What are the main aspects of the effects of airflow on pigs?

The movement of air molecules is called airflow, and airflow that moves horizontally is called wind. Atmospheric airflow is formed when uneven temperature distribution on the Earth's surface creates pressure differences, and air molecules flow from low-temperature high-pressure areas to high-temperature low-pressure areas. In meteorology, wind is commonly described by wind speed and wind direction. Wind speed is the distance the wind moves per unit time (m/s); wind direction is the direction from which the wind blows, expressed in 8 or 16 directions, and based on the probability of wind appearing in each direction, a wind direction frequency diagram (wind rose diagram) is drawn. The wind direction frequency diagram can clearly show the prevailing wind direction of a certain area over a certain period, and it is the basis for pig farm site selection, the layout of buildings within the farm, and the design of individual pig houses.

Different areas inside a pig house also have pressure differences, which likewise form airflow. Under natural conditions, the strength of airflow inside the house is affected by two factors: one is wind pressure, that is, wind outside the pig house flows in through doors, windows or openings on the windward side and flows out through doors, windows or openings on the leeward side; the other is thermal pressure, that is, the temperature inside the pig house is generally higher than outside, and the air entering the house is heated and moves upward, flowing out from the skylight or the upper part of doors and windows. If a fan is used as the ventilation power, the direction and size of its airflow depend on the number of fans, flow rate, position, air inlets, and other conditions. In pig house ventilation, in addition to considering wind direction and wind speed, ventilation volume (air exchange volume) is an important indicator, generally expressed as the volume of fresh air required per hour per pig or per unit live weight (1 kg or 100 kg); it can also be expressed as the multiple of the pig house volume that the air exchange volume per unit time represents, which is called the number of air exchanges. The airflow direction inside a pig house can be determined with smoke, while wind speed is measured with a kata thermometer or a hot-bulb micro-anemometer.

Airflow also affects the pig's body heat regulation together with other thermal environment factors, thereby affecting its productivity and health. When the air temperature is lower than skin temperature, airflow can promote convective heat loss, and at any other temperature, airflow can promote evaporative heat loss. Therefore, at low temperatures airflow is unfavorable to body heat regulation, while at warm temperatures airflow is favorable to body heat balance, but when the air temperature is higher than skin temperature (especially when relative humidity is high), high wind speed tends to be unfavorable to the body. Experiments have shown that when the air temperature remains unchanged, if the environmental wind speed for piglets increases from 0.1 m/s to 0.3 m/s, their sensation is equivalent to a 5°C decrease in environmental temperature. Someone trained pigs in an artificial climate chamber to learn to press a push-button switch to turn off the fan, and found that at different temperatures from -5°C to 40°C, the time that pigs allowed the fan to blow air increased as the temperature rose; when the environmental temperature reached 30°C or higher, the pigs no longer turned off the fan, which shows that airflow is beneficial to pigs under high temperature conditions. In addition, ventilation can remove water vapor and harmful gases such as carbon dioxide, ammonia, and hydrogen sulfide from the house, ensuring dryness inside the house and improving air hygiene. Therefore, pig houses need ventilation in any season, especially in winter when doors and windows are often closed to prevent cold, causing dampness inside the house and deterioration of air hygiene, so attention must be paid to proper ventilation. In winter, ventilation and heat preservation are a pair of contradictions. Enterprises with the conditions can control the ventilation volume according to the humidity requirements of the pig house while meeting the temperature supply; in order to reduce costs, ventilation measures should be taken while ensuring that the environmental temperature of the pig house is basically satisfied, and drafts must be prevented from invading and harming the health of the pig herd.