Heat stress and its impact occurs when a cow’s ability to dissipate heat is exceeded by heat generated from metabolic heat production and heat gain from environmental sources. Humidity is also a contributing factor to heat stress conditions and the Temperature Humidity Index, commonly referred to as the THI, is the primary method of measuring heat stress. A wide range of physiologic and behavioral changes result from heat stress: reduced dry matter intake; poor estrus expression; poor oocyte quality; and increased embryo loss. Reductions in milk and reproductive performance during the summertime have been well documented and can be particularly costly to the producer.

Lactating dairy cows are especially prone to metabolic heat increases due in part to their high energy diet, high feed intake, and the metabolic activity required for digestion and milk production. These factors lead to excessive core body temperature during heat stress periods and are reflected in 10-20% lower seasonal conception rates across the United States, with greater reduction in sub-tropical and desert regions of the world. Dairy cow reproductive performance declines during phases of heat stress in three ways. First, the estrus intensity lessens during heat stress, so heat detection becomes more difficult. Second, fertility is reduced, and finally, the survival of early embryos is compromised. Cows have a homokinetic mechanisms that work to regulate physiology during stress events like heat stress. One of these mechanisms for the dairy cow is the redirection of blood flow from the core to the periphery for cooling. This process can lead to cellular nutrient loss which in turn may hinder fetal development. Another adaptive mechanism is to regulate body temperature by reducing feed intake. It is well known that digestion generates heat, so in theory, reduced feed intake may reduce excessive body heat gain.

CowManager Activity System

What does it measure?

Rumination: Chewing the cud, standing, or lying (32 -53%)
Eating: Chewing, licking and/or swallowing feed (10 to 22%)
Inactive: Doing nothing for 60 seconds, could be lying or standing still (22 to 34%)
High active: Behavior related to estrus signs (4 to 12%)
Active: Non-of-the above, all other categories; walking, drinking water, etc. (7 to 16%)
Temperature: A real thermometer on the cows’ ear surface, measuring the ear skin temperature

Two temperature sensors on the upper left and right corner on the back of the tag record the ear skin temperature every 15 minutes (using the mean of the two sensors). For accurate temperature measurement the CowManager tag should be approximately in the middle of left ear and in contact with the skin, not above an identification tag.

Figure 1 characterizes a herd experiencing heat stress as noted by the elevated black line (temperature) and orange line (activity). February 2021 [in the southern hemisphere] the ear temperature was above 30 degrees Celsius.

There is a drop in both rumination (blue line) and eating (green line) times during the period of heat stress. This drop in dry matter intake will result in a cascade of negative events that occur during periods of heat stress.

Figure 1. A herd experiencing heat stress as measured by CowManager using the Group overview function.

Figure 2 characterizes a herd NOT experiencing heat stress as noted by the elevated black line (temperature) and stable orange line (activity). July of 2020 the ear temperature was above 30 degrees Celsius.

Figure 2. A herd with excellent use of fans and misters for heat abatement as measured by CowManager using the Group overview function.

In dairy cows, heat stress is one of the leading causes of reduced production and reproduction. Heat stress occurs when heat from metabolism and the environment are greater than the cow’s ability to dissipate heat. Cattle rely on panting to cool themselves due to the inability to sweat well. The CowManager activity system is the ONLY activity system that comprehensively and accurately measures rumination, eating, in-active, active, estrus activity, and temperature every fifteen minutes. Using the group overview, one can easily determine if the herd or individual groups (pens) or barns are experiencing heat stress. When adding heat abatement measures such as fans, misters, increased feeding times, and/or ration changes, CowManager can monitor their effectiveness in reducing heat stress.

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