Effects of Harmful Gases on Laying Hen Performance

　　Harmful gases in poultry houses are produced mainly by the birds’ respiration and excretion, and by the decomposition of organic waste during production. When ventilation is inadequate, levels of harmful gases such as ammonia, hydrogen sulfide, and carbon dioxide rise. Once they reach a certain concentration, the birds’ immunity may decline, and in severe cases disease may occur in the flock. Therefore, poultry farmers must pay close attention to harmful gases in poultry houses to avoid serious economic losses in production.

　　1, Ammonia

　　Ammonia is a colorless gas with a pungent odor. It mainly comes from the decomposition of nitrogen-containing substances such as manure and damp litter. In modern, enclosed, high-density poultry houses, excessive emphasis is often placed on insulation while ventilation is neglected. As the moisture content of the litter increases, ammonia levels in the poultry house can easily become too high. Because ammonia is highly soluble in water, it readily dissolves and is adsorbed onto damp floors and walls, as well as the birds’ mucous membranes and conjunctiva. Prolonged exposure to high ammonia concentrations in poultry houses not only damages the epithelial tissue of the birds’ respiratory tracts and destroys their natural barrier, but also reduces resistance to bacteria and viruses, increasing mortality. When inhaled into the lungs, ammonia can pass freely through the alveolar epithelium and enter the bloodstream, where it combines with hemoglobin and impairs the blood’s oxygen-carrying capacity, causing anemia in the birds. High ammonia concentrations can also reduce feed efficiency, delay sexual maturity, and lower the egg-laying rate. Studies have shown that when the ammonia concentration in a poultry house reaches 20 milligrams/cubic meter and remains at that level for 6 weeks, it can cause pulmonary congestion and edema in chickens, reduced appetite in the flock, and a lower egg-laying rate; at 50 milligrams/cubic meter, after several days it will cause laryngeal edema, necrotizing bronchitis, pulmonary hemorrhage, and a reduced respiratory rate in chickens, with deaths in severe cases; at 80 milligrams/cubic meter for 2 months, it can reduce the egg-laying rate of laying hens by 90% and increase mortality; at 100 milligrams/cubic meter, mortality will increase significantly. Therefore, the ammonia concentration in a poultry house should generally be kept below 20 milligrams/cubic meter.

　　2, Hydrogen sulfide

　　Hydrogen sulfide is a colorless, volatile gas with the foul odor of rotten eggs. Hydrogen sulfide in poultry-house air mainly comes from the decomposition of sulfur-containing organic matter such as manure, feed residues, and litter. When poultry consume protein-rich feed and develop digestive disorders, large amounts of hydrogen sulfide may be excreted from the intestinal tract. Hydrogen sulfide is heavier than air, so its concentration is higher closer to the ground. It is highly toxic. On contact with the birds’ respiratory mucous membranes, it reacts with alkalis in the tissues to form sodium sulfide. After sodium sulfide is absorbed by the mucous membranes and enters the bloodstream, it is hydrolyzed and releases hydrogen sulfide. In the body, hydrogen sulfide stimulates the nervous system, causing acute respiratory disease, pupil constriction, cardiac weakness, acute pneumonia, and pulmonary edema. Hydrogen sulfide can also bind to the iron in hemoglobin, causing hemoglobin to lose its ability to bind oxygen and resulting in tissue hypoxia. Long-term exposure to low concentrations of hydrogen sulfide weakens the birds’ condition, reduces their disease resistance, and affects growth and egg production. At higher concentrations, it stimulates the nervous system, constricts the birds’ pupils, weakens the heart, and causes tissue hypoxia.

　　3, Carbon dioxide

　　Carbon dioxide is a colorless, odorless gas with a slightly sour taste, produced mainly by chickens' respiration. The concentration of carbon dioxide in the atmosphere has no effect on livestock and poultry themselves. However, in the poultry industry, chickens exhale more carbon dioxide than other livestock. Poor ventilation in a chicken house can therefore cause the carbon dioxide concentration to become too high. If chickens are kept for a long time in an environment with a high concentration of carbon dioxide, they will suffer chronic poisoning. Symptoms include listlessness, reduced appetite, slow weight gain, declining physical condition, reduced productivity, and weakened resistance to disease. The permitted concentration of CO2 in a chicken house is 0.5%.