The perceived temperature is closely linked to the environmental one and to the relative humidity increase and increase significantly during summer. Every animal species can correctly dissipate endogenous heat to maintain the physiological temperature in a specific perceived temperature range: past this, the body temperature increases, and the animal experiences heat stress. Bovine in a thermos-neutral environment the rectal temperature is around 37.5-39.5°C. Perceived temperature is indicated by the THI (Temperature Humidity Index) that is:
THI (°C) = Tair – (0.55 – 0.55 × Hrel) × (Tair -58), where:
Tair = and Hrel = relative humidity (%).
When the THI is 68 (dairy cows) or 78 (beef cattle) bovine have serious difficulties in heat dissipation. The higher the environmental humidity, the higher also the perceived temperature (Table 1). High-producing dairy cows cannot dissipate correctly the endogenous heat and have the first heat stress symptoms. One of them is the reduction of the feed intake and the choice of concentrates (because this reduces the endogenous heat production by rumen fermentation). However, the reduction in productive performance has a number of causative factors, not only this first aspect.
Table 1: THI values for dairy cows calculated on the basis of the temperature (°C) and the relative humidity (%). Values are divided by risk classes: thermo-neutrality (green), medium risk (yellow), alert (orange), and emergency (red). The red square highlights the average Hrel in Italy between May and September, calculated on the basis of the monthly average data of the last 30 years.
Intestinal health and barrier function
Heat stress strongly affects the intestinal mucosa. There is a change in blood circulation in order to dissipate more heat. The enterocytes are very sensitive to slight hypoxia phenomenons and to the nutrient reduction caused by these perfusional changes. Thus, when the body temperature increases, there is a reduced ATP availability and an increase in oxidative and nitrosative stress (presence of nitrogen free radicals). The result is the disruption of the intestinal epithelial tight junction (intra- and inter-cellular protein structures with a pivotal role in membrane permeability regulation) and a lower barrier function at this level. In this case, it is important to underline that the vast majority of the intestinal microflora, both positive and pathogenic, is Gram-negative so that in the intestinal lumen there is a huge amount of LPS (lipopolysaccharides, endotoxins that are included in the Gram-negative cell membrane). During heat stress periods there are the contemporary presence of these LPS and a higher permeability of the mucosa: the direct consequence is an increase in toxicosis incidence during the summer period. At the same time, the disruption of epithelial integrity causes a reduction of dietary nutrients absorption by the animal. In a few hours, this malabsorption causes the reduction of the production: in 17 hours of exposure to a 68 THI, there may be up to a 2.2 kg/head/day loss in milk production.
Other heat stress consequences
Feed intake reduction is not the only heat stress consequence. To evaluate the energy metabolism changes and milk production two groups of dairy cows are compared. The first group is the heat-stressed on (HS): the feed intake of this group is recorded and used for the second group. The control group is a pair-fed thermal neutral (PFTN) group of animals, that receive the same amount of feed ingested by the HS cows to eliminate the bias related to reduced ingestion or dietary nutrient availability.
Hormonal asset related to milk production
Somatotropin and IGF-1 (insulin-like growth factor-1) are the most important hormones for milk production. Under physiological conditions, somatotropin distributes the absorbed nutrients between the udder and the other tissues. In high producing dairy cows this distribution is favorable to the udder. The same hormone stimulates the hepatic production of IGF-1 that consequently induces an increase in milk production. Negative energy balance (NEBAL) alters this hormonal equilibrium: somatotropin synthesis increases but not the IGF-1. Thus, both milk production and fertility are affected: energy sources are directed to the metabolic pathways fundamental for animal survival. Even if the reduced feed intake causes the NEBAL, it was proven that the HS group had a higher hormonal disequilibrium than the PFTN animals: HS cows had IGF-1 concentrations significantly lower than the PFTN ones. The consequence is that HS cows had both lower milk production and fertility.
The increased energy requirement during NEBAL is faced by the animal through adipose tissue mobilization and the increase of plasma NEFA. High concentrations of circulating fatty acids lead to ketone body synthesis. These molecules are used as an alternative energy source but can cause ketosis if they are produced excessively. Under heat stress, energy metabolism is not very efficient: HS bovines have basal NEFA levels lower than the PFTN ones. In HS animals also epinephrin (that induces lipolysis and the increase of NEFA concentration) is not so effective. Heat stress affects the expression and function of many enzymes and molecules involved in lipid metabolism, regardless of the lower feed intake of the hot periods. Animals are glucose-dependent for the energy supply, with quite a null lipid mobilization and consequently, difficulties in restoring their physiological energy balance.
Glucydic and muscle metabolism
Skeletric muscle is among the main culprits of endogenous heat production: if THI is high animals move less to reduce heat production. The body temperature increase also causes the increase of the glycogen lysis and of the carbohydrates oxidation. These two energy sources are not from the diet but from the animal endogenous storages. HS bovines have a higher glycemia than the PFTN ones but their skeletal muscle maintains a good sensitivity to insulin (while the PFTN risk insulin-resistance). Under heat stress, muscle metabolism uses anaerobic glycolysis producing high concentrations of lactate and lactic acid. Thus, the animal modifies its metabolism (and especially the Krebs cycle) trying to reduce damages due to ROS (reactive oxygen species) and metabolic processes (in particular the pyruvate-dehydrogenase complex is inhibited, reducing acetyl-CoA production from pyruvate). Even if more studies are needed, these modifications could be an additional cause for heat-stressed animals reduced growth.
How to sustain animals during the summer period?
Since heat stress is an inevitable condition, it is necessary to help cattle overcome this impasse and limit production losses. It is good to remember that the animals’ water requirement increases over 50%, especially after milking. Water must be easily accessible, fresh (20-30°C), clean, and always available. The summer diet (TMR) should contain palatable ingredients, high-quality forages, sugars, and digestible fibers from concentrates (maximum forage : concentrates ratio of 40% : 60%), and should contain balanced amino acids to compensate for the reduced production of bacterial proteins. Rumen B vitamins production is reduced, so it is necessary to reintegrate them with the diet, preferably in rumen-protected form. The increase in oxidative stress can be addressed by adding vitamins A and E to the ration. Another useful additive is niacin: it causes slight vasodilation allowing a better heat dissipation from the core to the peripheral tissues without affecting the animal’s intestinal health. Niacin is highly degraded by rumen microflora: microencapsulation allows to obtain the same benefits compared to the free form by using a significantly lower inclusion. Finally, it has been shown that the combination of organic acids and nature identical compounds leads to a significant improvement in intestinal health and epithelial barrier function, improving resistance to the passage of pathogens and toxins as well as nutrient absorption.
THI higher than 68:
- Alters intestinal integrity and permeability with an increased risk of toxemia and metabolic disorders;
- Reduces feed intake and nutrient absorption with a reduction in productive and reproductive animal performance;
- Reduces the IGF-1 synthesis with the consequent reduction of growth and milk production;
- Affects energy metabolism efficiency so that the animal has to use its endogenous glycogen storage.
A balanced diet and rumen-protected feed additives such as niacin, amino acids, vitamins, and the combination of organic acids and nature identical compounds can help ruminants in maintaining high performance and good health status even under heat stress.For more information: email@example.comOriginal article here.