Good research demonstrates that summer heat stress reduces the performance and health of lactating and dry dairy cows. The funny thing is most people forget that a good bout of heat stress during the summer can cause many problems that extend into the cooler autumn months. Therefore, we should be aware of the ways that post-heat stress trauma lingers and take immediate corrective steps. We should also plan more preventative actions against heat stress next summer, to shrink its dark shadow.
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Heat stress in cattle starts when the air temperature exceeds about 25 C at a humidity of about 55 to 60 per cent. Because dairy cows can’t sweat like us to dissipate heat, they must rely on panting if they are going to survive. As a dairy nutritionist, I often see severely heat-stressed dairy cows breathing heavily and drooling. Many are standing in place, rather than lying down, and most are not chewing their cud. As the summer becomes hotter and/or more humid, dairy performance declines rapidly without any cooling relief.
Effects of heat stress
Good reproduction records in heat-stressed dry and lactating cows virtually melt away (pun). University research proved a long time ago that dairy cows have two to three normal follicular waves during an estrus cycle, and heat stress retards growth and impairs the function of the dominant ovarian follicle. It is also believed that heat stress decreases the fertilization rate and increases early embryonic deaths before pregnancy status is confirmed. Erratic hormone and metabolic oxidative activities are thought to be contributing factors to such reproductive dysfunction.
From a practical standpoint, I speculate that the more severe and longer the heat stress, the harder it becomes to get cows pregnant with their next calf. For example, a typical hot and humid summer in Manitoba creates many groups of “open” cows. And if this happens during the last hot days of mid-September, I have witnessed lower pregnancy rates that will follow a cow herd well into November. That’s because the comfortable cow most likely resets proper follicular development/hormone functions during her next estrus cycles under much cooler conditions. Yet it could take one or two cycles to successfully complete.
Most also don’t realize that late-August/September heat stress usually causes adverse metabolic hormone changes in both faraway and close-up dry cows as well. Such phenomena may seem unrelated to the above primary reproductive problems, but they happen at the same time. These secondary hormonal changes reduce postpartum mammary cellular function, mammary development and milk synthesis in subsequent autumn lactation.
Reduced milk production
Case-in-point: Mississippi State University examined over 300 lactation records (over six sites) and found that heat stress in the early part of the cow’s dry period (re: 60 days prepartum) had profound milk production losses during subsequent early and mid-lactation periods. Similar evidence was reported in Mexico in which dairy cows were cooled during the entire dry period. As a result, they gave birth to calves of higher birthweights and then gave over three kilos more milk per day during their early-lactation stages, compared to non-cooled counterparts.
Although it is good to know that summer heat stress affects fall-time breeding and other aspects of dairy cow productive life, we should now should ask ourselves, “What can be done to correct this annual problem?” Luckily, dairy producers can utilize a few nutrition, reproduction and management tools during autumn and in subsequent summers to overcome post heat stress trauma.
- Implement a specific faraway and close-up dry cow program. It should promote good dry matter intake and cud-chewing. This won’t reverse the past reproductive autumn failures, but prepares otherwise fertile dairy cows for next summer.
- Set up an estrous-sync breeding program. Kansas State University showed heat-stressed anestrous cows (cows that did not show visible signs of estrus) assigned to a timed scheduled breeding program had a 40 per cent improvement in pregnancy rate at 27 days after insemination.
- Improve your heat stress prevention plan. Next summer, provide more waterers and shade to both dry and lactation cows. Implement a sprinkler system in both areas. Improve heat stress management such as reducing waiting times before the cows are milked.
I discussed these suggestions with a 150-cow dairy producer who thought they would be effective against post heat stress trauma that extends into his herd every autumn. He wished that he had focused upon them earlier this year (2023) against a typical Manitoba hot summer, which dipped to cooler temperatures in July and then bounced back for a second shot of heat stress at the end of August and well into September.
