Did you know that even if your farm is located in Canada or the northern U.S., heat stress will negatively impact your farm profitability? Although many dairy herds are in a higher northern parallel, they are suffering from high temperature and humidity during summer months.
The estimated loss of revenue seen in the U.S. is about $900 million per year; it is due to a reduction of milk production, butterfat concentration and reproductive performance.
A tool was developed to identify when heat stress will negatively affect the performance of the dairy cow. The Temperature-Humidity Index (THI) is a calculation using temperature and humidity. For the same temperature, the THI will increase with humidity and vice versa.
Figure 1 has been designed with a color code, showing various levels of heat stress severities with the heat stress threshold at a THI of 68 and a severe heat stress at 90.
At THI 68, dairy producers need to start adjusting management and nutrition to minimize the impact of heat stress on their dairy herd. An example of THI 68 is a temperature of 73.4ºF with a humidity of 40 percent.
The temperature and humidity measurements should be done in the cow barn. Do not use the local weather station. The length of the heat stress will impact performance, and recovery will take weeks.
There are some visual signs showing the extent of heat stress on dairy cows. At THI 68, respiration rate will increase to 60 breaths per minute (BPM) and rectal temperature will exceed 101.3ºF.
The metabolism of the cow is adapting to this situation, dry matter intake is reduced – but at the same time, maintenance energy requirement increases, since the cow needs to dissipate heat.
Dairy cows will be in a state where there is not enough energy for milk production since it does not become a priority. This is different for cows not suffering from heat stress during the transition period.
It is well known that those dairy cows are in negative energy balance and will start to mobilize body fat to use non-esterified fatty acids (NEFA) as a source of energy.
Some new findings show that it is not the same for cows under heat stress. Their adaptive metabolism is different since it will increase the uptake of glucose by the tissue cells as a source of energy instead of oxidizing fatty acids (NEFA), which is a reaction more costly in energy.
Experiments have shown that blood NEFA will not increase when cows are under heat stress compared to control cows, even if the dry matter intake was reduced to the same extent in both groups.
Only 50 percent of the decrease in milk production during heat stress is explained by the lower dry matter intake; the other 50 percent is due to the shift of glucose uptake by tissue cells instead of mammary gland cells. This means less glucose is available for lactose production, thus reducing milk production.
A study done at the University of Arizona, with cows producing 77 pounds of milk daily, showed a loss of 4.8 pounds of milk per day when the minimum THI was on average 68, with values between 65 and 73. This suggests that cooling dairy cows should start when the minimum daily THI is 65. The maximum impact on milk production will occur 24 to 48 hours following heat stress.
Heat stress during the dry cow period will negatively impact the future milk performance of the dairy cow by compromising its mammary gland development.
A study done in Florida with dairy cows under heat stress 46 days before calving showed reduced daily milk production (16.5 pounds), reduced milk components and feed efficiency during the subsequent lactation cycle compared to the control group, even if the heat stress period was resumed after calving.
The dry matter intake was lower for the heat stress group during the dry cow period but not during the lactation period.
Reproductive performance will be negatively affected during heat stress. Heat expression is depressed due to a lower production of estradiol, and the quality of the ova is reduced, which may influence the viability of the embryo.
Early embryo development up to day 6 is impaired when the body temperature of the dairy cow increases to 102.02ºF. This explains the lower pregnancy rate and conception rate observed during summer months when cows are under heat stress. It takes 40 to 60 days following heat stress before fertility returns to normal.
It is well known that the immune system is negatively impacted around parturition; this is even worse for dairy cows under heat stress. A group of researchers from the University of Florida did show an impaired immune response of dairy cows when they were under heat stress during their entire dry cow period compared to a control group.
The use of cooling systems and a reduction of stocking density are sound management tools to reduce the extent of heat stress during summer months. Providing clean water along with some nutritional adjustment to the ration will alleviate the risk of reduced performance.
Supplementing protected B vitamins during the transition and lactation period is also beneficial to reduce the negative impact of heat stress since those essential nutrients have specific function in the energy and protein metabolism of the dairy cow, its immune system and follicular development.
A blend of protected B vitamins, specially formulated for the transition period (folic acid, riboflavin and choline), improved liver health, which is necessary for glucose production. The same trial showed a 13 percent increase in dry matter intake pre-calving, resulting in a better energy balance.
The effect of folic acid on the preparation of the dominant follicle for ovulation and its role in early embryonic development may explain why there were more cows pregnant earlier when fed this protected B vitamins blend.
Moreover, supplemented cows had less mastitis and metritis, which may be connected to the function of riboflavin in the reduction of oxidative stress.
Milk and components production, along with feed efficiency, were improved when dairy cows were fed protected blends of B vitamins (folic acid, pyridoxine, pantothenic acid and biotin) during summer months in California.
Improved energy metabolism is explained by the specific roles of those B vitamins. As an example, biotin is necessary for liver glucose production from propionate, and pantothenic acid is part of all tissue cells as a co-factor for the conversion of carbohydrate, protein and fat to energy.
The dairy cow will adapt to heat stress by modifying its metabolism to reduce her heat expenditure. Milk production and reproductive performance will be negatively impacted, resulting in a loss of profit for the dairy producer.
Supplementing a blend of protected B vitamins during the transition and the lactation period is an innovative tool to provide ammunitions to dairy cows to counteract heat stress and improve performance. PD
References omitted due to space but are available upon request. Click here to email an editor.
Download your own copy of the THI chart . (PDF, 158K)
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Hélène Leclerc
- Technical Support and R&D - Ruminant Nutrition
- Jefo
- Email Hélène Leclerc
