With an annual cost of more than $900 million in lost production, heat stress takes a serious toll on U.S. dairy production. Despite large capital investments in cooling equipment, milk production and reproductive performance drastically decline on many dairies during periods of heat stress.
Producers first notice an immediate effect due to reduced milk production, but the impacts of heat stress linger beyond records maintained by meteorologists.
Heat stress also has a negative impact on a dairy farm’s future by reducing peak milk production of cows that go through the transition period during times of heat stress. The impact of reduced peak milk production often lingers into late fall or early winter.
Of course, cows can be managed and cooled to minimize the impact of heat stress; the key is to do so in a way that best fits each operation. The method used will vary depending on the severity of the climate and the ambient relative humidity.
Regardless of the cooling methods chosen, it’s essential to minimize variation in the cow’s core body temperature during periods of heat stress to maximize milk production and reproductive performance.
Metabolic impact
Genetic selection for milk production, as well as nutritional and management improvements, has contributed to increased milk yield and therefore increased metabolic heat output per cow. Increased metabolic heat output has considerably increased the susceptibility of lactating dairy cows to heat stress.
At issue is the fact that heat stress poses dramatic challenges to the metabolism of high-producing animals. It forces changes in many metabolic pathways as the animal attempts to maintain the equilibrium of its internal environment when faced with the external change of rising temperatures – all while adjusting its productive output.
For example, cows pant when heat-stressed, and panting leads to respiratory alkalosis in addition to rapidly reducing feed intake.
Plus, a 30 to 50 percent reduction in energy utilization efficiency for milk production occurs during heat stress. Lactating dairy cows subjected to heat stress go into negative energy balance as feed intake drops more quickly than milk yield.
Several studies indicate that lactating dairy cows losing greater than 0.5 units of body condition during the first 70 days in milk had longer postpartum intervals to first-detected estrus and ovulation.
When chronic heat stress occurs, cows acclimate metabolically. This leads to another set of challenges. These alterations include a shift in carbohydrate metabolism, including changes in insulin concentrations in blood. Production of glucose from the liver and metabolism of glucose by muscle are altered during heat stress.
Another potential heat-stress-related challenge to dairy cow fertility is increased circulating plasma urea nitrogen concentrations. Researchers theorize that these elevated plasma urea nitrogen concentrations may worsen the decrease in fertility frequently observed during periods of heat stress.
Implications for reproductive performance
Many studies reporting subtle effects of heat stress on subsequent fertility were published when milk yield was much less than it is today. It is well established that both heat stress and high production impact reproductive performance. Negative consequences of heat stress on dairy reproductive programs include:
- Reduced duration and intensity of estrus, increasing the difficulty of heat detection
- Impaired follicle selection
- Reduced pregnancy and conception rates, which can carry over into the fall months
- Negative effects on the early developing embryo post-fertilization
- Reduced embryonic growth up to day 17 after conception
- Increased embryonic loss
Take action
While you cannot completely remove heat stress for your herd, there are several steps you can take to reduce its detrimental impacts.
1.Cool dry cows. Dry cows traditionally receive little protection from heat stress because they are not lactating, and it is incorrectly assumed they are less prone to heat stress. But that’s not the case.
In addition, abrupt physiological, nutritional and environmental changes cause these cows additional stress. These changes can increase a cow’s susceptibility to heat stress and have a critical influence on postpartum health, milk production and reproduction.
Heat stress during this period can affect endocrine responses that may increase fetal abortions, shorten gestation, reduce calf birthweight and postpartum milk yield, and reduce follicle and oocyte maturation associated with the postpartum reproductive cycle.
2.Use more than one cooling method. It’s been shown that dry cows cooled with feed-line sprinklers, fans and shade compared with cows cooled with only feed-line sprinklers produced more 60-day milk with no difference in body condition score changes, incidence of postparturient disorders or serum nonesterified fatty acid (NEFA) concentrations.
3.Isolate problem areas. Use available tools to investigate the locations on your dairy where cows experience heat stress. For example, temperature monitoring equipment can be used to record cows’ core body temperatures.
Then you can analyze the data after several days of monitoring to see where on the dairy core body temperatures are elevated. This information can be used to improve cooling strategies in these locations.
4.Provide plenty of shade and cooling. A minimum of at least 38 to 45 square feet of shade per mature dairy cow is required to reduce solar radiation.
Spray and fan systems should be used in the holding pen, over the feeding areas (especially in some freestall barns) and under shades on drylot dairies in arid climates. Include exit-lane cooling as an inexpensive way to cool cows as they leave the parlor.
Access to clean water troughs when cows leave the milking parlor, at two locations in drylot housing and at every crossover between feeding and resting areas in freestall housing, is recommended. Also note that sand bedding provides a more comfortable resting surface and increases cow comfort during periods of heat stress.
Other adaptive options
Lastly, consider adapting your reproductive program to decrease the negative effects of heat stress. Repeated research shows that when timed-A.I. programs were used, pregnancy risk was improved under thermal stress conditions compared with A.I. based on expressed estrus.
Embryo transfer also has been reported to improve pregnancy rates during summer by removing the need to detect estrus. As a result, embryo transfer can significantly improve pregnancy rates during summer. Recent developments have improved embryo resistance to heat stress.
These developments include the use of both genotype manipulation and the addition of survival factors – such as insulin-like growth factor-1 that protects cells from a variety of stresses – and may further improve pregnancy rates associated with embryo transfer.
Additional work is warranted to improve reproductive performance of cattle in hot environments, as well as effects of heat stress during the transition period on subsequent lactations. However, the previous recommendations can help you significantly reduce the effects of heat stress on your herd. PD
This information was presented at the 2014 Dairy Cattle Reproduction Council Annual Meeting in Salt Lake City, Utah.
Todd Bilby is with Merck Animal Health dairy technical services and is also a Dairy Cattle Reproduction Council committee member.
Robert Collier
Professor of Animal Science
University of Arizona
