What Happened to Your Herd Last Summer?

4/28/2021

If there was a way to review your dairy’s results last summer in the areas of pregnancy rates, milk production, feed intakes and lying time, then make proactive changes to your heat abatement program before the dog days of summer arrive, would you do it? The good news is that this very action is possible. By assessing your herd’s pregnancy rates, milk production and feed intake changes, and lying time, for example, you get a good feel for whether your herd is experiencing heat stress. And, if so, about how much.

The first data point to look at is overall milk production. Heat stress can have a rapid impact on a herd, even when a mild elevation in temperature humidity index (THI) is experienced. THI is the measure of air temperature and relative humidity and should be used as a tool for measuring conditions of heat stress. Even if mild heat stress is encountered, overall milk production can fall significantly. For example, if last summer an animal in your herd spent more than 35% of her time at a THI greater than 65, milk production losses can reach roughly 2.9 pounds of milk, per cow, per day. That means, over the course of 90 days, if your herd is experiencing even moderate levels of heat stress, you could be looking at production declines of nearly 270 pounds of milk per cow during that period. Similarly, under those same conditions, reproduction issues begin to take hold.

Cows become susceptible to embryonic loss – the death of an embryo at any stage of its development – when their body temperature rises above 102.2ºF. While reproduction challenges don’t provide the same kind of immediate red flag for heat stress as milk production, if you find the calf barn becoming overpopulated outside of target calving seasons, that is a telltale sign heat stress is playing a negative role in your herd’s pregnancy rates. A good rule of thumb is that if there is a negative reproduction trend across the entire herd, there’s a very good chance it is related to heat stress during peak summer months.

That rule applies to dry cows, too. If milk production performance declines are observed, that indicates that after a cow’s dry period, she was likely exposed to heat stress. Since dry cows also have a weakened immune system after calving, they can also more easily develop mastitis, metritis, pneumonia and other diseases if under heat-stressed conditions. Finally, even calves born to heat-stressed cows are shown to have lower growth rates, poorer reproduction performance and less milk production in their first lactation compared to calves born to dry cows with proper cooling.

Lying time, lameness and overall wellness

In an optimal environment – one with a ventilation system implemented based on science and engineering, plus the presence of appropriate stall design – a cow will spend 50%-60% of her day lying down making her best milk. That time spent laying is first an indicator of proper cow comfort; it is also critical for milk production, as when a cow lies down, the blood flow to the udder increases, and it also prevents animal health issues such as lameness. If you are seeing a high incidence in lameness in the fall, your cows have been in heat stress over summer. These are all obvious indicators that summer cooling strategies need to be revised. When a cow is heat stressed, she will spend more time standing up in order to cool herself and have air circulating around her body, which cannot necessarily be achieved when lying down. While this helps cows reduce their heat stress, it does put more stress on their hooves.

When cows stand up, it puts pressure on the sole of their hoof. Continued pressure from standing due to heat stress reduces blood flow to the corium, decreases the thickness of the digital cushion and causes more wear on the sole of the hoof. Less blood flow to the corium decreases the rate at which new hoof horn is produced. Typically, it will take three months for new horn growth to reach the outside of the hoof. Given the increase in wear on the sole of the hoof from longer standing times, and the decrease in new growth, there will often be a lack of necessary new hoof horn, resulting in thin soles or sole ulcers. These painful conditions cause clinically lame cattle. Due to the time it takes to wear down hoof horn, and the rate at which new horn is produced, these negative effects of summer heat stress and lameness manifest themselves in the fall and can also have a prolonged impact on milk production. Among many things, milk production requires adequate feed and nutrition. A lame and uncomfortable cow will minimize walking to the feedbunk and will not want to stand long to eat. Additionally, as her dry matter intake goes down, so too will her milk production. Lameness also increases your animal health costs due to treatment, also increasing the risk of premature culling due to low production or lack of mobility, which warrants a cow’s removal from the herd.

Instead, when done properly, heat abatement allows cows to keep cool while lying down and mitigates the risk of heat stress-related lameness. Fresh, fast-moving air delivered where cows are lying down is essential to your cows’ and your herd’s health. When cows aren’t heat stressed, they stay in the herd longer, are comfortable all year round and produce consistently, irrespective of the season; a win-win for both cows and producers.

So, you’ve detected a problem, now what?

If any of the aforementioned production and health trends are being seen among your herd, there is still time before peak heat stress season to take action. In the battle against heat stress, it’s imperative that dairies provide animals with environments that are conducive to cow comfort and wellbeing. Creating such an environment can be distilled down to three main factors: air velocity, effective cooling and ambient temperature drop. The presence of appropriate fresh-air ventilation, a cooling strategy with high-pressure fogging and/or soaking (climate dependent), and lighting systems that can be programmed and adjusted to help cows perform at their best throughout pregnancy, development, calving, lactating and dry phases are essential.

Air quality is essential to the health and wellbeing of dairy cows. Research shows that the highest-performing cows in regard to average milk production actually produce more body heat compared to the “average” cow and are therefore more susceptible to heat stress. For example, a cow that produces 119 pounds of milk per day produces 152,000 BTUs of body heat per day, whereas a cow that produces 40 pounds of milk daily only produces 79,000 BTUs of body heat.

Specific areas that are most important for controlling core body temperature is in the milking parlor and the holding pen; the holding pen in particular, as it has the highest concentration of animals at any given time. Appropriately designed ventilation systems pull fresh air from outside the facility and direct it onto the animals, reducing core body temperature, while also removing hot humid air and gasses. The fan system, along with high-pressure fogging or soaking, will help remove heat being produced – especially from higher-producing cows. Additionally, the economics of targeting the parlor and holding pen is highly capital-effective, as the cost is amortized over all lactating cows, which enter the parlor two or three times daily.

When designing an animal-centered ventilation system for your operation, the first goal should be to return every cow to her basal core body temperature every 24 hours. Basal core body temperature is the lowest measure of an animal’s typical temperature – 102.2ºF. Another goal of an animal-centered ventilation system should be to minimize the level and duration of core body temperature increases during peak times, especially during time spent in the milking parlor and holding pen. Finally, mastitis risk is reduced by fast-moving air, as it removes moisture from bedding. By having the right animal-centered environment design, the air not only cools animals and provides fresh air, it also dries beds and helps reduce insects – promoting increased lying time.

For assistance in addressing your dairy’s heat abatement strategy, work with a veterinarian and connect with a reputable dairy industry environmental solutions provider who will take you through the process of developing a system that is scientifically best equipped to fit the infrastructure and exact needs of your herd.

Tags: Heat Stress