Heat stress is a serious challenge for dairy cows, which can harm the production, health and reproductive performance of dairy cows. To cope with heat stress, cows must adjust their metabolism. Two alternative mechanisms to deal with heat stress are:
1 First lose the heat in the body.
Including increasing respiratory rate, sweating, gasping, and the direction of the body's central blood changes to the peripheral tissue.
2 Minimize the generation of thermal energy.
Includes rumen activity, digestion, and dry matter intake. In heat stress conditions, cows change their metabolism and need more energy to cope with heat stress. As a result, maintenance needs to be increased to support the body's heat dissipation process. Cows also reduce the dry matter intake and digestion capacity of dairy cows under heat stress conditions, resulting in lower energy intake of dairy cows.
The contradiction between the need to maintain milk energy and the reduction in energy intake under heat stress results in a significant reduction in overall performance and lactation in dairy cows.
Reactive Oxygen Species (ROS)
In the context of discussing heat stress, the focus is on understanding heat stress, and in many livestock and laboratory animals, heat stress is directly related to the increase in reactive oxygen species (ROS) in the body. Therefore, heat stress has a direct relationship with oxidative stress.
ROS can cause serious damage to cells, protein molecules and DNA, resulting in increased energy consumption. Under heat stress conditions, the need for cows to maintain energy increases. In addition to increasing energy expenditure, heat stress also reduces energy intake. An increase in ROS production will cause an imbalance in the endogenous system of animals against oxidation.
In vitro and in vivo studies have shown that heat stress can increase oxidative stress in animals. Bernabucci et al. (2002) found that under heat stress conditions, red blood cell sampling was conducted on lactating Holstein cows in the transition period , and several indicators of oxidative stress were significantly increased.
Ordinate: TBARS ERYTHROCYTES thiobarbituric acid reactant in red blood cells nmol/ml
Horizontal coordinate: Days of calving
Summer, spring
Antioxidant system
Dairy cows have sophisticated enzyme systems that can cope with oxidative stress. One of the most recognized enzyme systems is the superoxide dismutase and glutathione peroxidase systems, which convert ROS into water. Other systems include vitamin systems (such as vitamin E and vitamin C) that repair damaged and damaged cell membranes and macromolecules caused by free radicals formed by unsaturated fatty acids.
These antioxidant systems require special nutrients to perform their functions, such as zinc, manganese, copper and methionine. Maintaining sufficient nutrients in the body to combat heat stress in dairy cows is particularly challenging under heat stress conditions. Stability of feed ingredients
The stability of feed ingredients is a relatively independent but very important part of the effects of hot weather on oxidative stress. The oxidation rate of fatty acids in feed ingredients increases exponentially with increasing temperature. Once the feed ingredients are oxidized, the energy value of the raw materials is usually lowered, odor is generated, the palatability is lowered, and the level of free radicals is increased. When animals eat oxidized feed ingredients, the free radicals carried will damage the intestinal epithelium. This damage further exacerbates the system imbalance caused by oxidative stress.
Abscissa: Temperature (°C)
Vertical coordinate: Oxygen consumption (ml/g/hr)
The total free radicals of the animal body system are mainly derived from the normal metabolic processes of the body (mitochondrial respiration and productivity, etc.) and oxidized feed ingredients. The effects of free radicals from these two sources will increase during heat stress. For example, a cow with a high respiratory rate consumes oxidized fat-fed cows when fed a diet that is fat-scarred due to excessive ambient temperature, compared to cows fed a diet that does not contain oxidized fat and has a normal respiratory rate. The oxidative stress is greater.
Heat stress has a significant effect on ruminant, rumen function and rumen pH, and rumen acidosis impairs the integrity of rumen epithelial cells. Damage to the rumen epithelial cells will cause bacteria to enter the animal through damaged tissue, causing liver abscesses. The formation of liver abscess will reduce the metabolic energy of the liver. The animal's endogenous immune system attacks ROS produced by bacteria and pathogens, thereby increasing oxidative stress in animals.
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