Heat stress has a hefty annual cost of more than $900 million in lost production in the United States. Despite considerable financial investments in cooling technology, many dairies' milk production and reproductive success plummet during periods of heat stress.

Producers initially observe an immediate effect owing to decreased milk production, but the impact of heat stress last beyond meteorological records.

Heat stress also harms a dairy farm's future by lowering cows' peak milk production from stress during the dry period. The consequences of lower peak milk production frequently extend into late fall or early winter.

Cows may, of course, be managed and cooled to reduce the impact of heat stress; the key is to do so uniquely for each business. The approach employed will differ based on the climate's severity and the environment's relative humidity.

Whatever cooling methods are used, it is critical to reduce the cow's core body temperature during heat stress periods in order to maximize milk production and reproductive function.

Milk yield has grown due to genetic selection for milk production, as well as nutritional and managerial improvements, as well as higher metabolic heat output per cow. Lactating dairy cows are more susceptible to heat stress due to increased metabolic heat output.

The difficulty is that heat stress significantly strains the metabolism of high-producing animals. It drives modifications in multiple metabolic pathways as the animal seeks to preserve the internal environment's equilibrium in the face of external changes such as rising temperatures - all while altering its productive output.

Cows, for example, pant when stressed by heat, and panting causes respiratory alkalosis and a quick decrease in feed intake.

Furthermore, heat stress reduces energy utilization efficiency for milk production by 30 to 50%. Lactating dairy cows exposed to heat stress experience negative energy balance because feed intake falls faster than milk output.

According to several projects, nursing dairy cows who lost more than 0.5 units of body condition during the first 70 days of milking had longer postpartum intervals to first-detected estrus and ovulation.

Cows acclimatize metabolically to chronic heat stress. This creates a new set of difficulties. These changes include a shift in glucose metabolism as well as changes in insulin concentrations in the blood. Heat stress alters glucose production from the liver and the metabolism of glucose by muscle.

Increased circulating plasma urea nitrogen concentrations are another potential heat-stress-related risk to dairy cow fertility. Researchers believe that the increased plasma urea nitrogen concentrations may exacerbate the reduction in fertility that is commonly observed during periods of heat stress.

Many studies that reported the modest effects of heat stress on later fertility were published when milk yield was substantially lower than it is now. It is generally known that both heat stress and high output have an impact on reproductive performance. 

Heat stress has the following negative effects on dairy reproduction.

Estrus duration and intensity are reduced, making heat detection more challenging.

Deficiency in follicle selection

Reduced pregnancy and conception rates, which may last into the autumn months

Post-fertilization adverse effects on the developing embryo

Reduced embryonic growth until the 17th day after conception; increased embryonic loss

Take action now

While you cannot totally eliminate heat stress in your herd, there are some methods you may take to mitigate its negative effects.

1.Dry cows that are cool. Dry cows have traditionally received little heat stress protection because they are "not milking" . It is mistakenly considered that they are less susceptible to heat stress. That, however, is not the case.

Furthermore, sudden physiological, dietary, and environmental changes stress these cows. These modifications can make a cow more susceptible to heat stress and significantly impact postpartum health, milk output, and reproduction.

Heat stress during this time period can alter endocrine responses, potentially increasing fetal abortions, shortening gestation, lowering calf birthweight and postpartum milk production, and decreasing follicle and oocyte maturation associated with the postpartum reproductive cycle.

2. Make use of more than one cooling method. Dry cows cooled with feed-line sprinklers, fans, and shade provided more 60-day milk than cows cooled simply with feed-line sprinklers, with no difference in body condition score changes, prevalence of postpartum problems, or serum nonesterified fatty acid (NEFA) concentrations.

3. Identify issue areas. Investigate the pens on your dairy where cows experience heat stress using accessible techniques. Temperature monitoring devices, for example, can be used to record cows' core body temperatures.

After several days of monitoring, you can evaluate the data to identify where the dairy core body temperatures are elevated. This data can be utilized to improve cooling tactics in these areas.

4.Provide ample shade and cooling. To minimize sun radiation, a minimum of 38 to 45 square feet of shade is necessary per mature dairy cow.

Spray and fan systems should be employed on drylot dairies in arid conditions in the holding pen, over feed lanes   (particularly in freestall barns), and under shade. Include exit-lane cooling as a low-cost method of cooling cows as they depart the parlor.

It is advised that cows have access to clean water troughs as they leave the milking parlor, at two places in drylot housing, and at every crossover between feeding and resting sections in freestall housing. Also, sand bedding provides a more pleasant resting surface and improves cow comfort under heat stress.

Other adaptability options

Finally, think about changing your reproductive program to mitigate the detrimental consequences of heat stress. Repeated studies suggest that using timed-A.I. programs improves pregnancy risk under thermal stress circumstances when compared to A.I. based on expressed estrus.

Embryo transfer has also been shown to increase pregnancy rates throughout the summer by eliminating the need to detect estrus. As a result, embryo transfer has the potential to increase pregnancy rates throughout the summer. Recent advancements have enhanced embryo heat stress resistance.

These advancements include the use of genotype manipulation as well as the addition of survival factors - such as insulin-like growth factor-1, which protects cells from a variety of stressors - and may enhance pregnancy rates linked with embryo transfer even more.

More research is needed to increase cattle reproductive performance in hot conditions and the impact of heat stress during the transition period on following lactations. The prior guidelines, on the other hand, can assist you lower the impacts of heat stress on your herd.