostrum by around 25% which will help to improve calf health.

If you require further advice, consider discussing historical issues with cow health around calving with a local on-farm specialist.

[1] How to manage minerals in suckler cows pre-calving – Farmers Weekly (